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2
Methods Instance (or member) method
Operates on a object (i.e., and instance of the class)
String s = new String("Help every cow reach its "+ "potential!");
int n = s.length();
Class (i.e. static) method Service provided by a class and it is not associated with a
particular object
String t = String.valueOf(n);
Instance method
Class method
3
Variables Instance variable and instance constants
Attribute of a particular object Usually a variable
Point p = new Point(5, 5);
int px = p.x;
Class variables and constants Collective information that is not specific to individual
objects of the class Usually a constant
Color favoriteColor = Color.MAGENTA;double favoriteNumber = Math.PI - Math.E;
Instance variable
Class constants
4
static and non-static rules Member/instance (i.e. non-static) fields and methods can
ONLY be accessed by the object name
Class (i.e. static) fields and methods can be accessed by Either the class name or the object name
Non-static methods can refer to BOTH class (i.e. static) variables and member/instance (i.e. non-static) variables
Class (i.e. static) methods can ONLY access class (i.e. static) variables
5
Task – Conversion.java Support conversion between English and metric values
d degrees Fahrenheit = (d – 32)/1.8 degrees Celsius 1 mile = 1.609344 kilometers 1 gallon = 3.785411784 liters 1 ounce (avdp) = 28.349523125 grams 1 acre = 0.0015625 square miles = 0.40468564 hectares
6
Conversion Implementationpublic class Conversion {
// conversion equivalenciesprivate static final double
KILOMETERS_PER_MILE = 1.609344;private static final double
LITERS_PER_GALLON = 3.785411784;private static final double
GRAMS_PER_OUNCE = 28.349523125;private static final double
HECTARES_PER_ACRE = 0.40468564;
7
Conversion implementation
public static double fahrenheitToCelsius (double f) {return (f - 32) / 1.8;
} }
Modifier public indicates other classes can use the method
Modifier static indicates the method is a class method
No use of member/instance variables!!!
8
Conversion Implementation// temperature conversions methodspublic static double fahrenheitToCelsius(double f) {
return (f - 32) / 1.8;}
public static double celsiusToFahrenheit(double c) {return 1.8 * c + 32;
}
// length conversions methodspublic static double kilometersToMiles(double km) {
return km / KILOMETERS_PER_MILE;}
9
Conversion Implementation// mass conversions methodspublic static double litersToGallons(double liters) {
return liters / LITERS_PER_GALLON;}
public static double gallonsToLiters(double gallons) {
return gallons * LITERS_PER_GALLON;}
public static double gramsToOunces(double grams) {return grams / GRAMS_PER_OUNCE;
}
public static double ouncesToGrams(double ounces) {return ounces * GRAMS_PER_OUNCE;
}
10
Conversion Implementation// area conversions methodspublic static double hectaresToAcres(double hectares) {
return hectares / HECTARES_PER_ACRE;}
public static double acresToHectares(double acres) {return acres * HECTARES_PER_ACRE;
}
11
Conversion useScanner stdin = new Scanner (System.in);
System.out.print("Enter a length in kilometers: ");double kilometers = stdin.nextDouble();double miles = Conversion.kilometersToMiles(kilometers);
System.out.print("Enter a mass in liters: ");double liters = stdin.nextDouble();double gallons = Conversion.litersToGallons(liters);
System.out.print("Enter a mass in grams: ");double grams = stdin.nextDouble();double ounces = Conversion.gramsToOunces(grams);
System.out.print("Enter an area in hectares: ");double hectares = stdin.nextDouble();double acres = Conversion.hectaresToAcres(hectares);
12
A Conversion use
System.out.println(kilometers + " kilometers = "+ miles + " miles ");
System.out.println(liters + " liters = "+ gallons + " gallons");
System.out.println(grams + " grams = "+ ounces + " ounces");
System.out.println(hectares + " hectares = " + acres + " acres");
2.0 kilometers = 1.242742384474668 miles3.0 liters = 0.7925161570744452 gallons4.0 grams = 0.14109584779832166 ounces5.0 hectares = 12.355269141746666 acres
