Lecture 5

Review (Attributes and Methods)

• Class will contain– a set of attributes and methods

public class Turtle{

///// Field (Attributes) ///// …

/////// Method /////// …

}

Review (Variables)• A variable is a symbol representing a value

public class Turtle{

///// Field (Attributes) ///// int length; int position = 100; double size = 2.5;

///// Method ///// …

}

Review (variable declaration)

• What does this line mean???

• We declare a variable, named length

• The type of value should be int

• Then, initialize the variable with 15

int length = 15;

data type variable name

length

15value

Review (arguments/parameters)• A argument is a variable that will be entered in

a method as input valuepublic class Turtle{

…

/////// Method /////// public void drawLine(int len)

{ penDown(); forward(len);

}}

Today’s topic

• More about methods Methods in a method (sub-method) Main method

• Arithmetic expression + , -, *, /, %

Methods in a method (“sub-methods”)• You can use (call) methods in a method

as sub-methods public class Turtle {

public void drawLine(int len){forward(len);}

public void drawSquare(int len){drawLine(len);turnRight();}

}

Practice• You already have two method

– drawTriangle, drawRectangle

• drawTriangle receives 1 parameter

drawTriangle(int len)

• drawRectangle receives 2 parameters

drawRectangle(int width, int height)

Practice (drawSimpleHouse)

Please write another method– Name: drawSimpleHouse– Receive 2 parameters, width and height– Use 2 methods in the previous slide

You can also use

penUp();

penDwon();

moveTo( x, y );

x

y

Practice (sample code)public void drawSimpleHouseLine(int w, int h){penUp();moveTo(200, 200);penDown();drawRectangle(w, h);drawTriangle(w);}

x

yFor example, you may type

t.drawSimpleHouse(150, 100);

Main method

public class Test{

public static void main(String[] args){

}

}

• Look at Test.java

Source code

Java compiler Java bytecode

Java interpreter Bytecodecomplier

Machine code

.java file

.class file

Execute!

Execute program from main method

We can test(in Interaction panel)

Main method

public class Test {

public static void main(String[] args){World w = new World();Turtle t = new Turtle(w);t.drawSimpleHouse();}

}

• Let’s write main method and RUN it !!!

How to execute a program?

• When a program is executed, it always starts from a main method

Right click onthe file containinga main method

• Arithmetic Expression!!!

How we can do Math in Java program ???

Arithmetic expression

• We can calculate arithmetic expressione.g., 2 + 3, 5 – 3, 4 * 5, 6 / 2

• An expression is a combination of one or more operators and operands

Addition +Subtraction -Multiplication *Division /Remainder %

5 + 3

Assignment

• The result of expression is assigned toa variable by using = symbol

x = 5 + 3;int x; Variable declaration

Assignment

Note: Somewhat different from math

Variable x contains a value of 8

Assignment (more)

• Variables can be operands• Variables may be re-assigned

x = 5 + 3;

int x; Variable declaration

Assignment

int y; Variable declaration

y = x – 4; Assignment

x = x / y; Assignment

Printout Method

• We can print out the variable value to a screen by calling a method

System.out.println( variable );

Practice (Printout Method) public class Test {

public static void main(String[] args){int x;int y;x = 145 + 25;y = 291 / 3;System.out.println( x );System.out.println( y );}

}Where the results appear ???

Exercise 1By running Test class, (i.e. calling a main method)

Calculate your age in minutesand show the result to a screen

use following conversions (assumptions)

1 year = 365 days1 day = 24 hours1 hour = 60 minutes

Exercise 1 (sample code)

public class Test {

public static void main(String[] args) { int minutes;

minutes = 17 * 365 * 24 * 60;

System.out.println( munutes ); }

}

How about this? (Useful!!!)

public class Test {

public static void main(String[] args) {

int age = 17; int minutes;

minutes = age * 365 * 24 * 60;

System.out.println( munutes ); }

}