Data Structures for Java William H Ford William

The Java Interface n n An interface is a pure abstract classlike structure. Contains

The Java Interface (concluded) n n n Interface reference variables can be assigned references

Declaring an Interface n Use the keyword interface. Example: | public interface Interface. Name

Implementing an Interface n n The relationship between a class and an interface is

The Measurement Interface n The Measurement interface is a template for the definition of

Circle Class public class Circle implements Measurement { private double radius; // creates an

Circle Class (concluded) // access and update methods public double get. Radius() { return

UML for the Measurement Hierarchy © 2005 Pearson Education, Inc. , Upper Saddle River,

Using an Interface Type n An interface reference variable can be assigned any object

resize() public static void resize(Measurement m, double pct) { if (m instanceof Rectangle) {

resize() (concluded) else if (m instanceof Circle) { // cast m as a Circle

Interface Inheritance Hierarchy n A parent interface may be used to derive a child

Extending Interface Implementations n A class that implements an interface can be extended. The

Multiple Interfaces and Inheritance n n Java only allows a class to extend one

Multiple Interfaces and Inheritance (continued) © 2005 Pearson Education, Inc. , Upper Saddle River,

Java Answer to Multiple Inheritance n Allow a superclass to implement multiple interfaces. ©

Javadoc n n Documentation is needed with any programming language. The javadoc utility produces

Javadoc (continued) n n n Javadoc comments begin with the marker /** and end

Javadoc Example /** * The <em>Circle</em> class provides measurement * operations for objects that

Javadoc Output for Circle Class © 2005 Pearson Education, Inc. , Upper Saddle River,

Javadoc Output for Circle Class (continued) © 2005 Pearson Education, Inc. , Upper Saddle

Design Pattern n n Documents the solution to a problem in a very general

Dice Class as Example of Singleton Design Pattern public class Dice { // static

Dice Class as Example of Singleton Design Pattern (continued) // if no object currently

Dice Class as Example of Singleton Design Pattern (continued) // toss the dice and

GUI Application Design n n GUI = "Graphical User Interface". GUI Components Frame -

Key Components of a GUI Application © 2005 Pearson Education, Inc. , Upper Saddle

Inheritance Hierarchy for Java GUI Components © 2005 Pearson Education, Inc. , Upper Saddle

GUI Application Design Pattern <import statements> public class Dice. Toss extends JFrame { <

GUI Design Pattern Import Statements n Import statements specify the class libraries that the

GUI Design Pattern Application Variables n Declare as application variables the GUI components and

Dice Class Variables class Dice. Toss extends JFrame { // application variables private JPanel

GUI Application Constructor n Initializes frame properties and loads the components. n Set title

GUI Application Constructor (concluded) n n Components in a frame reside in a region

Dice. Toss Constructor public Dice. Toss() {. . . // establish the content pane

Dice. Toss Constructor (continued) // panel with toss button, total label, // and text

Dice. Toss Constructor (concluded) // add panels and individual components to the // content

Event Listeners n n n An event is an action such as a mouse

Action Events n An event which indicates that a component-defined action occurred. This event

Action Events (continued) n n void add. Action. Listener(Action. Listener handler. Obj) Adds action

Action Events (concluded) n An action listener event handling class must implement the Action.

Action Event Sequence © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All

Dice. Toss Event Handler Class private class Toss. Event implements Action. Listener { public

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The Java Interface n n An interface is a pure abstract classlike structure. Contains only public abstract methods and public final static data. Classes implement the interface rather than extending it. Serves as a template for its implementing classes. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

The Java Interface (concluded) n n n Interface reference variables can be assigned references from any implementing class. Calling a method with an interface reference variable invokes polymorhpism. An interface may be implemented by more than one class. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Declaring an Interface n Use the keyword interface. Example: | public interface Interface. Name { public static final DATA_NAME = <value>; // method declaration uses only a signature public return. Type method. Name(<parameter list>); } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Implementing an Interface n n The relationship between a class and an interface is established by using the keyword implements. The implementing class must provide method declarations for each method in the interface. // class interface declaration public class Class. Name implements Interface. Name { <implement each interface method> } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