13
A preferred Conversion useNumberFormat style = NumberFormat.getNumberInstance();style.setMaximumFractionDigits(2);style.setMinimumFractionDigits(2);
System.out.println(kilometers + " kilometers = "+ style.format(miles) + " miles ");
System.out.println(liters + " liters = "+ style.format(gallons) + " gallons");
System.out.println(grams + " grams = "+ style.format(ounces) + " ounces");
System.out.println(hectares + " hectares = " + style.format(acres) + " acres");
2.0 kilometers = 1.24 miles3.0 liters = 0.79 gallons4.0 grams = 0.14 ounces5.0 hectares = 12.36 acres
Part of java.text
1414
A solution to commenting your A solution to commenting your codecode
The commentator: The commentator: http://www.cenqua.com/commentatohttp://www.cenqua.com/commentator/r/
15
Method invocations Actual parameters provide information that is otherwise
unavailable to a method
When a method is invoked Java sets aside memory for that particular invocation
Called the activation record Activation record stores, among other things, the
values of the formal parameters
Formal parameters initialized with values of the actual parameters After initialization, the actual parameters and formal
parameters are independent of each other
Flow of control is transferred temporarily to that method
16
Value parameter passing demonstrationpublic class Demo {
public static double add(double x, double y) {double result = x + y;return result;
}
public static double multiply(double x, double y) {x = x * y;return x;
}
public static void main(String[] args) {double a = 8, b = 11;
double sum = add(a, b);System.out.println(a + " + " + b + " = " + sum);
double product = multiply(a, b);System.out.println(a + " * " + b + " = " + product);
}}
18
8.0x
add()
11.0y
-result
8.0x
add()
11.0y
19.0result
Demo.java walkthrough
8.0a
main()
11.0b
-sum
-product
8.0a
main()
11.0b
19.0sum
-product
double sum = add(a, b);
public static double add (double x, double y) { double result = x + y;return result;
}
Initial values of formal parameterscome from the actual parameters
19
8.0x
multiply()
11.0y
88.0x
multiply()
11.0y
8.0a
main()
11.0b
19.0sum
-product
8.0a
main()
11.0b
19.0sum
88.0product
Demo.java walkthrough
double multiply = multiply(a, b);
public static double multiply (double x, double y) { x = x * y;
return x;}
Initial values of formal parameterscome from the actual parameters
20
PassingReferences.javaimport java.awt.*;public class PassingReferences {
public static void f(Point v) {v = new Point(0, 0);
}
public static void g(Point v) {v.setLocation(0, 0);
}
public static void main(String[] args) {Point p = new Point(10, 10);System.out.println(p);
f(p);System.out.println(p);
g(p);System.out.println(p);
}}
22
PassingReferences.javapublic static void main(String[] args) {
Point p = new Point(10, 10);System.out.println(p);
f(p);
p
main()Point
x: 10y: 10p
main()Point
x: 10y: 10
v
f()
Method main()'s variable pand method f()'s formal
parameter v have the samevalue, which is a reference to
an object representinglocation (10, 10)
java.awt.Point[x=10,y=10]
23
PassingReferences.javapublic static void f(Point v) {
v = new Point(0, 0);}
p
main()Point
x: 10y: 10p
main()Point
x: 10y: 10
v
f()Point
x: 0y: 0
24
PassingReferences.javapublic static void main(String[] args) {
Point p = new Point(10, 10);System.out.println(p);
f(p);
System.out.println(p);
g(p);
p
main()Point
x: 10y: 10
java.awt.Point[x=10,y=10]java.awt.Point[x=10,y=10]
p
main()Point
x: 10y: 10
v
g()
Method main()'s variable pand method g()'s formal
parameter v have the samevalue, which is a reference to
an object representinglocation (10, 10)
25
p
main()Point
x: 0y: 0p
main()Point
x: 0y: 0
v
g()
PassingReferences.javapublic static void g(Point v) {
v.setLocation(0, 0);}
p
main()Point
x: 10y: 10
v
g()
Method main()'s variable pand method g()'s formal
parameter v have the samevalue, which is a reference to
an object representinglocation (10, 10)
26
PassingReferences.javapublic static void main(String[] args) {
Point p = new Point(10, 10);System.out.println(p);
f(p);
