Intro to Computer Science I

1

Intro to Computer Science I

Chapter 4Classes, Objects, and Methods

OOP Concepts

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Strings (1)

A String is a sequence of 0 or more characters

Every string has a length associated with it

An empty string has length 0 Each character is stored internally as a

16-bit Unicode character (16-bit integer) Each character in a string has an index

associated with it.

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Strings (2)

Literal string: sequence of 0 or more characters delimited by double quote characters

Example: "Hello" Empty string: "" Memory model:

H e l l o

0 1 2 3 4

index begins at 0

The double quotesare not part of the string

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Substring

A substring is constructed by specifying a subsequence of characters

H e l l o

0 1 2 3 4

e l l

0 1 2

A substring is a string

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Constructing a literal string

Strings are objects String greeting = "Hello World";

Hello World

Stringgreeting

object referencevariable

object reference

object

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String expressions

Strings can be concatenated together Numeric values can be automatically

converted to strings The + sign is used to denote addition

and also string concatenation RULE: In a + b if one of a or b is a

string then the other will be converted to a string if necessary

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String expression examples

String first = "William";String middle = "James";String last = "Duncan";String fullName = first + " " + middle + " " + last;

fullname has the value "William James Duncan"

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Mixed string expressions (1)

String area = "Area: " + Math.sqrt(2.0);

The value of area is"Area: 1.4142135623730951"

You can try this in BeanShell and use print(area); to see the result

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Mixed string expressions (2)

"The sum of " + x + " and " +y + " is " + x + y

if x is 3 and y is 4 then the result of this expression is"The sum of 3 and 4 is 34"

What's wrong?

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Length of a string

Prototype: int length()

Example:

String name = "Harry";int len = name.length();

The value of lenwill be 5

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Number to string conversion

Examples

String s1 = "" + age;String s2 = "" + area;

If age is 34 and area is 2.34 then s1 and s2 will have the

values "34" and "2.34"

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Extracting characters

Prototype: char charAt(int index)

Example

String s = "Hello";char c = s.charAt(1);

Here the value of c is 'e'(index begins at 0)

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Substring construction

Prototypes String substring(int firstIndex) String substring(int firstIndex, int lastIndexPlusOne)

int d = 531452;String sd = "" + d;int len = sd.length();sd = "$" + sd.substring(0,len-3) + "," + sd.substring(len-3);

result is"$531,452"

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Trimming spaces

Prototype String trim()

String s = " Hello ";s = s.trim();

result is"Hello"

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String case conversion

upper/lower case conversion Prototypes

String toLowerCase() String toUpperCase()

String test = "Hello";String upper = test.toUpperCase();String lower = test.toLowerCase();char first = test.toLowerCase().charAt(0);

results are "HELLO", "hello" and "h"

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String search methods

Searching for one string inside another Prototypes

int indexOf(char ch) int indexOf(char ch, int startIndex) int indexOf(String sub) int indexOf(String sub, int startIndex)

Method Overloading: these methods all have the same name but

they have different signatures

signature

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String search examples

String target = "This is the target string";

target.indexOf('u')

target.indexOf("the")

target.indexOf("target",13)

value is -1

value is 8

value is -1

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Displaying numbers, strings

Some prototypes: void print(int n) void print(double d) void print(String s) void println(int n) void println(double d) void println(String s) void println()

print but don'tmove to next line

print and moveto next line

Output appears in the terminal window

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Examples:

double area = 3.14159;System.out.println("Area: " + area);

double area = 3.14159;System.out.print("Area: ");System.out.println(area);

These produce the same results

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Examples:

System.out.println("Hello");

System.out.print("Hello\n");

These produce the same results since \n is interpreted as the newlinecharacter

\ is called the escape

character

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Examples:

System.out.println("\"\\Hello\\\"");

Displayed result is"\Hello\"

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The BlueJ terminal window

public void display(){ System.out.println("Radius = " + radius); System.out.println("Area = " + area); System.out.println("...");}

put this method inCircleCalculator

class

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The toString method

Prototype public String toString()

Purpose return a string representation of an object

Default representation toString is special: every class has a

default toString method with a string representation that is not very useful so we normally provide our own version of this method.

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Default toString example

bsh % addClassPath("c:/book-projects/chapter3");bsh % CircleCalculator circle = new CircleCalculator(3.0);bsh % String rep = "toString gives " + circle;bsh % print(rep);toString gives CircleCalculator@4d1d41bsh % rep = "toString gives " + circle.toString();bsh % print(rep);toString gives CircleCalculator@4d1d41bsh % print(circle);CircleCalculator@4d1d41

Using the object name circle in a string expression isequvalent to using circle.toString()

Not verymeaningful

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Define our own toString

public String toString(){ return "CircleCalculator[radius=" + radius + ", area=" + area + ", circumference=" + "]";}

add this methodto CircleCalculator cass

print(circle);CircleCalculator[radius=3.0, area=28.274333882308138,circumference=18.84955592153876]

now the displayed result is meaningful

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Why toString ?

It is useful to write a toString method for every class. The toString representation of an object obj can be displayed using System.out.println(obj);

This is useful for debugging classes (finding logical errors) since it provides an easy way to display the state of any object.

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Formatting data (Java 5)

public static String format(String f, Object... args)

The String class contains a static method with prototype

Here f is the format string and Object... args represents theargument list of values to be formatted.

