1. Math and String Classes
Lecture 9
Naveen Kumar

2. Constructing Objects
 new Rectangle(5, 10, 20, 30)
Detail:
– The new operator makes a Rectangle object
– It uses the parameters (5, 10, 20, and 30) to initialize
the data of the object
– returns the object
 Usually the output of the new operator is stored
in a variable
Rectangle box = new Rectangle(5, 10, 20, 30);

3. Constructing Objects
Default constructor
 new Rectangle()
// constructs a rectangle with its top-left corner
// at the origin (0, 0), width 0, and height 0

4. Example
import java.awt.Rectangle;
public class Move
{
public static void main(String[] args)
{
Rectangle box = new Rectangle(5, 10, 20, 30);
// Move the rectangle
box.translate(15, 25);
// Print information about the moved rectangle
System.out.println("After moving, the top-left corner is:");
System.out.println(box.getX());
System.out.println(box.getY());
}
}

5. Random class
 Random r = new Random();
 int i = r.nextInt(int n) Returns random int ≥ 0 and < n
 int i = r.nextInt() Returns random int (full range)
 long i = r.nextLong() Returns random long (full range)
 float f = r.nextFloat() Returns random float ≥0.0 and <1.0
 double d = r.nextDouble() Returns random double ≥ 0.0 and < 1.0
 boolean b = r.nextBoolean() Returns random double (true ,false)
 double x;
x = Math.random(); // assigns random number to x
5
import java.util.Random;

6. Object References
 Describe the location of objects
 The new operator returns a reference to a new object
Rectangle box = new Rectangle();
 Multiple object variables can refer to the same object
Rectangle box = new Rectangle(5, 10, 20, 30);
Rectangle box2 = box;
box2.translate(15, 25);
 Primitive type variables ≠ object variables

7. Cast
 (typeName) expression
Example:
 (int) (balance * 100)
Purpose:
 To convert an expression to a different type
7

8. Constants: final
 A final variable is a constant
 Once its value has been set, it cannot be changed
 Named constants make programs easier to read and maintain
 Convention: use all-uppercase names for constants
final double QUARTER_VALUE = 0.25;
final double DIME_VALUE = 0.1;
8

9. Constants: static final
 static is used with class members
 If constant values are needed in several methods, declare them
together and tag them as static and final
 Give static final constants public access to enable other classes to
use them
public class Math
{
. . .
public static final double E = 2.7182818284590452354;
public static final double PI = 3.14159265358979323846;
}
double circumference = Math.PI * diameter; (call without object)
9

10. Syntax : Constant Definition
 In a method:
final typeName variableName = expression ;
 In a class:
accessSpecifier static final typeName variableName =
expression;
Example:
 final double NICKEL_VALUE = 0.05;
 public static final double LITERS_PER_GALLON = 3.785;
Purpose:
 To define a constant in a method or a class
10

11. Increment, and Decrement
 items++ is the same as items = items + 1
 items-- subtracts 1 from items
 ++a;
 --a;
 a++;
 a--;
11

12. Arithmetic Operations
 / is the division operator
 If both arguments are integers, the result is an
integer. The remainder is discarded
– 7.0 / 4 yields 1.75
– 7 / 4 yields 1
 Get the remainder with % (pronounced "modulo")
7 % 4 is 312

13. The Math class
 Math class: contains methods like sqrt and pow
 To compute xn, you write Math.pow(x, n)
 However, to compute x2 it is significantly more efficient simply to
compute x * x
 To take the square root of a number x, use the Math.sqrt;
for example,
Math.sqrt(x)
13
Java.lang.Math

14. Mathematical Methods in Java
14
Math.sqrt(x) square root
Math.pow(x, y) power xy
Math.exp(x) ex
Math.log(x) natural log
Math.sin(x), Math.cos(x), Math.tan(x) sine, cosine, tangent (x in radian)
Math.round(x) closest integer to x
Math.min(x, y), Math.max(x, y) minimum, maximum

15. java.lang.Math methods
Constants: E, PI
 sin(_), cos(_), abs(_), tan(_),
 ceil(_), floor(_), log(_),
 max(_,_), min(_,_), pow(_,_), sqrt(_),
 round(_), random(),
 toDegrees(_), toRadians(_)
15

16. Self Check
 What is the value of 1729 / 100? and 1729 % 100?
 What does the following statement compute ?
double average = 20 + 25 + 30 / 3;
 What is the value of Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2))
in mathematical notation?
Answers
 17 and 29
 Only 30 is divided by 3. To get the correct result, use
parentheses. Moreover, if all three are integers, you must divide
by 3.0 to avoid integer division:
(20 + 25 + 30) / 3.0
16

17. Calling Static Methods
 A static method does not operate on an object
double x = 4;
double root = x.sqrt(); // Error
 Static methods are defined inside classes
 Naming convention: Classes start with an uppercase
letter; objects start with a lowercase letter
Math
System.out
17

18. Static Method Call
 ClassName. methodName(parameters)
Example:
 Math.sqrt(4)
Purpose:
 To invoke a static method (a method that
does not operate on an object) and supply its
parameters18

19. Self Check
 Why can't you call x.pow(y) to compute xy ?
 Is the call System.out.println(4) a static
method call?
Answers
 x is a number, not an object, and you cannot
invoke methods on numbers
 No–the println method is called on the object
System.out19

20. Strings
 A string is a sequence of characters
 Strings are objects of the String class
 String constants:
"Hello, World!"
 String variables:
String message = "Hello, World!";
 String length:
int n = message.length();
 Empty string: ""
20

21. Concatenation
 Use the + operator:
String name = "Dave";
String message = "Hello, " + name;
// message is "Hello, Dave"
 If one of the arguments of the + operator is a
string, the other is converted to a string
String a = "Agent";
int n = 7;
String bond = a + n; // bond is Agent7
21

22. Converting between Strings and
Numbers
 Convert to number:
int n = Integer.parseInt(str);
double x = Double.parseDouble(x);
int num = Integer.parseInt("1234");
 Convert to string:
String str = "" + n;
str = Integer.toString(n);
String str = String.valueOf(num);22

23. Substrings
 String greeting = "Hello, World!";
String sub = greeting.substring(0, 5);
// sub is "Hello"
 Supply start and end positions, end is not
included
 First position is at 0
23

24. Self Check
 Assuming the String variable s holds the value "Agent", what is
the effect of the assignment s = s + s.length() ?
 Assuming the String variable river holds the value "Mississippi",
what is the value of
river.substring(1, 2)? and
river.substring(2, river.length() - 3)?
Answers
 s is set to the string Agent5
 The strings "i" and
 "ssissi"
24

25. Frame Windows
 The JFrame class JFrame frame = new JFrame();
 frame.setSize(300, 400);
frame.setTitle("An Empty Frame");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
 import javax.swing.*;
25

26. Frame program
import javax.swing.*;
public class frame
{
public static void main(String[] args)
{
JFrame frame = new JFrame();
final int FRAME_WIDTH = 300;
final int FRAME_HEIGHT = 400;
frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("An Empty Frame");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}
26

27. Self Check
 How do you display a square frame with a title bar that
reads "Hello, World!"?
 How can a program display two frames at once?
Answers
 Modify the EmptyFrameViewer program as follows:
frame.setSize(300, 300);
frame.setTitle("Hello, World!");
 Construct two JFrame objects, set each of their sizes,
and call setVisible(true) on each of them
27

28. Drawing Shapes
 paintComponent: called whenever the component
needs to be repainted:
public class frame1 extends JComponent
{
public void paintComponent(Graphics g)
{
// Recover Graphics2D
Graphics2D g2 = (Graphics2D) g;
. . .
}
}
28

29. Drawing Shapes
 Graphics class lets you manipulate the graphics state
(such as current color)
 Graphics2D class has methods to draw shape objects
 Use a cast to recover the Graphics2D object from the
Graphics parameter
Rectangle box = new Rectangle(5, 10, 20, 30);
g2.draw(box);
 java.awt package29

30. Rectangle Drawing Program Classes
 frame1: its paintComponent method produces the drawing
 frame: its main method constructs a frame and a frame1, adds the
component to the frame, and makes the frame visible
– Construct a frame
– Construct an object of your component class:
frame1 component = new frame1();
– Add the component to the frame
frame.add(component);
However, if you use an older version of Java (before Version 5), you
must make a slightly more complicated call:
frame.getContentPane().add(component);
– Make the frame visible
30

31. An example (produce a drawing
with two boxes)
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JPanel;
import javax.swing.Jcomponent; /** A component that draws two rectangles. */
public class frame1 extends Jcomponent {
public void paintComponent(Graphics g) {
Graphics2D g2 = (Graphics2D) g; // Recover Graphics2D
Rectangle box = new Rectangle(5, 10, 20, 30); // Construct a rectangle and
g2.draw(box); // draw it
box.translate(15, 25); // Move rectangle 15 units to the right and 25 units down
g2.draw(box); // Draw moved rectangle
} }
31

32. Create frame and add drawing
import javax.swing.JFrame;
public class frame {
public static void main(String[] args) {
JFrame frame = new JFrame();
final int FRAME_WIDTH = 300;
final int FRAME_HEIGHT = 400;
frame.setSize(FRAME_WIDTH,FRAME_HEIGHT);
frame.setTitle("Two rectangles");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame1 component = new frame1();
frame.add(component);
frame.setVisible(true);
} }32

33. Self Check
 How do you modify the program to draw two squares?
 How do you modify the program to draw one rectangle and one
square?
 What happens if you call g.draw(box) instead of g2.draw(box)?
Answers
 Rectangle box = new Rectangle(5, 10, 20, 20);
 Replace the call to box.translate(15, 25) with
box = new Rectangle(20, 35, 20, 20);
 The compiler complains that g doesn't have a draw method
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34. Applets
 This is almost the same outline as for a component, with two minor
differences:
– You extend JApplet, not JComponent
– You place the drawing code inside the paint method, not inside
paintComponent
 To run an applet, you need an HTML file with the applet tag
 You view applets with the appletviewer
34

