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Java Basics

Java Programs and Byte-Code

 Source cod e is the program/instructions written in

Java (high level language).

 The Java compiler converts source code into byte-

code (low level language).

 The Java virtual machine (JVM) converts the byte-

code into machine code and executes it.

Source code: HelloWorld.java

Byte code: HelloWorld.class

Hello World

Compile (javac) Run (java)

Example Program

Test1.java:

public class Test

public static void main ( String[] args )

System.out.println (“Hello World”);

output :

Hello World

What It Means

This class/program is accessible from outside (the file)

We are going to define a class / program

The class/program is to be called Test

This set of instructions is accessible from outside (the class) and can be used immediately

The set of instructions is to be called main

Print Hello World to the screen

How we find the extra data typed in after the name of the program

Once it is done, no data is sent back for more processing

public class Test {

public static void main ( String[] args ) { System.out.println (“Hello World”); } }

Problem

 Write a program to print out the lyrics of the chorus of

Britney Spears' song “Gimme More”.

Classes and Objects

Instruction instruction instruction ...

classY

Instructiona instructionb instructionc ...

classX

Instruction instruction instruction ...

object

Instruction instruction instruction ...

object

Instructiona instructionb instructionc ...

object

Classes:

• Templates to create objects.
• Written by programmers and compiled.
• Stored in files.

Objects:

• Correspond to classes.
• Created by JVM.
• Stored in memory.

Program Syntax and Style

 Semicolon needed after every statement.

 Case-sensitivity

 (^) STUFF vs stuff vs STuff vs stUFF

 Indented programs are easier to read.

 Everything after // is a comment

// a sample method

public void test

System.out.println (“Hi”); // write Hi on screen

Comments

 Brief description, author, date at top of class.

 Brief description of purpose of each method (if more

than one).

 Short explanation of non-obvious parts of code within

methods.

// test program to print to screen

// hussein suleman

// 16 february 2009

public class HelloWorld

Identifiers

 In source file, HelloWorld is an identifier.

 Identifiers are used to name parts of the program.

 start with _ or letter, and followed by zero or more

of _, letter or digit

 preferred style: ClassName, everythingElseLikeThis

 Reserved words :

 class, public, void, …

 Not reserved but has special meaning:

 main, String, ...

Identifiers: Quick Quiz

 Which are valid identifiers:

 (^12345)

 (^) JanetandJustin

 (^) lots_of_money

 (^) “Hello world”

 (^) J

 (^) cc:

 Which are good identifiers?

Integers: Literals

 Literals are actual data values written into a program.

 Numerical literals can be output just like text, but after

sensible conversions:

 (^) System.out.println (12);  (^12)  (^) System.out.println (“No:” + 12);  (^) No:  (^) System.out.println (12 + 13);  (^25)  (^) System.out.println (“No:” + (12 + 13));  (^) No:

Integers: Expressions

 Common operations

 + (plus), - (minus), / (divide), * (times), % (mod)

 11 + 11 / 2 = 16 … how?

 precedence of operators:

 high: ( )

 middle: * / %

 low: + -

 left associative if equal precedence.

 integer operations when both “operands” are integers.

Integers: Types

name size smallest largest

byte 1 byte -128 127

short 2 bytes -32768 32767

int 4 bytes -2147483648 2147483647

long 8 bytes approx. -910^18 approx. 910^18

Floating-point numbers

2.6e12, 5.34e-

 Two types:

 float 4 bytes 1.4e-45 … 3.4e+

 double 8 bytes 4.9e-324 … 1.7e+

 Same precedence and meaning of operations, except

for mixed type expressions

 (^) (10 / 4.0f) * 4

 (^) Must use suffix to force calculations to be floating point!