Lec 7 28_aug [compatibility mode]

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Lec 7 28_aug [compatibility mode]

  1. 1. Java Data types and Variables Lecture 7 Naveen Kumar
  2. 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: "" 2
  3. 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 3
  4. 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. 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 5
  6. 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" 6
  7. 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.*; 7
  8. 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); } } 8
  9. 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 9
  10. 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; . . . } } 10
  11. 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. 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 12
  13. 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 } } 13
  14. 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. 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 15
  16. 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 16
  17. 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); } } 17
  18. 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); 18
  19. 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); 19
  20. 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); 20 Upper-left corner, Width , Height
  21. 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 21
  22. 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; 22
  23. 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); 23
  24. 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); 24
  25. 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. 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

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