There is also a new printf method that can be used with aformat string that has the prototype

public void printf(String f, Object... args)

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Format codes (Java 5)Format codes begin with %Here are some useful ones (see javadoc for complete list)

%5d format an integer right justified in field of width 5%-5d format at integer left justified in a field of width 5%-20s format s string left justified in a field of width 20%15.5f format a floating point number right justified in a field

of width 15 using fixed format rounded to 5 digits afterthe decimal point

%.5f format a floating point nuber in a field that just fitsusing fixed format rounded to 5 digits after the decimalpoint

%20.8e format a floating point number right justified in a fieldof width 20 using exponential (scientific) formatrounded to 8 digits after the decimal point

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Format example (1)

The statements

int i = 3;double pi = Math.PI;String end = "End";String f = String.format("answer: %5d%15.5f%10s\n", i, pi, end);System.out.println(f);

produce the output

answer: 3 3.14159 End

%5d %15.5f %10s

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Format example (2)

The statements

int i = 3;double pi = Math.PI;String end = "End";System.out.printf("answer: %5d%15.5f%10s\n", I, pi, end);

produce the same output using printf

answer: 3 3.14159 End

%5d %15.5f %10s

To reuse formats use the format method in the String class

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Format example(3)

In the CircleCalculator class we could include the followingdisplay method that formats the results.

public void display(){ System.out.printf("Radius = %.5f\n", radius); System.out.printf("Area = %.5f\n", area); System.out.printf("Circumference = %.5f\n", circumference);}

to display the values rounded to 5 digits after the decimal pointin fields that just fit.

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Intro to Computer Science I

Chapter 4Example classes that use the

String class

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Design, Implement, Test

Begin with an English description of a class

Design the class by deciding what constructors and methods it should have. This is called writing the public specification

or public interface. Write the complete class by providing the

implementation Test the class by itself using BlueJ

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BankAccount class description

A BankAccount object should represent a bank account using an account number, an owner name, and a current balance.

There should be a constructor for creating a bank account given these values.

There should be methods to withdraw or deposit a given amount and the usual "get methods" for retrurning the account number, owner name, and balance.

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BankAccount class design

public class BankAccount{ // instance data fields go here

public BankAccount(int accountNumber, String ownerName, double initialBalance) {...} public void deposit(double amount) {...} public void withdraw(double amount) {...} public int getNumber() {...} public String getOwner() {...} public double getBalance() {...} public String toString() {...}}

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How will we use the class

BankAccount account = new BankAccount(123, "Fred", 125.50);

account.withdraw(100);

System.out.println("Balance is " + account.getBalance());

Construct an object

withdraw $100

display balance

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formal and actual arguments

BankAccount myAccount = new BankAccount(123, "Fred", 125.50);

public BankAccount(int accountNumber, String ownerName, double initialBalance) { ...}

Actual arguments in constructor call expression

Formal arguments in constructor prototype

Same idea applies to method call expressions and prototypes

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Implementing the class (1)

instance data fields

private int number;private String name;private double balance;

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constructor implementation

public BankAccount(int accountNumber, String ownerName, double initialBalance){ number = accountNumber; name = ownerName; balance = initialBalance;}

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deposit, withdraw (mutator methods)

public void deposit(double amount){ balance = balance + amount;}

public void withdraw(double amount){ balance = balance - amount;}

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get methods (enquiry methods)

public int getNumber(){ return number;}

public String getName(){ return name;}

public double getBalance(){ return balance;}

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toString method

public String toString(){ return "BankAccount[" + "number=" + number + ", name=" + name + ", balance=" + balance + "]";}

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BankAccount class (1)

public class BankAccount{ private int number; private String name; private double balance;

public BankAccount(int accountNumber, String ownerName, double initialBalance) { number = accountNumber; name = ownerName; balance = initialBalance; }

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public void deposit(double amount) { balance = balance + amount; }

public void withdraw(double amount) { balance = balance – amount; }

public int getNumber() { return number; }

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public String getName() { return name; }

public double getBalance() { return balance; }

public String toString() { return "BankAccount[number=" + number + ", name=" + name + ", balance=" + balance + "]"; }}

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BankAccount with BeanShell

addClassPath("c:/book-projects/chapter4/bank-account");BankAccount account = new BankAccount(123, "Fred", 125.50);account.withdraw(100);print(account.getBalance());25.5account.deposit(100);print(account.getBalance());125.5print(account);BankAccount[number=123, name=Fred,balance=125.5]

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BankAccount with BlueJ

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InitialsMaker description

An InitialsMaker object uses the first and last name of a person to produce the initials in uppercase. For example, if the name is Henry James or henry james then the initials are HJ.

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InitialsMaker design

public class InitialsMaker{ // instance data fields go here

public InitialsMaker(String firstName, String lastName) {...}

public String getInitials() {...} public String toString() {...}}

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instance data field

Another possibility

private String initials;

private String firstName;private String lastName;private String initials;

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Constructor

public InitialsMaker(String firstName, String lastName){ initials = firstName.substring(0,1); + lastName.substring(0,1); initials = initials.toUpperCase();}

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get method

toString method

public String getInitials(){ return initials;}

public String toString(){ return "InitialsMaker[initials=" + initials + "]";}

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InitialsMaker with BeanShell

addClassPath("c:/book-projects/chapter4/strings");InitialsMaker maker = new InitialsMaker("harry", "James");print(maker.getInitials());HJprint(maker);InitialsMaker[initials=HJ]

Test all four possibilities for the names:"Henry James", "Henry james", "henry James'", "henry james"

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PasswordGenerator description

A PasswordGenerator object generates random 7 character passwords. The first four characters should be lower case letters and the last three characters should be digits 0 to 9.

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Random number generation

In the Math class there is the random() method that returns a real number

In the java.util package there is a Random class that does what we want.