The Measurement Interface n The Measurement interface is a template for the definition of geometric classes that define the area and perimeter of shapes. public interface Measurement { public static final double PI = 3. 14159265; double area(); double perimeter(); } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Circle Class public class Circle implements Measurement { private double radius; // creates an instance with // the specified radius public Circle(double radius) { this. radius = radius; } // interface method area() returns // PI * radius (squared) public double area() { return PI * radius; } // interface method perimeter() returns // 2 * PI * radius public double perimeter() { return 2 * PI * radius; } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Circle Class (concluded) // access and update methods public double get. Radius() { return radius; } public void set. Radius(double radius) { this. radius = radius; } // returns a description of the object public String to. String() { return "Circle with radius " + radius; } } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Using an Interface Type n An interface reference variable can be assigned any object from a class that implements the interface, and polymorphism applies. Interface. Name ref; Implementing. Class obj = new Implementing. Class(. . . ); ref = obj; // calls method. Name() in Implementing. Class ref. method. Name(); © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

resize() public static void resize(Measurement m, double pct) { if (m instanceof Rectangle) { // cast m as a Rectangle r = (Rectangle)m; // use set. Sides() to resize the // length and width r. set. Sides(r. get. Length()*pct, r. get. Width()*pct); } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

resize() (concluded) else if (m instanceof Circle) { // cast m as a Circle c = (Circle)m; // use set. Radius() to resize the radius c. set. Radius(c. get. Radius()*pct); } } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Interface Inheritance Hierarchy n A parent interface may be used to derive a child interface. Use the extends keyword. Example: public interface Diagonal. Measurement extends Measurement { public double diagonal(); } public class Circle implements Diagonal. Measurement { <declarations of area(), perimeter(), and diagonal()> } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Extending Interface Implementations n A class that implements an interface can be extended. The subclass implements the interface also. Example: public class Rectangle implements Measurement {. . . } public class Square extends Rectangle { <methods area(), perimeter() inherited from Rectangle } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Multiple Interfaces and Inheritance n n Java only allows a class to extend one superclass (single inheritance). Some languages like C++ allow a class to inherit multiple superclasses. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Javadoc n n Documentation is needed with any programming language. The javadoc utility produces HTML documentation for classes and interfaces. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Javadoc (continued) n n n Javadoc comments begin with the marker /** and end with the marker */ Can use HTML tags in a javadoc comment. Javadoc tags that produce HTML include: @param n @return type n @throws when n - parameter for a method identifies method return specifies exception thrown an error occurs © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Javadoc Example /** * The <em>Circle</em> class provides measurement * operations for objects that are determined by their radius. */ public class Circle implements Measurement { /** * Creates a circle with the specified radius * @param radius the radius of this object * @throws Illegal. Argument. Exception if the argument * is negative. */ public Circle(double radius) {. . . } /** * Returns the area of the circle. * @return a <code>double</code> having value PI *r (squared). */ public double area() {. . . } } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Design Pattern n n Documents the solution to a problem in a very general way. Singleton design pattern describes a model for building a class that may only have one instance. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice Class as Example of Singleton Design Pattern public class Dice { // static reference that identifies // the single instance private static Dice dice = null; private Random rnd; private int dice 1, dice 2; // private constructor is called by // the method get. Dice() to create // a single instance of the class private Dice() { // create the random number generator rnd = new Random(); } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice Class as Example of Singleton Design Pattern (continued) // if no object currently exists, the // method calls the private constructor // to create an instance; in any case, the // method returns the static reference // variable dice public static Dice get. Dice() { if (dice == null) { dice = new Dice(); } return dice; } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice Class as Example of Singleton Design Pattern (continued) // toss the dice and update values // for dice 1 and dice 2 public void toss() { dice 1 = rnd. next. Int(6) + 1; dice 2 = rnd. next. Int(6) + 1; } // access methods get. One(), get. Two(), // and get. Total(). . . } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

GUI Application Design n n GUI = "Graphical User Interface". GUI Components Frame - Main application window n Panel - Container to which other components are added n Label - Displays a string and/or an image n Text field - Allows input/output of text line n Button - Responds to pressing with mouse n © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

GUI Application Design Pattern <import statements> public class Dice. Toss extends JFrame { < component and application variables> public static void main(String[] args) { Dice. Toss app = new Dice. Toss(); app. set. Visible(true); } // constructor creates components and adds them to the frame public Dice. Toss() { <initialize frame attributes, create components> } // inner class defines an event handler object private class Event. Handler implements <event interface> { } } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

