User Interface

java UI: awt and events 

Any program that utilizes GUI (graphic user interface) like windows-written Java application is event oriented. Event defines any object's change in status. For example: press a button, enter a character in textbox, click or drag a mouse, etc. 

Components of Event Handling 

Event handling has three main components, 

• Events:  An event is a change in state of an object. 
• Events Source: An event source is an object that generates an event. 
• Listeners:  A listener is an object that listens to the event. A listener gets notified when an event occurs. 

How areEvents  handled? 

A source produces an event and sends it with the source to one or more listeners. Once the listener receives the event, the event will be processed and returned. Many Java packages, such as java.util, java.awt and java.awt.event, support events. 

Important Event Classes and Interface 

Steps to handle events: 

• Implement appropriate interface in the class. 
• Register the component with the listener. 

Example of Event Handling

import java.awt.*; 
import java.awt.event.*; 
import java.applet.*; 
import java.applet.*; 
import java.awt.event.*; 
import java.awt.*; 
public class Test extends Applet implements KeyListener 
{ 
String msg=""; 
public void init() 
{ 
    addKeyListener(this); 
} 
public void keyPressed(KeyEvent k) 
{ 
   showStatus("KeyPressed"); 
} 
public void keyReleased(KeyEvent k) 
{ 
    showStatus("KeyRealesed"); 
} 
public void keyTyped(KeyEvent k) 
{ 
msg = msg+k.getKeyChar(); 
  repaint(); 
} 
public void paint(Graphics g) 
{ 
    g.drawString(msg, 20, 40); 
  } 
} 

HTML code:

<applet code="Test" width=300, height=100> 
</applet> 

Java Abstract Window Toolkit(AWT) 

AWT includes a big amount of classes and techniques to build and handle apps such as windows, buttons, scroll bars, etc. The AWT was intended to provide a prevalent collection of GUI design instruments capable of working on a multitude of platforms.The tools supplied by the AWT are introduced using the indigenous GUI toolkit of each platform, thus maintaining each platform's look and feel. This is an benefit of using AWT.But the disadvantage of such an strategy is that when displayed on another platform, GUI built on one platform may look distinct. 

AWT is the basis for Swing, i.e. Swing is a set of GUI interfaces that extend the AWT. But nowadays AWT is merely used as most GUI Java programs are being implemented using Swing, due to its rich implementation of GUI controls and lightweight nature. 

Java AWT Hierarchy 

• Component class 

At the top of the AWT hierarchy is the component category. Component is an abstract class encapsulating all the visual component characteristics. Recalling the present foreground and background colors and the presently chosen text font is accountable for a component item. 

• Container 

Container is an AWT element containing a different element such as button, text field, tables, etc. Container is a component class subclass. Container class keeps track of parts added to a different element. 

• Panel 

Panel class is a container specific subclass. There is no title bar, menu bar or boundary in the panel. It is a container used to hold parts. 

• Window class 

Window class produces a window of the highest standard. Window has no boundaries and menu bar. 

• Frame 

Frame is a Window subclass and can be resized. It is a container containing various parts such as button, title bar, textfield, label, etc. Most AWT apps are developed in Java using the Frame window. Frame class is made up of two distinct builders. 

Frame() throws HeadlessException 
Frame(String title) throws HeadlessException

Creating a Frame 

There are two ways to create a Frame. They are, 

• By Instantiating Frame class 
• By extending Frame class 

Creating Frame Window by Instantiating Frame class 

import java.awt.*; 
public class Testawt 
{ 
Testawt() 
  { 
    Frame fm=new Frame();    //Creating a frame 
    Label lb = new Label("welcome to java graphics");   //Creating a label 
    fm.add(lb);//adding label to the frame 
    fm.setSize(300, 300);   //setting frame size. 
    fm.setVisible(true);     //set frame visibilty true 
  } 
public static void main(String args[]) 
  { 
    Testawt ta = new Testawt(); 
  } 
} 

Creating Frame window by extending Frame class

package testawt; 
import java.awt.*; 
import java.awt.event.*; 
public class Testawt extends Frame 
{ 
  public Testawt() 
    { 
        Button btn=new Button("Hello World"); 
        add(btn); //adding a new Button. 
        setSize(400, 500);        //setting size. 
        setTitle("StudyTonight");  //setting title. 
        setLayout(new FlowLayout());//set default layout for frame. 
        setVisible(true);           //set frame visibilty true. 
    } 
     public static void main (String[] args) 
     { 
        Testawt ta = new Testawt();   //creating a frame. 
    } 
} 

Points to note: 

• While creating a frame (either by instantiating or extending Frame class), the following two attributes are a must for visibility of the frame: 
• setSize(int width, int height); 
• setVisible(true); 
• When you create other components like Buttons, TextFields, etc, you need to add it to the frame by using the method - add(Component's Object); 
• You can add the following method also for resizing the frame - setResizable(true);