CSE 131 Computer Science 1Module 1: (basics of Java)

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Welcome to the World of Java

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Eclipse

Integrated Development Environment (IDE)» “smart” editing of Java programs – continuous syntax checks» compiles program files automatically as necessary» integrated program testing and debugging tools» has plug-ins for version control system (Subversion)

Can be a powerful tool for managing large projects» but, can also be a bit overwhelming and mysterious at first» watch the first few Eclipse videos on the web site before lab

works the same way in Windows, Mac and Linux

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Your First Program

public class HelloWorld {public static void main(String[] args) {

System.out.println("Hello, world!");}

}» defines a class called HelloWorld (a class defines an object)» class contains a single method called main

• in Java, the main method is the starting point of a program• the main method has an array of arguments (not used here)• Any program has one and only one main() method

» program “prints” a brief message and halts

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Today: understand basic building blocks of JAVA

You will be able to» Output a slogan on the screen » Tell if a year is a Leap Year» Calculate the area of a circle

» Output anything you like» Do all kinds of calculation

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Basic unit: variable A variable in Java is a piece of

information stored in a computer’s memory: like a jar holding food

Each variable has a type» Like different kinds of jars (different food,

size, usage, etc.) Main types:

int a; double b;

String s; boolean x;

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Basic Data Types in Java

Type (keyword) Meaning Examples Primitive Operators

int Integer 3, 0, -5, 256 +, -, *, /

double Decimal number 1.5, -3.1415, 0.0 +, -, *, /

boolean true or false true, false &&, ||, !(return boolean: <,>,<=,>=,!=, ==)

String a sequence of characters

“hello world” + (concatenation)

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Basic Definitions

Type. A category of values and a set of operations on such values.Variable. A name that refers to a value of a declared type.Assignment statement. Associates a value with a variable.

type

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The int type

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Integers

This surprises most students. Don’t feel bad:

experts forget this too sometimes.

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Key points

Assignment statement Variable = Expression ; (For example: a = b+ 5/3; )

Expression’s type must match the variable’s type (will come back to this point)

Sequential execution » The order of statements matters

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The double type

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Example: compute the area of a circle

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String: a sequence of characters

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String operation

Meaning of characters depends on context.

Example: report +, -, *, / of two integers

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Boolean

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Booleansboolean data type. Useful to control the logic of a program.

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ComparisonsComparisons. Take two operands of one type (int or double) and produce a result of type boolean.

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Leap YearQ. Is a given year a leap year?A. Yes if either (i) divisible by 400 or (ii) divisible by 4 but not 100.

public class LeapYear { public static void main(String[] args) { int year = 2012;

boolean isLeapYear;

// divisible by 4 but not 100 isLeapYear = (year % 4 == 0) && (year % 100 != 0); // or divisible by 400 isLeapYear = isLeapYear || (year % 400 == 0);

System.out.println(isLeapYear); }}

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Data Type Conversion

Operations must be applied to same types, with the following exceptions

Java automatically converts some types of values to make them compatible

int i, j; double x, y;i = 5; j = i/2; y = 2; x = i/y; - As long as there is a double in an expression, the type of the

expression is promoted to double Combining strings and numeric values

int i = 5; String S = “two plus two =” + i;

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Basic Expressions in JavaExpression Value Type

3+5 8 int

7/2.0 3.5 double

7/2 3 int

3+5-2/4+2 10 int

3+5-2.0/4+2 9.5 double

(3+5-2)/(4+2) 1 int

“Hello”+”there” “Hellothere” String

“I have “+5+5+” toes” “I have 55 toes” String

true && false false boolean

false || true true boolean

!true false boolean

(true&&false)||(!false) true boolean

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Variable types A variable can only hold values of the proper type

int x; (default 0) int x = 1; … x = false; (error! Type-checking problem)

Any expression of type T can appear in an expression where a value of type T is required» E.g. int x = 3+5;

int x = 7/2.0; (type error) int x= (int) (7/2.0) (cast, x has value 3)

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Method

What if we need to find areas of different circles?» Do we have write the code over and over again?

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Procedural abstraction (method) Find areas of circles A method is like a blackbox (input output)

» Hide details of computation from outside A java method

Use it: double area = circleArea(10.0);

double circleArea(double radius) {return Math.PI * radius * radius;

}

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Method Call and Return

Java methods used to decompose program into parts» allows us to solve a problem once and conveniently re-use the

solution many times

You can invoke the method by using it in an expression» Example: area = circleArea(2.0);» The number of input parameters should match the method

definition» The types of input parameters should match the method

definition

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Method for Length of Hypotenuse

We know C=√A2+B2

Forms the basis of the Java method

double hypotenuse(double A, double B) {return Math.sqrt(A * A + B * B);

}» Math.sqrt(x) returns the square root of x

• part of a library of mathematical functions, includingsin(x), cos(x), tan(x), log(x), exp(x)

» Signature of a method: double hypotenuse(double, double)defines method name, return type, types of arguments

» Invocation of a method must match its signature Example: double h = hypotenuse(3, 4);

C

B

A