35. JApplet
/*
<APPLET CODE="frameapplet.class" WIDTH=350 HEIGHT=200>
</APPLET>
*/
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JApplet; /** An applet that draws two rectangles. */
public class frameapplet extends JApplet
{
public void paint(Graphics g)
{ // Prepare for extended graphics
Graphics2D g2 = (Graphics2D) g; // Construct a rectangle and draw it
Rectangle box = new Rectangle(5, 10, 20, 30);
g2.draw(box); // Move rectangle 15 units to the right and 25 units down
box.translate(15, 25); // Draw moved rectangle
g2.draw(box);
}
}
35

36. Graphical Shapes
 Rectangle, Ellipse2D.Double, and Line2D.Double describe graphical
shapes
 We won't use the .Float classes
 These classes are inner classes–doesn't matter to us except for the
import statement:
import java.awt.geom.Ellipse2D; // no .Double
 Must construct and draw the shape
Ellipse2D.Double ellipse = new Ellipse2D.Double(x, y, width, height);
g2.draw(ellipse);
36

37. Drawing Lines
To draw a line:
 Line2D.Double segment = new Line2D.Double(x1, y1, x2, y2);
or
 Point2D.Double from = new Point2D.Double(x1, y1);
Point2D.Double to = new Point2D.Double(x2, y2);
Line2D.Double segment = new Line2D.Double(from, to);
37

38. Self Check
 Give instructions to draw a circle with center (100,100) and radius 25
 Give instructions to draw a letter "V" by drawing two line segments
 Give instructions to draw a string consisting of the letter "V"
Answers
 g2.draw(new Ellipse2D.Double(75, 75, 50, 50);
 Line2D.Double segment1 = new Line2D.Double(0, 0, 10, 30);
g2.draw(segment1);
Line2D.Double segment2 = new Line2D.Double(10, 30, 20, 0);
g2.draw(segment2);
 g2.drawString("V", 0, 30);
38
Upper-left corner, Width , Height

39. Colors
 Standard colors Color.BLUE, Color.RED, Color.PINK etc.
 Specify red, green, blue between 0.0F and 1.0F
 Color magenta = new Color(1.0F, 0.0F, 1.0F); // F = float
Set color in graphics context
 g2.setColor(magenta);
Color is used when drawing and filling shapes
 g2.fill(rectangle); // filled with current color
39

40. Self Check
 What are the RGB color values of Color.BLUE?
 How do you draw a yellow square on a red background?
Answers
 0.0F, 0.0F, and 0.1F
 First fill a big red square, then fill a small yellow square inside:
g2.setColor(Color.RED);
g2.fill(new Rectangle(0, 0, 200, 200));
g2.setColor(Color.YELLOW);
g2.fill(new Rectangle(50, 50, 100, 100));
Note: Use import java.awt.Color;
40

41. Drawing Graphical Shapes
 Rectangle leftRectangle
= new Rectangle(100, 100, 30, 60);
Rectangle rightRectangle
= new Rectangle(160, 100, 30, 60);
Line2D.Double topLine
= new Line2D.Double(130, 100, 160, 100);
Line2D.Double bottomLine
= new Line2D.Double(130, 160, 160, 160);
41

42. Reading Text Input
 A graphical application can obtain input by displaying a
JOptionPane
 The showInputDialog method displays a prompt and
waits for user input
 The showInputDialog method returns the string that the
user typed
String input = JOptionPane.showInputDialog("Enter x");
double x = Double.parseDouble(input);
42

43. An Example
import java.awt.Color; import javax.swing.Jframe;
import javax.swing.JOptionPane; import javax.swing.JComponent;
public class ColorViewer {
public static void main(String[] args) {
JFrame frame = new JFrame(); final int FRAME_WIDTH = 300; final int FRAME_HEIGHT = 400;
frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
String input; // Ask the user for red, green, blue values
input = JOptionPane.showInputDialog("red:");
double red = Double.parseDouble(input);
input = JOptionPane.showInputDialog("green:");
double green = Double.parseDouble(input);
input = JOptionPane.showInputDialog("blue:");
double blue = Double.parseDouble(input);
Color fillColor = new Color( (float) red, (float) green, (float) blue);
ColoredSquarecomponent = new ColoredSquare (fillColor);
frame.add(component); frame.setVisible(true); } }43

44. Example cont.
import java.awt.Color; import java.awt.Graphics; import java.awt.Graphics2D;
import java.awt.Rectangle; import javax.swing.JComponent;
public class ColoredSquare extends Jcomponent {
private Color fillColor;
public ColoredSquare (Color aColor) { fillColor = aColor; }
public void paintComponent(Graphics g) {
Graphics2D g2 = (Graphics2D) g; // Select color into graphics context
g2.setColor(fillColor); // Const and fill a square whose center is center of the window
final int SQUARE_LENGTH = 100;
Rectangle square = new Rectangle((getWidth() - SQUARE_LENGTH) / 2, (getHeight()
SQUARE_LENGTH) / 2, SQUARE_LENGTH, SQUARE_LENGTH);
g2.fill(square); }
}44