GUI Design Pattern Import Statements n Import statements specify the class libraries that the application will use import java. awt. *; import javax. swing; import java. awt. event. *; // original Java AWT // Swing component classes // Java event classes and interfaces © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

GUI Design Pattern Application Variables n Declare as application variables the GUI components and any data that will be accessed by event handlers, utility methods, and inner classes. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice Class Variables class Dice. Toss extends JFrame { // application variables private JPanel panel. A, panel. B; private JLabel die. Label 1, die. Label 2, total. Label; private JText. Field total. Field; private JText. Area total. Area; private JButton toss. Button; private Dice d = Dice. get. Dice(); private Image. Icon[] die. Pict = {null, new Image. Icon("die 1. gif"), new Image. Icon("die 2. gif"), new Image. Icon("die 3. gif"), new Image. Icon("die 4. gif"), new Image. Icon("die 5. gif"), new Image. Icon("die 6. gif")}; . . . } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

GUI Application Constructor n Initializes frame properties and loads the components. n Set title with set. Title(title. Str). n set. Bounds(pos. X, pos. Y, width, height) n set. Default. Close. Operation(JFrame. EXIT_ON_CLOSE) sets the size of the frame and its position on the screen. closes the frame and exits the application when the user clicks the close box in the frame. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

GUI Application Constructor (concluded) n n Components in a frame reside in a region called the content pane. Use get. Content. Pane() to access the content pane and assign it to a Container reference variable. Container content = get. Content. Pane(); n Create and organize components and add them to the content pane. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice. Toss Constructor public Dice. Toss() {. . . // establish the content pane for the window Container content = get. Content. Pane(); content. set. Layout(new Border. Layout()); // panel and labels for the two die pictures JPanel panel. A = new JPanel(); die. Label 1 = new JLabel(); die. Label 2 = new JLabel(); panel. A. add(die. Label 1); panel. A. add(die. Label 2); // text area to store results of // successive dice tosses total. Area = new JText. Area(10, 15); © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice. Toss Constructor (continued) // panel with toss button, total label, // and text field for the total // the toss button has an action listener // (see next section) JPanel panel. B = new JPanel(); toss. Button = new JButton("Toss"); toss. Button. add. Action. Listener(new Toss. Event()); total. Label = new JLabel("Total"); total. Field = new JText. Field(4); panel. B. add(toss. Button); panel. B. add(total. Label); panel. B. add(total. Field); © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice. Toss Constructor (concluded) // add panels and individual components to the // content pane using compass point constants // for class Border. Layout. // the text area is embedded in a JScroll. Pane // to add scroll bars content. add(panel. A, Border. Layout. NORTH); content. add(new JScroll. Pane(total. Area), Border. Layout. CENTER); content. add(panel. B, Border. Layout. SOUTH); } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Event Listeners n n n An event is an action such as a mouse click, a button press, or a keystroke. An event listener is an object that resides in a component and waits for the occurrence of an event and handles the actions required by the event. Add a listener to a component using add. Type. Listner(. . . ) where Type is the type of the event. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Action Events n An event which indicates that a component-defined action occurred. This event is generated by a component (such as a Button) when the component-specific action occurs (such as being pressed). The event is passed to every Action. Listener object that registered to receive such events using the component's add. Action. Listener method © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Action Events (continued) n n void add. Action. Listener(Action. Listener handler. Obj) Adds action listener to a component. The parameter is the event handler that responds to the event. Normally use a private inner class to define an event handler. An inner class has access to all the variables in the outer class and can update them. © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Action Events (concluded) n An action listener event handling class must implement the Action. Listener interface. The interface has only one method void action. Performed(Action. Event ae) Invoked when an action occurs © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.

Dice. Toss Event Handler Class private class Toss. Event implements Action. Listener { public void action. Performed(Action. Event ae) { d. toss(); die. Label 1. set. Icon(die. Pict[d. get. One()]); die. Label 2. set. Icon(die. Pict[d. get. Two()]); total. Area. append("Total is " + d. get. Total() + "n"); total. Field. set. Text("" + d. get. Total()); } } © 2005 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved.