Chapter 5: Keyboard I/O & Simple GUI’s Copyright 2002, Matthew Evett. These slides are based on...
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Transcript of Chapter 5: Keyboard I/O & Simple GUI’s Copyright 2002, Matthew Evett. These slides are based on...
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