Skip to main content

Steps to perform Event Handling

Steps to perform Event Handling

Following steps are required to perform event handling:

  1. Register the component with the Listener

Registration Methods

For registering the component with the Listener, many classes provide the registration methods. For example:

  • Button
    • public void addActionListener(ActionListener a){}
  • MenuItem
    • public void addActionListener(ActionListener a){}
  • TextField
    • public void addActionListener(ActionListener a){}
    • public void addTextListener(TextListener a){}
  • TextArea
    • public void addTextListener(TextListener a){}
  • Checkbox
    • public void addItemListener(ItemListener a){}
  • Choice
    • public void addItemListener(ItemListener a){}
  • List
    • public void addActionListener(ActionListener a){}
    • public void addItemListener(ItemListener a){}

Java Event Handling Code

We can put the event handling code into one of the following places:

  1. Within class
  2. Other class
  3. Anonymous class

Java event handling by implementing ActionListener

  1. import java.awt.*;  
  2. import java.awt.event.*;  
  3. class AEvent extends Frame implements ActionListener{  
  4. TextField tf;  
  5. AEvent(){  
  6.   
  7. //create components  
  8. tf=new TextField();  
  9. tf.setBounds(60,50,170,20);  
  10. Button b=new Button("click me");  
  11. b.setBounds(100,120,80,30);  
  12.   
  13. //register listener  
  14. b.addActionListener(this);//passing current instance  
  15.   
  16. //add components and set size, layout and visibility  
  17. add(b);add(tf);  
  18. setSize(300,300);  
  19. setLayout(null);  
  20. setVisible(true);  
  21. }  
  22. public void actionPerformed(ActionEvent e){  
  23. tf.setText("Welcome");  
  24. }  
  25. public static void main(String args[]){  
  26. new AEvent();  
  27. }  
  28. }  

public void setBounds(int xaxis, int yaxis, int width, int height); have been used in the above example that sets the position of the component it may be button, textfield etc.

event handling in java

2) Java event handling by outer class

  1. import java.awt.*;  
  2. import java.awt.event.*;  
  3. class AEvent2 extends Frame{  
  4. TextField tf;  
  5. AEvent2(){  
  6. //create components  
  7. tf=new TextField();  
  8. tf.setBounds(60,50,170,20);  
  9. Button b=new Button("click me");  
  10. b.setBounds(100,120,80,30);  
  11. //register listener  
  12. Outer o=new Outer(this);  
  13. b.addActionListener(o);//passing outer class instance  
  14. //add components and set size, layout and visibility  
  15. add(b);add(tf);  
  16. setSize(300,300);  
  17. setLayout(null);  
  18. setVisible(true);  
  19. }  
  20. public static void main(String args[]){  
  21. new AEvent2();  
  22. }  
  23. }  
  1. import java.awt.event.*;  
  2. class Outer implements ActionListener{  
  3. AEvent2 obj;  
  4. Outer(AEvent2 obj){  
  5. this.obj=obj;  
  6. }  
  7. public void actionPerformed(ActionEvent e){  
  8. obj.tf.setText("welcome");  
  9. }  
  10. }  

3) Java event handling by anonymous class

  1. import java.awt.*;  
  2. import java.awt.event.*;  
  3. class AEvent3 extends Frame{  
  4. TextField tf;  
  5. AEvent3(){  
  6. tf=new TextField();  
  7. tf.setBounds(60,50,170,20);  
  8. Button b=new Button("click me");  
  9. b.setBounds(50,120,80,30);  
  10.   
  11. b.addActionListener(new ActionListener(){  
  12. public void actionPerformed(){  
  13. tf.setText("hello");  
  14. }  
  15. });  
  16. add(b);add(tf);  
  17. setSize(300,300);  
  18. setLayout(null);  
  19. setVisible(true);  
  20. }  
  21. public static void main(String args[]){  
  22. new AEvent3();  
  23. }  
  24. }  
Anurag Rana

Comments

Post a Comment

Popular posts from this blog

Standard and Formatted Input / Output in C++

The C++ standard libraries provide an extensive set of input/output capabilities which we will see in subsequent chapters. This chapter will discuss very basic and most common I/O operations required for C++ programming. C++ I/O occurs in streams, which are sequences of bytes. If bytes flow from a device like a keyboard, a disk drive, or a network connection etc. to main memory, this is called   input operation   and if bytes flow from main memory to a device like a display screen, a printer, a disk drive, or a network connection, etc., this is called   output operation . Standard Input and Output in C++ is done through the use of  streams . Streams are generic places to send or receive data. In C++, I/O is done through classes and objects defined in the header file  <iostream> .  iostream  stands for standard input-output stream. This header file contains definitions to objects like  cin ,  cout , etc. /O Library Header Files There are...
Post a Comment
Read more

Genetic Algorithm: Population, Fitness Function, Parent Selection, Cross over, Mutation

Genetic Algo Population Population is a subset of solutions in the current generation. It can also be defined as a set of chromosomes. There are several things to be kept in mind when dealing with GA population − The diversity of the population should be maintained otherwise it might lead to premature convergence. The population size should not be kept very large as it can cause a GA to slow down, while a smaller population might not be enough for a good mating pool. Therefore, an optimal population size needs to be decided by trial and error. The population is usually defined as a two dimensional array of –  size population, size x, chromosome size . Population Initialization There are two primary methods to initialize a population in a GA. They are − Random Initialization  − Populate the initial population with completely random solutions. Heuristic initialization  − Populate the initial population using a known heuristic for the problem. It has been observed that the e...
Post a Comment
Read more

C++ (Object and Class)

The major purpose of C++ programming is to introduce the concept of object orientation to the C programming language. Object Oriented Programming is a paradigm that provides many concepts such as  inheritance, data binding, polymorphism etc. The programming paradigm where everything is represented as an object is known as truly object-oriented programming language.  Smalltalk  is considered as the first truly object-oriented programming language. OOPs (Object Oriented Programming System) Object  means a real word entity such as pen, chair, table etc.  Object-Oriented Programming  is a methodology or paradigm to design a program using classes and objects. It simplifies the software development and maintenance by providing some concepts: Object Class Inheritance Polymorphism Abstraction Encapsulation C++ Object In C++, Object is a real world entity, for example, chair, car, pen, mobile, laptop etc. In other words, object is an ent...
Post a Comment
Read more