System.out.println(p);
g(p);System.out.println(p);
p
main()Point
x: 0y: 0
java.awt.Point[x=10,y=10]java.awt.Point[x=10,y=10]java.awt.Point[x=0,y=0]
2727
This may be a bit disturbing…This may be a bit disturbing…
Lots of piercings…Lots of piercings…
29
Static vs. non-static Consider the following code:
public class Staticness {
private int a = 0;private static int b = 0;
public void increment() {a++;b++;
}
public String toString() {return "(a=" + a + ",b=" + b + ")";
}
}
30
Static vs. non-static And the code to run it:
public class StaticTest {
public static void main (String[] args) {Staticness s = new Staticness();Staticness t = new Staticness();
s.increment();t.increment();t.increment();
System.out.println (s);System.out.println (t);
}}
32
Staticness s = new Staticness();Staticness t = new Staticness();s.increment();t.increment();t.increment();System.out.println (s);System.out.println (t);
Staticness s = new Staticness();Staticness t = new Staticness();s.increment();t.increment();t.increment();System.out.println (s);System.out.println (t);
Static vs. non-static: memory diagram
Staticness
- a = 0
t
Staticness
- a = 0
s
0b
Staticness
- a = 1
Staticness
- a = 1
Staticness
- a = 2
123
33
Task – Triple.java Represent objects with three integer attributes
public Triple() Constructs a default Triple value representing three zeros
public Triple(int a, int b, int c) Constructs a representation of the values a, b, and c
public int getValue(int i) Returns the i-th element of the associated Triple
public void setValue(int i, int value) Sets the i-th element of the associated Triple to value
34
Task – Triple.java
Represent objects with three integer attributes
public String toString() Returns a textual representation of the associated Triple
public Object clone() Returns a new Triple whose representation is the same as
the associated Triple
public boolean equals(Object v) Returns whether v is equivalent to the associated Triple
35
Triple.java implementation
// Triple(): default constructorpublic Triple() {
this (0, 0, 0);} The new Triple object (the this object) is constructed
by invoking the Triple constructor expecting three intvalues as actual parameters
public Triple() {int a = 0;int b = 0;int c = 0;this (a, b, c);
}
Illegal this() invocation. A this() invocationmust begin its statement body
36
Triple.java implementation// Triple(): default constructorpublic Triple(int a, int b, int c) {
this (0,0,0);}
// Triple(): specific constructorpublic Triple(int a, int b, int c) {
setValue(1, a);setValue(2, b);setValue(3, c);
}
// Triple(): specific constructor - alternative definition
public Triple(int a, int b, int c) {this.setValue(1, a);this.setValue(2, b);this.setValue(3, c);
}
38
Triple.java implementation Class Triple like every other Java class
Automatically an extension of the standard class Object Class Object specifies some basic behaviors common to
all objects These behaviors are said to be inherited
Three of the inherited Object methods toString() clone() equals()
39
Recommendation Classes should override (i.e., provide a class-specific
implementation) toString() clone() equals()
By doing so, the programmer-expected behavior can be provided
System.out.println(p); // displays string version of // object referenced by p System.out.println(q); // displays string version of
// object referenced by q
40
Triple.java toString() implementationpublic String toString() {
int a = getValue(1);int b = getValue(2);int c = getValue(3);
return "Triple[" + a + ", " + b + ", " + c+ "]";}
Consider
Triple t1 = new Triple(10, 20, 30);System.out.println(t1);
Triple t2 = new Triple(8, 88, 888);System.out.println(t2);
Produces
Triple[10, 20, 30]Triple[8, 88, 888]
41
Triple.java clone() implementationpublic Object clone() {
int a = getValue(1);int b = getValue(2);int c = getValue(3);
return new Triple(a, b, c);}
Consider
Triple t1 = new Triple(9, 28, 29);Triple t2 = (Triple) t1.clone();
System.out.println("t1 = " + t1);System.out.println("t2 = " + t2);
Produces
Triple[9, 28, 29]Triple[9, 28, 29]
Must cast!
