Lecture 3

Review (What is Class and Object ?) The world of JAVA contains objects The world of JAVA

Review (What is Class and Object ?) Objects are generated from The world of JAVA classes

Class Car Review (What is Class and Object ?) … HondaFord TOYOTA

Today Let’s write a method in the class Turtle –We are going to write our own methods –So far, we just used methods written by other programmers (e.g. forward(), turnLeft(), etc)

Writing Java program Application (or Software) is written in programming language –Java application is constructed by Java programs written in Java programming language –Generally, when we say that “writing java program”, it means that “writing classes” –i.e. we are writing blueprints for applications

Attributes and Methods Class will contain –a set of attributes and methods Attribute –State of object generated from the class E.g. name, color, size, etc … Methods –Behavior of object generated from the class E.g. moveTo(int x, int y), turnLeft(), etc …

Class Car HondaFord TOYOTA Example (Attributes and Methods) … Attribute color, size, name, etc Methods move, turn, stop, etc

Java Source Code (source file) public class SimpleTurtle { ///// Field (Attributes) ///// … /////// Method /////// … } Each class is programmed in source file named ClassName. java For example, SimpleTurtle.java

How to run the program? How do computers understand source codes written in Java programming language ? Basically, there are 3 steps 1.Edit source code 2.Compile source code 3.Execute the compiled code Repeat!

Compile Once we write source codes (i.e..java file), we have to translate it to the codes which computers can understand –The process of translation is called “compile” Java compiler performs this task The compiled code is called java Bytecode Source code (.java) Bytecode (.class) Java Compiler

Execute (Run) In order to execute bytecodes –bytecode compiler will translate the codes into machine codes (sequences of 0 and 1) Finally, computers understand what we write in Java programming language –i.e. computers understand what we want Bytecode (.class) Bytecode Compiler Machine Code

Level of programming language Java source code –Human readable language –High-level programming language Java bytecode –Human readable notation of machine code Machine code –Binary string (0 and 1) x = x + y.data x:.word 0x42 y:.word 0x43.text start: set x, %r1 ld [%r1], %r2 ! $x$ --> %r2 set y, %r1 ld [%r1], %r3 ! $y$ --> %r3 add %r2, %r3, %r2 ! $x+y$ --> %r2 set x, %r1 st %r2, [%r1] ! $x+y$ --> x end: ta (omit…) Example

Source code Java compiler Java bytecode Java interpreter Bytecode complier Machine code.java file.class file Execute! Summary * We can test (in Interaction panel)

Practice –Let’s write our own method!!!

Edit source code - Open Turtle.java We gonna write Methods Here!!! After this line

Write a method, named drawOne ////////// method ////////// public void drawOne() { }

Write a method, named drawOne ////////// method ////////// public void drawOne() { forward(100); turn(225); forward(40); }

Save source code –Save Turtle.java Note: File name should be the same as Class name Extension of the file is.java

Compile it –Right click on the source file Success?

Let’s Test the compiled code –In an interaction panel, create a turtle in the world, then try to call the method drawOne()

Challenge!!! Write a method for Turtle –The method name is drawSquare –The method draws the one square with size 50