Constructors and methods public Random() public Random(long seed) public int nextInt()

10 r

repeatable by using the same

seed again

sequence generateddepends on currenttime in milliseconds

10 ni

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Random digits and characters

nextInt(10) generates random integer 0,1,...,9 which

can be converted to a character nextInt(26)

generates random integer 0,1, ...,25 which can be used as an index into the string

"abcdefghijklmnopqrstuvwxyz"

to generate a random letter

index 0

index 25

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PasswordGenerator design

public class PasswordGenerator{ // instance data fields go here

public PasswordGenerator() {...} public PasswordGenerator(long seed) {...} public String next() {...}}

Each call to next producesa random password

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Using the class

First construct PasswordGenerator object PasswordGenerator gen = new PasswordGenerator();

Each call to next returns a random password String p1 = gen.next(); String p2 = gen.next(); String p3 = gen.next();

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Implementation (1)

data fields

private static final String LETTERS = "abcdefghijklmnopqrstuvwxyz";

private Random random;

Instance data feld

static class data feld(not associated with any object)

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Implementation (2)

Constructors

public PasswordGenerator(){ random = new Random();}

public PasswordGenerator(long seed){ random = new Random(seed);}

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Implementation (3)

The next method

public String next(){ int index; String password = ""; index = random.nextInt(26); password = password + LETTERS.substring(index, index + 1); // repeat these two statements three more times index = random.nextInt(10); password = password + index; // repeat these two statements two more times return password;}

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Import statement (1)

The Random class is not in the standard java.lang package that is automatically imported into every java class

It is in java.util and needs to be imported excplicity using import java.util.Random;

This statement goes OUTSIDE the class declaration at the top

of the file

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Import statement (2)

Another variation is import java.util.*;

This can import and class in the java.util package

If the import statement is not used then it is necesary to use the fully qualified name java.util.Random

of the Random class instead of the short name

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Testing the class

addClassPath("c:/book-projects/chapter4/strings");PasswordGenerator gen = new PasswordGenerator();print(gen.next());avfi637print(gen.next());iqde665gen = new PasswordGenerator(); // make a new oneprint(gen.next());zuwe456gen = new PasswordGenerator(123);print(gen.next());eomt574gen = new PasswordGenerator(123);print(gen.next());eomt574

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Test with BlueJ

Construct an object Use the next method repeatedly from

the object menu Can also insert the simple method

public test(){ System.out.println(next());}

this.next()

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The "this" object

When you write a class and call a method from the same class what is the object on which the method is invoked?

Compare gen.next(); this.next(); next();

Method call expression with an explict object called gen

Method call expression from within the

PasswordGenerator class

If object name is omitted for an instance method the compiler assumes that the

method is in the same class

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Association & Aggregation (1)

Programs normally consist of interacting objects from several classes.

Types of classes built-in classes such as String or Random classes obtained from someone else classes you write yourself such as CircleCalculator

The terms association and aggregation are used to describe how classes can relate to each other.

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Association (1)

Some classes such as CircleCalculator from Chapter 3 are not related with any other classes.

Others depend on other classes Example

PasswordGenerator uses (depends on) the String and Random classes

This dependence of one class on others is called association

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Association (2)

The PasswordGenerator class is associated with, or uses, or depends on the String and Random classes.

The relationship is not symmetric For example, we don't say that the String class is associated with the PasswordGenerator class

The String class is designed to be used by other classes

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Association (3)

Class A is associated with class B if A uses B. This can occur in several ways.

An object of B is used as a local variable in a constructor or method in A

An object of B is used as a method or constructor argument in A

An object of B is used as a return value of a method in A

An object of B is an instance data field in A

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Aggregation

Aggregation (sometimes called composition) refers to the important last case that an object of B is used as an instance data field of A.

This is gives rise to an object hierarchy Example

Random class is used as instance data fields in PasswordGenerator

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TriangleCalculatorTester

public class TriangleCalculatorTester{ public TriangleCalculatorTester() { } public void doTest(double a, double b, double g) { TriangleCalculator tri = new TriangleCalculator(a,b,g); System.out.println("Sides: " + tri.getA() + ", " + tri.getB() + ", " + tri.getC()); System.out.println("Angles: " + tri.getAlpha() + ", " + tri.getBeta() + ", " + tri.getGamma()); System.out.println("Angle sum is " + tri.checkAngleSum()); }}

Association

Can omit constructor

In BlueJ a dotted arrrow indicates association

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Association in BlueJ

doTest produces this

output

the dotted arrowindicates

association

TriangleCalculatorTester is associatedwith (uses) TriangleCalculator

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Point class design

public class Point{ private double x, y; public Point() {...} public Point(double x, double y){...} public double getX() {...} public double getY() {...} public String toString() {...}}

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Point class implementation (2)

public class Point{ private double x, y;

public Point() { x = y = 0.0; } public Point(double x, double y) { this.x = x; this.y = y; }

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Point class implementation (2)

public double getX() { return x; } public double getY() { return y; }

public String toString() { return "Point[" + x + ", " + y +"]"; }} // end of class

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Testing the Point class

addClassPath("c:/book-projects/chapter4/geometry");import Point; // necessary or we get java.awt.PointPoint origin = new Point(0,0);Point p = new Point(1,2);print(origin);Point[0.0, 0.0]print(p);Point[1.0, 2.0]print(p.getX());1.0print(p.getY());2.0

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Designing a circle class

We want to develop a simple Circle class to describe geometric circles in terms of their center (x,y) and radius r

Method 1 Use three double type instance data fields

for the center coordinates and the radius Method 2 (using aggregation)

Use two instance data fields. One is a Point object for the center and the other is the radius