42
Triple.java equals() implementationpublic boolean equals(Object v) {
if (v instanceof Triple) {int a1 = getValue(1);int b1 = getValue(2);int c1 = getValue(3);
Triple t = (Triple) v;int a2 = t.getValue(1);int b2 = t.getValue(2);int c2 = t.getValue(3);
return (a1 == a2) && (b1 == b2) && (c1 == c2);}else {
return false;}
}
Can’t be equal unless it’s a Triple
Compare corresponding attributes
43
Triple.java equals()
Triple e = new Triple(4, 6, 10);Triple f = new Triple(4, 6, 11);,Triple g = new Triple(4, 6, 10); Triple h = new Triple(4, 5, 11); boolean flag1 = e.equals(f);
eTriple
x1: 4
f
g
h
x2: 6 x3: 10
Triple
x1: 4 x2: 6 x3: 11
Triple
x1: 4 x2: 6 x3: 10
Triple
x1: 4 x2: 5 x3: 11
44
Triple.java equals()
Triple e = new Triple(4, 6, 10);Triple f = new Triple(4, 6, 11);,Triple g = new Triple(4, 6, 10); Triple h = new Triple(4, 5, 11); boolean flag2 = e.equals(g);
eTriple
x1: 4
f
g
h
x2: 6 x3: 10
Triple
x1: 4 x2: 6 x3: 11
Triple
x1: 4 x2: 6 x3: 10
Triple
x1: 4 x2: 5 x3: 11
45
Triple.java equals()
Triple e = new Triple(4, 6, 10);Triple f = new Triple(4, 6, 11);,Triple g = new Triple(4, 6, 10); Triple h = new Triple(4, 5, 11); boolean flag3 = g.equals(h);
eTriple
x1: 4
f
g
h
x2: 6 x3: 10
Triple
x1: 4 x2: 6 x3: 11
Triple
x1: 4 x2: 6 x3: 10
Triple
x1: 4 x2: 5 x3: 11
46
What’s wrong with this code?class Scope {
public static void f(int a) {int b = 1; // local definitionSystem.out.println(a); // print 10a = b; // update aSystem.out.println(a); // print 1
}
public static void main(String[] args) {int i = 10; // local definitionf(i); // invoking f() with i as
parameterSystem.out.println(a); System.out.println(b);
}}
Variables a and b do not exist in the scope of method main()
47
Blocks and scope rules A block is a list of statements nested within braces
A method body is a block A block can be placed anywhere a statement would be legal
A block contained within another block is a nested block
A formal parameter is considered to be defined at the beginning of the method body
A local variable can be used only in a statement or nested blocks that occurs after its definition
An identifier name can be reused as long as the blocks containing the duplicate declarations are not nested one within the other
Name reuse within a method is permitted as long as the reuse occurs in distinct blocks
4848
BiologyBiology
PhysicsPhysics
InterdisciplinaryInterdisciplinary ChemistryChemistry MathematicsMathematics LiteratureLiterature
PeacePeace
HygieneHygiene EconomicsEconomics
MedicineMedicine
The 2002 Ig Nobel PrizesThe 2002 Ig Nobel Prizes““Courtship behavior of ostriches towards humans underCourtship behavior of ostriches towards humans under
farming conditions in Britain”farming conditions in Britain”
““Demonstration of the exponential decay law using beer Demonstration of the exponential decay law using beer
froth”froth”
A comprehensive study of human belly button lintA comprehensive study of human belly button lint
Creating a four-legged periodic tableCreating a four-legged periodic table
““Estimation of the surface area of African elephants”Estimation of the surface area of African elephants”
““The effects of pre-existing inappropriate highlighting on The effects of pre-existing inappropriate highlighting on
reading comprehension”reading comprehension”
For creating Bow-lingual, a computerized dog-to-human For creating Bow-lingual, a computerized dog-to-human
translation devicetranslation device
For creating a washing machine for cats and dogsFor creating a washing machine for cats and dogs
Enron et. al. for applying imaginary numbers to the Enron et. al. for applying imaginary numbers to the
business worldbusiness world
““*** asymmetry in man in ancient sculpture”*** asymmetry in man in ancient sculpture”
49
Legalclass Scope2 {
public static void main(String[] args) {int a = 10; f(a); System.out.println(a);
}
public static void f(int a) {System.out.println(a);a = 1; System.out.println(a);
}}
50
Legal but not recommendedpublic void g() {
{int j = 1; // define jSystem.out.println(j); // print 1
}{
int j = 10; // define a different j
System.out.println(j); // print 10}{
char j = '@'; // define a different j
System.out.println(j); // print '@'}
}
51
What’s the output?
