Chapter 5: Keyboard I/O & Chapter 5: Keyboard I/O & Simple GUI’s Simple GUI’s

Copyright 2002, Matthew Evett. These slides are based on slides copyrighted by John Lewis and William Loftus, 2002, and

used with permission. All rights reserved.

Keyboard Input RevisitedKeyboard Input Revisited

The Keyboard class was introduced in Chapter 2 to facilitate capturing input from the keyboard

Recall that the Keyboard class was written by the authors of the book

The Keyboard class hides various aspects of Java input processing

But it is important to understand how that processing works

Reading Keyboard InputReading Keyboard Input

Java I/O is accomplished using objects that represent streams of data

A stream is an ordered sequence of bytes

The System.out object represents a standard output stream, which defaults to the monitor screen

Reading keyboard input is more complicated

The input stream is made up of multiple objects:

BufferedReader in = new BufferedReader (

new InputStreamReader (System.in));

Reading Keyboard InputReading Keyboard Input

The System.in object is used to create an InputStreamReader object

The InputStreamReader object is used to create a BufferedReader object

This creates an input stream that treats input as characters and buffers them so that input can be read a line at a time

The readLine method of the BufferedReader class reads an entire line of input as a String

Wrapper class methods can be used to convert numeric input

Reading Keyboard InputReading Keyboard Input

Problems that arise in reading or converting a value manifest themselves as exceptions

The throws clause of a method header indicates what exceptions it may throw

See Wages2.java (page 289)

I/O and exceptions are explored further in Chapter 8

Nested ClassesNested Classes

In addition to containing data and methods, a class can contain other classes

A class declared within another class is called a nested class

Enclosing Class

Nested Class

Nested ClassesNested Classes

A nested class has access to the variables and methods of the enclosing class, even if they are declared private

In certain situations this makes the implementation of the classes easier because they can share information easily

Furthermore, the nested class can be protected by the enclosing class from external use

This is a special relationship and should be used with care

Nested ClassesNested Classes

A nested class produces a separate bytecode file

If a nested class called Inside is declared in an outer class called Outside, two bytecode files are produced:

Outside.class

Outside$Inside.class

Nested classes can be declared as static, in which case they cannot refer to instance variables or methods

Inner ClassesInner Classes

A nonstatic nested class is called an inner class

An inner class instance is associated with each instance of the enclosing class

An instance of an inner class can exist only within an instance of an enclosing class

See TestInner.java (page 291)

See Outer.java (page 292)

InterfacesInterfaces

A Java interface is a collection of abstract methods and constants

An abstract method is a method header without a method body

An abstract method can be declared using the modifier abstract, but because all methods in an interface are abstract, usually it is left off

An interface is used to establish, as a formal contract, a set of methods that a class will implement

InterfacesInterfaces

public interface Doable{ public void doThis(); public int doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num);}

interface is a reserved wordNone of the methods inan interface are given

a definition (body)

A semicolon immediatelyfollows each method header

InterfacesInterfaces

An interface cannot be instantiated

Methods in an interface have public visibility by default

A class formally implements an interface by

• stating so in the class header

• providing implementations for each abstract method in the interface

If a class asserts that it implements an interface, it must define all methods in the interface

InterfacesInterfaces

public class CanDo implements Doable{ public void doThis () { // whatever }

public void doThat () { // whatever }

// etc.}

implements is areserved word

Each method listedin Doable is

given a definition

InterfacesInterfaces

A class that implements an interface can implement other methods as well

See Complexity.java (page 294) See Question.java (page 295) See MiniQuiz.java (page 297)

In addition to (or instead of) abstract methods, an interface can contain constants

When a class implements an interface, it gains access to all its constants

InterfacesInterfaces

A class can implement multiple interfaces

The interfaces are listed in the implements clause

The class must implement all methods in all interfaces listed in the header

class ManyThings implements interface1, interface2

{

// all methods of both interfaces

}

InterfacesInterfaces

The Java standard class library contains many helpful interfaces

The Comparable interface contains an abstract method called compareTo, which is used to compare two objects

The String class implements Comparable, giving us the ability to put strings in lexicographic order

The Iterator interface contains methods that allow the user to move easily through a collection of objects