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Circle class design (method 1)

public class Circle{ private double x, y, radius; public Circle() {...} public Circle(double x, double y, double r) {...} public double getX() {...} public double getY() {...} public double getRadius() {...} public String toString() {...}}

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Circle class design (method 2)

public class Circle{ private Point center; private double radius;

public Circle() {...} public Circle(double x, double y, double r) {...} public Circle(Point c, double r) {...} public Point getCenter() {...} public double getRadius() {...} public String toString() {...}}

aggregation

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Implementation (1)

Constructors

public Circle(Point p, double r){ center = p; radius = r;}public Circle(double x, double y, double r){ center = new Point(x,y); radius = r;}public Circle(){ center = new Point(); radius = 1;}

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Implementation (2)

Methods

public double getRadius(){ return radius;}

public Point getCenter(){ return center;}

public String toString(){ return "Circle[" + center + ", " + radius + "]";}

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Testing the class (BeanShell)

addClassPath("c:/book-projects/chapter4/geometry");import Point;Point center = new Point(3,4);Circle c1 = new Circle();Circle c2 = new Circle(center, 5);Circle c3 = new Circle(3,4,5);print(c1);Circle[Point[0.0, 0.0], 1.0]print(c2);Circle[Point[3.0, 4.0], 5.0]print(c3);Circle[Point[3.0, 4.0], 5.0]double x = c2.getCenter().getX();print(x);3.0

multiple methodcall expressions

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Testing the class (BlueJ)

Point object for (3,4)

use it here as

constructor argument

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CircleTester (1)

public class CircleTester{ public CircleTester() { }

public void doTest() { Point center = new Point(3,4); Circle c1 = new Circle(); Circle c2 = new Circle(center, 5); Circle c3 = new Circle(3,4,5);

Construct 3 circle objects

can omit constructor

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CircleTester (2)

System.out.println("c1 = " + c1); System.out.println("c2 = " + c2); System.out.println("c3 = " + c3);

double radius = c2.getRadius(); double x = c2.getCenter().getX(); double y = c2.getCenter().getY(); System.out.println("Radius = " + radius); System.out.println("Center x = " + x); System.out.println("Center y = " + y);

} // end of doTest method

} // end of CircleTester class

display using

toString

aggregation

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Other Library classes

Dates and timesCalendars

Currency formattingnumeric formatting

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Date class

Represents dates as the number of milliseconds since January 1, 1970, 00:00:00 GMT as a long value

Example 1055681816162 =

Sun Jun 15 08:56:57 EDT 2003

toString provides this

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Date class specification

Some of the constructors and methods arepublic Date()

public Date(long date)

public long getTime()public void setTime(long date)public String toString()

date right now

Date class is in package java.util

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Date example (BeanShell)

import java.util.Date;Date now = new Date();print(now);Sun Jun 15 08:56:56 EDT 2003long t = now.getTime();print(t);1055681816162Date first = new Date(0L);print(first);Wed Dec 31 19:00:00 EST 1969first.setTime(0L + 1000L * 60L * 60L * 24L);print(first);Thu Jan 01 19:00:00 EST 1970

We are 5 hours ahead of

midnight GMT

milliseconds in a day

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SimpleDateFormat class

We need to be able to display dates in various formats. SimpleDateFormat can do this

Constructors and methods

public SimpleDateFormat()public SimpleDateFormat(String pattern)public String format(Date d);

Convert a Date object to the format specified by the

constructor

This class is inpackage

java.text

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Example (BeanShell)

import java.util.Date;import java.text.SimpleDateFormat;Date now = new Date();SimpleDateFormat f1 = new SimpleDateFormat();String n1 = f1.format(now);print(n1);6/15/03 8:56 AMSimpleDateFormat f2 = new SimpleDateFormat("dd/MM/yyyy");print(f2.format(now));15/06/2003SimpleDateFormat f3 = new SimpleDateFormat("HH:mm:ss z");print(f3.format(now));08:56:56 EDT

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Calendar class

A higher level class than Date that knows about year, month, day of month, day of year, etc.

Java can use any of the world's calendars depending on the locale.

For us this is the Gregorian calendar

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Constructing a Calendar

public static Calendar getInstance()

A constructor is not used to create a Calendar object.Instead a static class methodis used. Method prototype is

Recall that a static method in a class is not associated with any objectsof the class. The Math class methods were like this.

Compare Math.sqrt with Calendar.getInstance

Calendar now = Calendar.getInstance();

This is a static methodcall expression

For a static (class) method the class name is used

instead of an object name

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Instance versus static method

Instance method call expression

Static method call expression

objectName.methodName(actualArgumentList)

ClassName.methodName(actualArgumentList)

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Some Calendar methods

public Date getTime()

public int get(int field)

public void set(int field, int value)

public void set(int year, int month, int day)

Calendar fields are integer constants such as Calendar.YEAR,

Calendar.MONTH, Calendar.DAY_OF_MONTH

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Using Calendar class (1)

import java.util.Date;import java.util.Calendar;Calendar now = Calendar.getInstance();Date time = now.getTime();print(time);Sun Jun 15 10:04:12 EDT 2003print(now.get(Calendar.YEAR));2003print(now.get(Calendar.MONTH)); // Jan is month 05print(now.get(Calendar.DAY_OF_MONTH));15print(now.get(Calendar.DAY_OF_WEEK)); // Sunday is 11

factorymethod

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Using Calendar class (2)

print(now.get(Calendar.DAY_OF_YEAR));166

Calendar christmas = Calendar.getInstance();

int year = christmas.get(Calendar.YEAR);

christmas.set(year, Calendar.DECEMBER, 25);print(christmas.getTime());Thu Dec 25 11:13:01 EST 2003