for (int i = 0; i < 3; ++i) {int j = 0;++j;System.out.println(j);
}
The scope of variable j is the body of the for loop j is not in scope when ++i j is not in scope when i < 3 are evaluated j is redefined and re-initialized with each loop iteration
52
Overloading Have seen it often before with operators
int i = 11 + 28;double x = 6.9 + 11.29;String s = "April " + "June";
Java also supports method overloading Several methods can have the same name Useful when we need to write methods that perform
similar tasks but different parameter lists Method name can be overloaded as long as its signature
is different from the other methods of its class Difference in the names, types, number, or order of
the parameters
53
Legal
public static int min(int a, int b, int c) {return Math.min(a, Math.min(b, c));
}
public static int min(int a, int b, int c, int d) {return Math.min(a, min(b, c, d));
}
54
Legalpublic static int power(int x, int n) {
int result = 1;for (int i = 1; i <= n; ++i) {
result *= x;}return result;
}
public static double power(double x, int n) {double result = 1;for (int i = 1; i <= n; ++i) {
result *= x;}return result;
}
55
What’s the output?public static void f(int a, int b) {
System.out.println(a + b);}
public static void f(double a, double b) {System.out.println(a - b);
}
public static void main(String[] args) {int i = 19;double x = 54.0;
f(i, x); }
56
Random numbers To generate random numbers, use the Random class
In java.util
Usage: Random random = new Random(); random.nextInt(7)
Generates a random number between 0 and 6 random.nextDouble()
Returns a number x where 0 ≤ x < 1
Technically, they are not true random numbers They are called ‘pseudo-random’ numbers But that’s close enough to random numbers for us
57
Using random number generatorspublic class RandomTest {
public static void main (String[] args) {
int foo[] = new int[10];double bar[] = new double[10];Random random = new Random();
for ( int i = 0; i < 10; i++ ) { foo[i] = random.nextInt(100); bar[i] = random.nextDouble();}
for ( int i = 0; i < 10; i++ ) { System.out.println (i + ": " + foo[i] + ", " +
bar[i]);}
}}
58
Output
0: 5, 0.0275801681392713331: 15, 0.56095496869018052: 67, 0.74667532510126573: 42, 0.80872782971275394: 55, 0.316677953697143475: 72, 0.50574974452290486: 32, 0.51378279036818767: 85, 0.143706513720959338: 1, 0.102707388788240379: 54, 0.9212840891576871
But it varies each time!
59
Summary of key points The this keyword
Can be used to call another constructor Must be the FIRST thing called
Can be used as a reference to the current object Static vs. non-static
A static variable means there is only one such variable regardless of how many objects have been declared
A static method does not care about the “state” of the object
Various methods we may want to override: clone() toString() equals()
Using random numbers
61
Quick surveyQuick survey
I felt I understood the material in this I felt I understood the material in this slide set…slide set…
a)a) Very wellVery well
b)b) With some review, I’ll be goodWith some review, I’ll be good
c)c) Not reallyNot really
d)d) Not at allNot at all
62
Quick surveyQuick survey
The pace of the lecture for this The pace of the lecture for this slide set was…slide set was…
a)a) FastFast
b)b) About rightAbout right
c)c) A little slowA little slow
d)d) Too slowToo slow
63
Quick surveyQuick survey
How interesting was the material in How interesting was the material in this slide set? Be honest!this slide set? Be honest!
a)a) Wow! That was SOOOOOOO cool!Wow! That was SOOOOOOO cool!
b)b) Somewhat interestingSomewhat interesting
c)c) Rather boringRather boring
d)d) ZzzzzzzzzzzZzzzzzzzzzz