This document discusses Java data types and variables including strings, concatenation, converting between strings and numbers, and substrings. It also covers drawing shapes using Graphics2D, colors, reading text input using JOptionPane, and an example program that reads RGB values from the user and draws a colored square on a JFrame.

1. Java Data types and Variables
Lecture 7
Naveen Kumar

2. 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: ""
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3. 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
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4. 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);4

5. 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
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6. 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"
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7. 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.*;
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8. 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);
}
}
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9. 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
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10. 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;
. . .
}
}
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11. 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 package11

12. 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
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13. 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
} }
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14. 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);
} }14

15. 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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16. 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
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17. 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);
}
}
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18. 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);
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19. 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);
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20. 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);
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Upper-left corner, Width , Height

21. 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
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22. 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;
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23. 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);
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24. 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);
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25. 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); } }25

26. 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); }
}26