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Leap years

import java.util.Calendar;

Calendar feb2003 = Calendar.getInstance();feb2003.set(2003, Calendar.FEBRUARY, 1));

Calendar feb2004 = Calendar.getInstance();feb2004.set(2004, Calendar.FEBRUARY, 1));

print(feb2003.getActualMaximum( Calendar.DAY_OF_MONTH));28print(feb2004.getActualMaximum( Calendar.DAY_OF_MONTH));29

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Person class (1)

import java.util.Calendar;

public class Person{ private String name; private int birthYear;

public Person(String name, int birthYear) { this.name = name; this.birthYear = birthYear; }

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Person class (2)

public String getName() { return name; }

public int getBirthYear() { return birthYear; }

public int age() { Calendar now = Calendar.getInstance(); return now.get(Calendar.YEAR) - birthYear; }} // end of class

Using a Calendarobject to determine

the person's age

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CalendarMonth class (1)

Suppose we want to display a calendar for a given year and month.

We don't need the full complexity of the Calendar class

We can write a simpler version called CalendarMonth called an adapter class

An adpater class is a class which makes another class easier to use in specific problems

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CalendarMonth class (2)

We need the following functionality only the year and month parts of a Date are

required We need the day of the week for the first day of the

month We need to know the number of days in the month

properly accounting for leap years in February We need the names of the months so that we can

print headings. This class is called an adapter class since it

provides a simpler more specialized version of a more complicated class.

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Class design (specification)

public class CalendarMonth{ public CalendarMonth() {...} public CalendarMonth(int year, int month) {...} public int getYear() {...} public int getMonth() {...} public int dayOfWeek() {...} public int daysInMonth() {...} public String monthName() {...} public String toString() {...}}

for first dayof month(1 to 7)

calendarfor this month

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Class implementation (1)

import java.util.Calendar;import java.util.SimpleDateFormatpublic class CalendarMonth{ private Calendar calendar;

public static final int JANUARY = Calendar.JANUARY; ... public static final int SATURDAY = Calendar.SATURDAY.

adapt this class

Constants for month and day names

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public CalendarMonth() { calendar = Calendar.getInstance(); calendar.set(Calendar.DAY_OF_MONTH, 1); }

public CalendarMonth(int year, int month) { calendar = Calendar.getInstance(); calendar.set(year, month, 1); }

set calendar tofirst day of month

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public int getYear() { return calendar.get(Calendar.YEAR); }

public int getMonth() { return calendar.get(Calendar.MONTH); }

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public int dayOfWeek() { return calendar.get( Calendar.DAY_OF_WEEK); }

public int daysInMonth() { return calendar.getActualMaximim( Calendar.DAY_OF_MONTH); }

1 = Sunday

calendar is already set to first day of

month

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public String monthName() { SimpleDateFormat f = new SimpleDateFormat("MMMM"); return f.format(calendar.getTime()); }

public String toString() { SimpleDateFormat f = new SimpleDateFormat("MMMM yyyy"); return f.format(calendar.getTime()); }} // end of class

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Using CalendarMonth class (1)

addClassPath("c:/book-projects/chapter4/calendar");

CalendarMonth thisMonth = new CalendarMonth();print(thisMonth.dayOfWeek());2print(thisMonth.daysInMonth());30print(thisMonth.monthName());Septemberprint(thisMonth);September 2003

Monday

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Using CalendarMonth class (2)

// continued from previous slide

CalendarMonth feb2004 = new CalendarMonth(2004, CalendarMonth.FEBRUARY);print(feb2004.dayOfWeek());1print(feb2004.daysInMonth());29print(feb2004.monthName());Februaryprint(feb2004);February 2004

our constants

leap year

Sunday

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CalendarMonthTester (1)

public class CalendarMonthTester{ public CalendarMonthTester() {} public void doTest() { CalendarMonth thisMonth = new CalendarMonth(); System.out.println("First day of month is " + thisMonth.dayOfWeek()); System.out.println("Number of days in month is " + thisMonth.daysInMonth()); System.out.println("Month name is " + thisMonth.monthName()); System.out.println("Calendar name is " + thisMonth);

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CalendarMonthTester (2)

System.out.println(); CalendarMonth feb2004 = new CalendarMonth(2004, CalendarMonth.FEBRUARY);

System.out.println("First day of month is " + thisMonth.dayOfWeek()); System.out.println("Number of days in month is " + thisMonth.daysInMonth()); System.out.println("Month name is " + thisMonth.monthName()); System.out.println("Calendar name is " + thisMonth); } // end of doTest} // end of class

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CalendarMonth in Bluej

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Currency formatting

Depends on locale English: $100,000.56 French: 100000,56

$ NumberFormat is in java.text package Construct a NumberFormat object

Format it as a string

NumberFormat currency = NumberFormat.getCurrencyInstance();

double salary = 4000.56String value = currency.format(salary);

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BeanShell example

import java.text.NumberFormat;double salary = 100000.555;NumberFormat currency = NumberFormat.getCurrencyInstance();print(currency.format(salary));$100,000.56NumberFormat currencyCF = NumberFormat.getCurrencyInstance( Locale.CANADA_FRENCH);print(currencyCF.format(salary));100000,56 $

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Formatting fixed numbers

Formatting numbers with a fixed number of digits after decimal point

import java.text.DecimalFormat;DecimalFormat fix = new DecimalFormat(" 0.00000;-0.00000);

print(fix.format(Math.PI)); 3.14159print(fix.format(-Math.PI));-3.14159

format forpositive numbers

format fornegative numbers

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Formatting scientific numbers

