CS211Fall 20002■ Declaring the characteristics of an ADT● Examples: Collection, Set, List■ Declaring a single characteristic that is shared by several top-level, stand-alone classes (e.g., String, Integer)● Examples: Comparable, Cloneable■ Declaring a single helper method that is thus attached to an object; the object can be stored or passed as an argument● Examples: Comparator, various Event Listeners3■ In general, a class that implements Comparable has many other methods■ In general, a class that implements Comparator contains just the single method compare( )■ This isn’t quite true because a single class can be made to serve multiple purposes (i.e., you can define a single class that implements Comparable, Comparator, Set, andActionListener)Comparing Objects x and y■ Intuitionif (x < y)…■ When you want the natural order// Can throw ClassCastExceptionComparable cx = (Comparable)x;if (cx.compareTo(y) < 0)…■ When using a Comparator (com)// Can throw a ClassCastExceptionif (com.compare(x,y) < 0)…■ When you might be using a Comparatorif (com == null) {Comparable cx = (Comparable)x;if (cx.compareTo(y) < 0)…}elseif (com.compare(x,y) < 0)…4■ java.awt.Point defines a 2D point with integer coordinates■ If p is a Point then● p.x is the x-coordinate● p.y is the y-coordinate■ Goal: use lexicographic ordering (i.e., the first coordinate determines the order, but if they’re tied, use the second coordinate)import java.awt.Point;class MyPointComparator implements Comparator {public int compare (Object a, Object b) {Point pa = (Point) a;Point pb = (Point) b;if (pa.x <p b.x) return –1;if (pa.x > pb.x) return 1;if (pa.y < pb.y) return –1;if (pa.y > pb.y) return 1;return 0;}}5■ If we store Points in a SortedSet then it’s easy to print them in lexicographic order■ Assume we start with an array p of Points■ Attempt #1:SortedSet s = new SortedSet(java.util.Arrays.asList(p));● This fails (throws a ClassCastException) because Points are not Comparable■ Attempt #2:SortedSet s = new SortedSet(new MyPointComparator());s = s.addAll(java.util.Arrays.asList(p)); // Bulk add● This succeeds because we provided a Comparator● If I try to put a non-Point into s then my compare() method throws a ClassCastException6■ Layout● How items are arranged● There are lots of predefined GUI items JButton, JLabel, JCheckbox, JList, JScrollbar,…● You have to write the code that determines layout● In Java, you use LayoutManagers to help with layout■ Event Handling● An event is (generally) a user input or action● The JVM (Java Virtual Machine) takes care of generating eventsButton pushed, text typed, mouse clicked,…● You have to write the code that determines how your program responds to an event7public static void main (String[ ] args) {JFrame frame = new JFrame();JPanel panel = new JPanel();JPanel topPanel = new JPanel();JPanel botPanel = new JPanel();topPanel.add(new JButton("One"));topPanel.add(new JButton("Two"));botPanel.add(new JButton("Three"));botPanel.add(new JButton("Four"));botPanel.add(new JButton("Five"));panel.setLayout(new BorderLayout());panel.add(topPanel,BorderLayout.NORTH);panel.add(botPanel,BorderLayout.SOUTH);frame.getContentPane().add(panel);frame.setSize(250,300);frame.setVisible(true);}8import javax.swing.*;import java.awt.GridLayout;import java.awt.BorderLayout;class GUITest {public static void main (String[ ] args) {JFrame frame = new JFrame();JPanel panel = new JPanel();panel.setLayout(new GridLayout(0,1));panel.add(new JButton("One"));panel.add(new JButton("Two"));panel.add(new JButton("Three"));panel.add(new JButton("Four"));panel.add(new JButton("Five"));frame.getContentPane().add(panel,BorderLayout.WEST);frame.setSize(250,300);frame.setVisible(true);} }9■ The JVM (Java Virtual Machine) determines the event’s source and type● The source is the component from which the event originated● Each source has certain types of events it can generate■ The JVM looks for one or more event listeners that have registeredwith the source● An event listener is an object that implements one of the Listener interfaces in java.awt.event or in javax.swing.event● You register an event listener by using one of the component’saddListener methods■ The JVM creates an event object using one of the classes in java.awt.event or in javax.swing.event■ For each registered event listener, the JVM invokes the listener's event-handling method and passes the event object as the parameter10import javax.swing.*; import java.awt.event.*;import java.awt.Color;class GUITest {static String[ ] name = {"red","blue","green","magenta","cyan","yellow"};static Color[ ] color = {Color.red,Color.blue,Color.green,Color.magenta,Color.cyan,Color.yellow};public static void main (String[ ] args) {JFrame frame = new JFrame();JPanel panel = new JPanel();for (int i = 0; i < name.length; i++) {JButton button = new JButton(name[i]);panel.add(button);button.addActionListener(new MyListener(panel,color[i]));}frame.getContentPane().add(panel);frame.setSize(250,300);frame.setVisible(true);}11■ Class for color manipulation■ Constants● black, blue, cyan, darkGray, gray, green, lightGray, magenta, orange, pink, red, white, yellow■ Constructors (2 of several)● Color (int r, int g, int b); // 0 to 255● Color (float r, float g, float b); // 0.0 to 1.0■ Methods● lighter( ), darker( ), getRed( ), getGreen( ), getBlue( ), getHSBColor( ),…12static class MyListener implements ActionListener {Color myColor;JPanel myPanel;MyListener (JPanel panel, Color color) {myColor = color;myPanel = panel;}public void actionPerformed (ActionEvent event) {myPanel.setBackground(myColor);}}}13■ The user clicks a button■ The JVM examines the button to see if any ActionListeners have registered with the button● If the button is supposed to do something, an ActionListener must have registered with the button prior to the button-click● Use button.addActionListener(…) to register● You can register a Listener at any time, but this is typically done when the button is created (as in the example)■ The JVM creates an ActionEvent (call it e)■ For each registered ActionListener x, the JVM calls x.actionPerformed(e)● This is where/when your code (telling what should happen when the button is clicked) is executed● In the example, the ActionListener remembers the panel and a color when it’s created; when the action is performed (i.e., the button is
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