1. Constructor
Laboratory 7
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2. Review Class definition Declaring Objects
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Class definition
Declaring Objects
Assigning object reference variables
Introducing methods
Constructors

3. The General Form of a class
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Class classname {
Type instance-variable1; //
Type methodname1(parameter-list) {
//body of method }
Type methodname2 (parameter-list {
//body of method } ….
Class person {
Double height;
Double weight; }

4. Creating the object box
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Person theperson = new person();
Here, theperson will be an instance of person.
To assign a value, use
theperson.weight = 70;
theperson.height = 170;

5. Declaring the object – page 135
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Statement Effect
Person theperson;
theperson = new Person();
Null
theperson
Weight
theperson
Height
theperson.height

6. Example of creating and assigning value
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7. Introducing Methods Classes usually consist of two things:
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Classes usually consist of two things:
Instance variables (int x, y) and methods (pair())

8. Passing the values x = 1 and y = 2 will pass to Pair
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x = 1 and y = 2
will pass to Pair

9. Returning a value – from myarea.area()
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10. 2018/12/42018/12/4

11. Return maximum
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12. Call maximum (loop) double max =
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double max =
maximum(maximum( number1, number2, number3 ),
maximum( number4, number5, number6 ),
maximum( number7, number8, number9 ));

13. 2018/12/42018/12/4 import java.awt.Container; import javax.swing.*;
public class MaxMin9Test extends JApplet {
public void init() {
String s1 = JOptionPane.showInputDialog("Enter first floating-point value" );
String s2 = JOptionPane.showInputDialog("Enter second floating-point value" );
String s3 = JOptionPane.showInputDialog("Enter third floating-point value" );
String s4 = JOptionPane.showInputDialog("Enter first floating-point value" );
………. //fill in value
double number1 = Double.parseDouble( s1 );
double number2 = Double.parseDouble( s2 );
double number3 = Double.parseDouble( s3 );
………..// fill in value
double max = maximum(maximum( number1, number2, number3 ),
maximum( number4, number5, number6 ),
maximum( number7, number8, number9 ));
double min = ……….. Fill in value
JTextArea outputArea = new JTextArea();
outputArea.setText( "number1: " + number1 +
"\nnumber2: " + number2 +
"\nnumber3: " + number3 +
"\nnumber2: " + number4 +
"\nnumber3: " + number5 +
"\nnumber2: " + number6 +
"\nnumber3: " + number7 +
"\nnumber2: " + number8 +
"\nnumber3: " + number9 +
"\nmaximum is: " + max +
"\nminimum is: " + min );
Container container = getContentPane();
container.add( outputArea ); }
public double maximum( double x, double y, double z )
……………// fill in value
public double minimum( double x, double y, double z )
…………… // fill in value

14. Constructor
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It is tedious to initialise all variables in a class each time an instance is created.
Constructors are a special sort of method that initialise freshly minted objects.
They are like static methods in that we don't need an existing object to use them, just new.

15. Example
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the values
are fixed

16. How to draw
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17. Star
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18. Background and position
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19. Example 2018/12/42018/12/4 import java.awt.*;
import java.applet.Applet;
import javax.swing.*;
public class lab72 extends Applet {
String s1 = JOptionPane.showInputDialog("Enter R" ); //enter the color code
String s2 = JOptionPane.showInputDialog("Enter G" );
String s3 = JOptionPane.showInputDialog("Enter B" );
// convert user input to integer
int i = Integer.parseInt(s1);
int j = Integer.parseInt(s2);
int k = Integer.parseInt(s3);
public void paint( Graphics g ) {
int[] xArr = {28,63,73,81,115,87,98,73,46,55};
int[] yArr = {55,55,24,55,55,80,108,89,108,80};
int[] x1Arr = {153,161,165,166,174,165,165,160,153,156};
int[] y1Arr = {20,22,16,24,27,30,39,32,33,27};
int[] x2Arr = {179,187,193,191,198,189,187,184,176,181};
int[] y2Arr = {66,69,65,73,78,79,85,79,78,74};
int[] x3Arr = {167,173,177,180,189,181,183,178,171,173};
int[] y3Arr = {118,118,110,118,118,125,132,128,133,125};
int[] x4Arr = {128,136,141,142,150,140,142,137,129,133};
int[] y4Arr = {158,161,154,162,165,168,177,171,173,166};
g.setColor(new Color(i, j, k));
g.fillRect(0, 0, 452, 300);
g.setColor(Color.yellow);
g.fillPolygon(xArr,yArr,10);
g.fillPolygon(x1Arr,y1Arr,10);
g.fillPolygon(x2Arr,y2Arr,10);
g.fillPolygon(x3Arr,y3Arr,10);
g.fillPolygon(x4Arr,y4Arr,10); }

20. HTML <html> <head>
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<html>
<head>
<title>simple applet</title>
</head>
<body>
<applet code = "lab72.class" width = 452 height = 302>
</applet>
</body>
</html>

21. 2018/12/42018/12/4 import java.awt.*; import java.applet.Applet;
import javax.swing.*;
public class lab72 extends Applet {
Color backGroundColor; /define
public void init() {
// fill in the three numbers here
String firstNumber, secondNumber, thirdNumber;
int number1, number2, number3;
backGroundColor = new Color(…………..); // fill in the value }
public void paint( Graphics g ) {
int[] xArr = {28,63,73,81,115,87,98,73,46,55};
int[] yArr = {55,55,24,55,55,80,108,89,108,80};
int[] x1Arr = {153,161,165,166,174,165,165,160,153,156};
int[] y1Arr = {20,22,16,24,27,30,39,32,33,27};
int[] x2Arr = {179,187,193,191,198,189,187,184,176,181};
int[] y2Arr = {66,69,65,73,78,79,85,79,78,74};
int[] x3Arr = {167,173,177,180,189,181,183,178,171,173};
int[] y3Arr = {118,118,110,118,118,125,132,128,133,125};
int[] x4Arr = {128,136,141,142,150,140,142,137,129,133};
int[] y4Arr = {158,161,154,162,165,168,177,171,173,166};
g.setColor(…………….); //fill in the value by calling background
g.fillRect(0, 0, 472, 322); // use same attributes of applet
g.setColor(Color.red);
g.fillRect(10, 10, 452, 302); // for a border of size 10
g.setColor(Color.yellow);
g.fillPolygon(xArr,yArr,10);
g.fillPolygon(x1Arr,y1Arr,10);
g.fillPolygon(x2Arr,y2Arr,10);
g.fillPolygon(x3Arr,y3Arr,10);
g.fillPolygon(x4Arr,y4Arr,10);