Formatting numbers with a an exponent and five digits after decimal pointimport java.text.DecimalFormat;

DecimalFormat fix = new DecimalFormat(" 0.00000E000;-0.00000E000);double d = 1.2345678E-23;print(sci.format(d)); 1.23457E-023print(fix.format(d));-1.23457E-023

format forpositive numbers

format fornegative numbers

119

Review of OOP concepts

constructing objectsobject referencesusing references

data encapsulation and integrityinstance variables and methods

static variables,constants,methods

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Constructing objects

Objects are constructed in two ways Using a constructor

Circle c1 = new Circle(3,4,5); or using a static factory method which

returns an object of the class using a static method call expression

Calendar now = Calendar.getInstance();

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Constructor Examples

Circle c1 = new Circle(new Point(3,4), 5);Point p = new Point(3,4);Point q = new Point();Circle c1 = new Circle(3, 4, 5);BankAccount a = new BankAccount(123, "Fred", 4000);SimpleDateFormat f = new SimpleDateFormat("MMMM yyyy");

public Circle(Point p, double radius)public Point(double x, double y)public Circle(double x, double y, double radius)public BankAccount(int number, String name, double balance)public SimpleDateFormat(String pattern)

Constructor call expressions (underlined)

Constructor prototypes

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Static Factory Examples

Calendar now = Calendar.getInstance();NumberFormat currency = NumberFormat.getCurrencyInstance();

public static Calendar getInstance()public static NumberFormat getCurrencyInstance()

Static method call expressions (underlined)

Static method prototypes

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public Circle(double x, double y double r){ center = new Point(x,y); radius = r;}

public Circle(){ center = new Point(); radius = 1;}

Using this as a constructor call

public Circle(Point p, double r){ center = p; radius = r;}

this(new Point(),1);

this(new Point(x,y),r);

"this" will callthis constructor

WARNING

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The Default constructor

If you don't put any constructors in a class the default no-arg constructor

is automatically provided by the compiler.

For example in CircleTester we could have omitted the no-arg constructor that was included in the class.

public ClassName(){}

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Miranda convention

You have a right to a constructor. If you do not have one, a default one will be provided to you by the compiler.

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What does it do?

The default constructor simply supplies default initialization for any uninitialized instance data fields: A value of zero is assigned to all

uniinitialized numeric data fields. A reference value of null is assigned to all

uninitialized data fields of object type (explained later)

Try it: see page 142 of text book

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Object references

Constructing an object is a three step process Memory space is allocated for the object

and its instance data fields. A reference (address) to the object is

returned so that it can be located This reference is assigned as the value of

an object reference variable This variable is the object reference

variable

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Pictorial representation

ObjectData

ClassNameobjectName

nullobjectName

ClassName objectName =

new ClassName(actualArguments)

ClassName objectName = null;

ClassName objectName;

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Circle Example

Circle c;

Circle c = null;

Circle c = new Circle(3,4,5);

Uninitializedreference

null reference

InitializedReference

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Primitive and reference types

Primitive types numeric types such as int, long, float,

double

Reference types object types such as String, Circle,

BankAccount

Hello World

Stringgreeting

17.902area

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Why do we need both types?

Types such as int and double are primitive types for efficiency reasons. There is a certain amount of overhead in following a reference to find an object.

There are wrapper classes that let us convert the primitive types to object types

Example: Integer class encapsulates an int value

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Assignment for reference types

Primitive types: the assignment statement a = b; means to assign the value of b as the value of a.

Reference types: the assignment statement a = b; means to assign the reference b as the value of the reference a and this means that a and b now both reference the same object

assignment does not copy objects.

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a=b for primitive types

17a

19b

19a

19b

Before a = b After a = b

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a=b for reference types

Before a = b After a = b

a objectA

b objectB

a objectA

b objectB

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BankAccount example

BankAccount fred = new BankAccount(123, "Fred", 150.0);BankAccount mary = new BankAccount(345, "Mary", 250.0);mary = fred; // both reference fredmary.withdraw(100.0);print(mary);BankAccount[number=123, name=Fred, 50.0]print(fred);BankAccount[number=123, name=Fred, 50.0]

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Where are references used?

Local variables in the constructor or method Example: TriangleCalculator tri = new TriangleCalculator(a,b,g);

Constructor or method arguments Example: public Circle(Point p, double r)

method return values Example: return center;

instance data fields (aggregation) Example: private Calendar calendar;

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Data encapsulation (1)

In OOP the data is spread out over the instance data fields of one or more objects

Each object is only responsible for its own data

This data is normally private so that it cannot be directly changed outside the class

This is called data encapsulation.

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Data encapsulation (2)

Classes can be mutable or immutable For mutable classes the constructors

and methods that change the values of one or more instance data fields can be written to check that objects are not in an inconsistent state.

Example: See page 146 for discussion of public data fields and set methods.

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Side effects

Changing the state of an object indirectly from outside the class

Sometimes this is desirable Sometimes side-effects are undesirable Side-effects are normally created by

returning references from methods that can be used outside the class to change the state of an object.

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The MPoint class

Our Point class is immutable: it is not possible to change the state of a Point object after it has been constructed

Consider a mutable version called MPoint like Point but with following set methodspublic void setX(double x){ this.x = x;}public void setY(double y){ this.y = y;}

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The MCircle class

Identical to Circle except that it uses aggregation with the mutable MPoint class instead of the immutable Point class

The following constructor has side-effectspublic MCircle(MPoint p, double r){ center = p; radius = r;}

center can be changed through p

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Example of side-effect (1)

addClassPath("c:/book-projects/chapter4/side-effects");MPoint p = new MPoint(3,4);MCircle c = new MCircle(p, 5);print(c);MCircle[MPoint[3.0, 4.0], 5.0]p.setX(999);print(c);MCircle[MPoint[999.0, 4.0], 5.0]

The change in the x coordinate of point p outside the MCircle class hasthe side-effect of changing the x coordinate of the center of the circle.