The Comparable InterfaceThe Comparable Interface

The Comparable interface provides a common mechanism for comparing one object to another

if (obj1.compareTo(obj2) < 0)

System.out.println (“obj1 is less than obj2”);

The result is negative is obj1 is less that obj2, 0 if they are equal, and positive if obj1 is greater than obj2

When a programmer writes a class that implements the Comparable interface, it should follow this intent

It's up to the programmer to determine what makes one object “less” than another

The Iterator InterfaceThe Iterator Interface

The Iterator interface provides a means of moving through a collection of objects, one at a time

The hasNext method returns a boolean result (true if there are items left to process)

The next method returns the next object in the iteration

The remove method removes the object most recently returned by the next method

Dialog BoxesDialog Boxes

A dialog box is a graphical window that pops up on top of any currently active window for the user

The Swing API contains a class called JOptionPane that simplifies the creation and use of basic dialog boxes

There are three categories of JOptionPane dialog boxes• A message dialog displays an output string• An input dialog presents a prompt and a single input text

field• A confirm dialog presents the user with a simple yes-or-

no question

See EvenOdd.java (page 302)

The EvenOdd ProgramThe EvenOdd Program

Graphical User InterfacesGraphical User Interfaces

A Graphical User Interface (GUI) is created with at least three kinds of objects• components• events• listeners

A GUI component defines a screen element to display information or allow the user to interact with the program• buttons, text fields, labels, menus, etc.

A container is a special component that holds and organizes other components• dialog boxes, applets, frames, panels, etc.

EventsEvents

An event is an object that represents some activity to which we may want to respond

For example, we may want our program to perform some action when the following occurs:• the mouse is moved• a mouse button is clicked• the mouse is dragged• a graphical button is clicked• a keyboard key is pressed• a timer expires

Events often correspond to user actions, but not always

Events and ListenersEvents and Listeners

The Java standard class library contains several classes that represent typical events

Components, such as an applet or a graphical button, generate (fire) an event when it occurs

Other objects, called listeners, wait for events to occur

We can write listener objects to do whatever we want when an event occurs

A listener object is often defined using an inner class

Events and ListenersEvents and Listeners

Component

This object maygenerate an event

Listener

This object waits for andresponds to an event

Event

When an event occurs, the generator callsthe appropriate method of the listener,

passing an object that describes the event

Listener InterfacesListener Interfaces

We can create a listener object by writing a class that implements a particular listener interface

The Java standard class library contains several interfaces that correspond to particular event categories

For example, the MouseListener interface contains methods that correspond to mouse events

After creating the listener, we add the listener to the component that might generate the event to set up a formal relationship between the generator and listener

Creating GUIsCreating GUIs

To create a program with a GUI:• define and set up the components• create listener objects• set up the relationships between the listeners and the

components that generate the events of interest• define what happens in response to each event

A push button is a component that allows the user to initiate an action with the press of the mouse button• defined by the JButton class• generates an action event

A label is a component that displays a line of text (or an image, or both)• defined by the JLabel class

Creating GUIsCreating GUIs

The init method of an applet can be used to set up the GUI and add each component to the applet container

The Swing version of the Applet class is called JApplet

In a JApplet, components are added to the applet's content pane

The content pane is retrieved using the getContentPane method

A JButton generates an action event

See PushCounter.java (page 305)

The PushCounter ProgramThe PushCounter Program

Action ListenersAction Listeners

The interface corresponding to an action event is called ActionListener, which defines only one method, called actionPerformed

The ButtonListener inner class implements the ActionListener interface in the PushButton program

When the button is pushed, the JButton object invokes the actionPerformed method, passing it an ActionEvent

The listener method may or may not make use of the event object passed to it

GUI ApplicationsGUI Applications

A frame is a container component used for stand-alone GUI-based applications

A panel is a container, but unlike a frame, it cannot be displayed on its own

• it must be added to another container

• it helps organize the components in a GUI

See Fahrenheit.java (page 309)

See FahrenheitGUI.java (page 310)

The Fahrenheit ProgramThe Fahrenheit Program

SummarySummary

Chapter 5 has focused on:

• object references and aliases• passing objects references as parameters• the static modifier• wrapper classes• nested classes and inner classes• interfaces• dialog boxes• GUI components, events, and listeners