This is normally an undesirable side-effect since it violates the dataencapsulation.

Also try itin BlueJ

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addClassPath("c:/book-projects/chapter4/side-effects");MCircle c = new MCircle(3, 4, 5);print(c);MCircle[MPoint[3.0, 4.0], 5.0]MPoint p = c.getCenter();p.setX(999);print(c);MCircle[MPoint[999.0, 4.0], 5.0]

Also try itin BlueJ

reference tocenter isreturned

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The problems occurs with the assignment statement

and with the statement

This gives two references to the same point object. One reference inside the class (center) and one outside the class (p).

center = p;

return center;

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999x

MPoint

center

5radius

4y

MCircle

p

c

There are two references toone MPoint object

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No side-effect in original classes

addClassPath("c:/book-projects/chapter4/geometry");import Point;Point p = new Point(3,4);Circle c = new Circle(p, 5);print(c);Circle[Point[3.0, 4.0], 5.0]MPoint p = c.getCenter();p = new Point(999,4);print(c);Circle[Point[3.0, 4.0], 5.0]

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Fixing the side-effect (1)

Include a copy constructor in MPoint class

General form of copy constructor

public MPoint(MPoint p){ x = p.x; y = p.y;}

public ClassName ( ClassName objectName )

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Fixing the side-effect (1)

Revise the constructor that takes an MPoint object as an argument

Revise getCenter method

public MCircle(MPoint p, double r){ center = new MPoint(p); radius = r;}

public MPoint getCenter(){ return new MPoint(center);}

Now centerreferences its ownprivate copy of the

MPoint object

Now we return acopy of the private

MPoint object

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Side-effects are gone

addClassPath( "c:/book-projects/chapter4/no-side-effects");MPoint p = new MPoint(3,4);MCircle c = new MCircle(p, 5);print(c);MCircle[MPoint[3.0, 4.0], 5.0]p.setX(999);print(c);MCircle[MPoint[3.0, 4.0], 5.0]

This has noeffect on MCircle

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Pictorial Representation

999x

MPoint

center

5radius

4y

MCircle

p

c

3x

MPoint

4y

Now p and c arereferences to differentobjects

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BankAccount transfer (1)

public class TransferAgent{ public TransferAgent(BankAccount from, BankAccount to) {...} public void transfer(double amount) {...}}

The constructor takes references to the two BankAccount objectsthat participate in the transfer.

The transfer method does the transfer of a given amount fromone account to the other

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BankAccount transfer (2)

public class TransferAgent{ private BankAccount from; private BankAccount to;

public TransferAgent(BankAccount from, BankAccount to) { this.from = from; this.to = to; }

public void transfer(double amount) { from.withdraw(amount); to.deposit(amount); }}

Aggregation

bank-account project

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Side-effects (1)

The BankAccount class is mutable and there will be side-effects in the TransferAgent class

The side-effects are desired here since the purpose of the transfer method is to change the balances in the two accounts referenced by from and to suplied by the user.

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Side-effects (2)

addClassPath("c:/book-projects/chapter4/bank-account");

BankAccount fred = new BankAccount(123,"Fred",1000);BankAccount mary = new BankAccount(345,"Mary",1000);

TransferAgent agent = new TransferAgent(fred, mary);agent.transfer(500);

print(fred);BankAccount[number=123, name=Fred, balance=500]print(mary);BankAccount[number=345, name=Mary, balance=1500]

try it in BlueJ

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Instance variables

Instance variables are also called instance data fields

They are declared in the class but outside any method or constructor

Only instance methods have access to these variables

Each object has its own set of instance variables

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Instance methods

Instance methods are associated with objects of the class

Instance methods can access the instance data fields

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Using an instance method

To use or call an instance method means to invoke it on some object using an instance method call expression

We also say that instance methods are used to send messages to objects.

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Examples

int length = name.length();char first = test.toUpperCase.charAt(0);password = password + LETTERS.substring(index,index+1);String upper = test.toUpperCase();account.deposit(100));String firstInitial = firstName.substring(0,1).toUpperCase();double x = c2.getCenter().getX();doCalculations(); // this is the implied objectlong t = now.getTime();String n3 = f3.format(now);

Method call expressions are underlined in red

objectName.methodName(actualArguments)this.methodName(actualArguments)

message composition

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prototypes and call expressions

String letter = LETTERS.substring(index, index + 1);

public String substring(int first, int lastPlusOne)

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Method composition

Also called message composition

objectName.methodName(args1) . methodName(args2) ... methodName(argsN)

this.methodName(args1) . methodName(args2) ... methodName(argsN)

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Static variables (1)

Also called class variables Declared using the static modifier

inside a class but outside any method or constructor declaration.

We have not used them yet Example

private static int count = 0;

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Static variables (2)

They are not associated with any objects.

They belong to the class. Each static variable is shared by all the

objects of the class (shared variable) This is very different from instance

variables1 2 3

0count

4

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Static constants

Constants are declared to be static since there is no need to have separate copies of a constant, one for each object.

Examplespublic static final String LETTERS = "abcdefghijklmnopqrstuvwxyz";

public static final int JANAURY = Calendar.JANUARY;

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Static methods (1)

They are methods that are not associated with any objects of a class.

To use them in another class prefix the name of the method with the class namebeta = Math.toDegrees(beta);

System.out.println("Area: " + area);Calendar now = Calendar.getInstance();NumberFormat currency = NumberFormat.getCurrencyInstance();

Static method call expressions are underlined in red

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Static Methods (2)

General syntax instance method call expressions

General syntax for static method call expressions

(1) objectName.methodName(actualArguments)(2) this.methodName(actualArguments)

(1) className.methodName(actualArguments)(2) methodName(actualArguments)

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Counting objects (1)

We can illustrate static variables (shared varibles) and static methods by writing a version of the Point class that keeps track of the number of Point objects that have been counted so far.

Add following static data field to the Point class

public static int count = 0;

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Counting objects (2)

Add the following statement to the body of each different constructor (but not if "this" is used)

Add the following static method to the classpublic static int getCount(){ return count;}

count++;

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Testing Point (BeanShell)

addClassPath( "c:/book-projects/chapter4/static-variable");import Point;Point p1 = new Point(1,2);Point p2 = new Point(3,4);Point p3 = new Point(4.5);Point p4 = p1;int objectCount = Point.getCount();print(objectName);3

this does notcreate a new object

Also try it with BlueJNote that static methods appear on the Constructor menunot the object menu.

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Classifying variables and args

Instance data fields instance variables associated with objects

Static (class) data fields static variables not associated with objects

Local variables declared in the body a constructor or method and

are only accessible there Formal arguments

declared in method or constructor prototype and are local variables

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Call by value

Argument passing mechanism is always call by value in Java: if an actual argument is a variable then a

copy of its value is supplied as the value of the formal argument.

If an actual argument is a literal (literal string or number for example) then this value is supplied as the value of the formal argument

If an actual argument is an expression then it is evaluated and its value is supplied as the value of the formal argument.

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Call by value (primitive type)

If a constructor or method argument is a primitive type and a variable is used as an actual argument, then the value of the variable can never be changed by the method or constructor since only a copy of the variable's value, not its location, is passed to the method.

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Example (BeanShell)

void addOne(int k) { k = k + 1; }int m = 5;addOne(m);print(m);5print(k);// Error: indefined variable

The value of m is not changed by the addOne methodsince only the value 5 is passed to the method so themethod simply adds 1 to the local variable k whichdisappears anyway when the method exits.

static methods can be defined directly

in BeanShell

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Example (BlueJ)

Write the following simple class

public class ArgumentTester1{ public void doTest() { int m = 5; addOne(m); System.out.println(m); } private static void addOne(int k) { k = k + 1; }} arguments project in Chapter 4

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Call by value (reference type)

For reference type (object type) the situation is quite different

We still use "call by value" but now the value passed is a copy of a reference to the caller's object so there will now be two references to the caller's object

In the case of mutable classes this means that the caller's object can be changed (side-effect).

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Example (BeanShell)

addClassPath("c:/book-projects/chapter4/arguments");void addOne(BankAccount b) { b.deposit(1) }BankAccount a = new BankAccount(123,"Fred",1000);addOneDollar(a);print(a);BankAccount[number=123, name=Fred, balance=1001.0]

The value of a is not changed by the addOneDollar methodbut since a is a reference to a BankAccount then the depositstatement does change the balance of the caller's accountas referenced by a

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Example (BlueJ)

Write the following simple class

public class ArgumentTester2{ public void doTest() { BankAccount a = new BankAccount(123, "Fred", 1000); addOneDollar(a); System.out.println(a); } private static void addOneDollar(BankAccount b) { b.deposit(1); }}

arguments project in Chapter 4

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Pictorial Representation

BankAccount

1001

balance

a

BankAccount

1000

balance

a

BankAccount

1001

balance

a

b

Beforemethodcall

Duringmethodcall

AftermethodcallReference b is local to method

so it has disappeared

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The main method (1)

Executing a class outside the BlueJ environment

The JRE (java run time environment) has an interpreter that can be run from the command line using a command prompt

For this to work the class must have a special static method called the main method

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Main method (2)

Form of the main method must be

public static void main(String[] args){ ...}

We won't usethis argumentuntil Chapter 8but it must be

there

The java interpreter will beginexecuting your program at thefirst statement in the main method.

This method is normally used onlyto create an object of the class thatstarts your program

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Adding a main method

Add a main method to the CircleTester classpublic class CircleTester

{ ... public void doTest() { ... }

public static void main(String[] args) { CircleTester tester = new CircleTester(); tester.doTest(); }}

main will constructa CircleTester object

and run the test

see main-method project

Can also execute main method from inside BlueJ

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Another approach

If you don't want to add a main method to the class you want to run you can write a special class that only has a main method

This main method can construct an object of the other class and execute its methods.

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BankAccountRunner

public class BankAccountRunner{ public void doTest() { BankAccount account = new BankAccount(123, "Fred", 125.50); System.out.println("Initial balance is " + account.getBalance(); account.withdraw(100); System.out.println("Balance is " + account.getBalance(); ... }

public static void main(String[] args) { BankAccountRunner runner = new BankAccountRunner(); runner.doTest(); }}

uses defaultconstructor

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Run from command line

cd to the directory containing the BankAccountRunner class c:\book-projects\chapter4\main-method

Compile the class (if you haven't used BlueJ to do it) javac BankAccountRunner.java

Now run the class using its main method java BankAccountRunner

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BankAccountRunner output

java BankAccountRunner

Initial balance is 125.5Balance is 25.5Balance is 125.5BankAccount[number=123, name=Fred, balance=125.5]

interpretercommand

output produced bySystem.out.println

statements

Documents

Intro to Computer Science I