what are java events

Java event mechanism consists of three parts: event, event source, and event listener.

1. Events

Events generally inherit from the java.util.EventObject class and encapsulate the event source object and event-related information.

import java.util.EventObject;  
  
/** 
 * 事件类,用于封装事件源及一些与事件相关的参数. 
 */  
public class CusEvent extends EventObject {  
    private static final long serialVersionUID = 1L;  
    private Object source;//事件源  
      
    public CusEvent(Object source){  
        super(source);  
        this.source = source;  
    }  
  
    public Object getSource() {  
        return source;  
    }  
  
    public void setSource(Object source) {  
        this.source = source;  
    }  
}

2. Event source

The event source is the place where the event occurs. Due to a change in a property or state of the event source (such as BUTTON being clicked, The value of TEXTBOX changes, etc.) causing an event to occur. In other words, the corresponding event object is generated. Because the event listener needs to be registered on the event source, the event source class should have a container (List, Set, etc.) to hold the listener.

/** 
 * 事件源.  
 */  
public class EventSourceObject {  
    private String name;  
    //监听器容器  
    private Set<CusEventListener> listener;  
    public EventSourceObject(){  
        this.listener = new HashSet<CusEventListener>();  
        this.name = "defaultname";  
    }  
    //给事件源注册监听器  
    public void addCusListener(CusEventListener cel){  
        this.listener.add(cel);  
    }  
    //当事件发生时,通知注册在该事件源上的所有监听器做出相应的反应（调用回调方法）  
    protected void notifies(){  
        CusEventListener cel = null;  
        Iterator<CusEventListener> iterator = this.listener.iterator();  
        while(iterator.hasNext()){  
            cel = iterator.next();  
            cel.fireCusEvent(new CusEvent(this));  
        }  
    }  
    public String getName() {  
        return name;  
    }  
    //模拟事件触发器，当成员变量name的值发生变化时，触发事件。  
    public void setName(String name) {  
        if(!this.name.equals(name)){  
            this.name = name;  
            notifies();  
        }        
    }  
}

3. Event listener

The event listener implements the java.util.EventListener interface and is registered on the event source. When the properties or status of the event source change , get the corresponding listener to call its internal callback method.

import java.util.EventListener;  
  
/** 
 * 事件监听器，实现java.util.EventListener接口。定义回调方法，将你想要做的事 
 * 放到这个方法下,因为事件源发生相应的事件时会调用这个方法。 
 */  
public class CusEventListener implements EventListener {  
      
    //事件发生后的回调方法  
    public void fireCusEvent(CusEvent e){  
        EventSourceObjecteObject = (EventSourceObject)e.getSource();  
        System.out.println("My name has been changed!");  
        System.out.println("I got a new name,named \""+eObject.getName()+"\"");
    }  
}

Test class

public class MainTest {  
   
    public static void main(String[] args) {  
        EventSourceObject object = new EventSourceObject();  
        //注册监听器  
        object.addCusListener(new CusEventListener(){  
            @Override  
            public void fireCusEvent(CusEvent e) {  
                super.fireCusEvent(e);  
            }  
        });  
        //触发事件  
        object.setName("AiLu");  
    }  
}

4, the connection between events, event sources, and listeners

Event source----- Generate ----->Event------>Discovered by the event listener------>Enter the event processing code

import java.awt.Frame;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
public class DemoAction extends Frame {//窗口就是事件源
    public DemoAction() {
        this.setTitle("窗口关闭");
        this.setLocation(400, 200);
        this.setSize(360, 280);   
        //给窗口绑定一个事件监听器
        this.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {//事件WindowEvent
                System.exit(0);//关闭窗口事件, 那么退出jvm
            }
        });
        this.setVisible(true);
    }
 
    public static void main(String[] args) {
        new DemoAction();
    }
}

5. Main points of event processing Steps:

(1) Generate a listener;

(2) Register the listener on the corresponding component where the event occurs;

(3) Register the listener When an event occurs on the component, the event object is returned to the listener;

(4) The listener calls the corresponding method to handle the event.

Code examples:

public class LoginListener implements ActionListener{
String text;
public void actionPerformed(ActionEvent e){
if("willow".equals(text)){
system.out.println("登录成功！\n");
}
}
}
public class Login(){
public void showFrame(){
//创建窗体
JFrame LFrame = new JFrame();
LFrame.setSize(1000,800);
LFrame.setLocationRelativeTo(null);
LFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
LFrame.setLayout(new FlowLayout());
//创建监听器对象
LoginListener lg = new LoginListener();
//给窗体添加按钮组件
JTextField tf = new JTextField();
LFrame.add(tf);
//给输入框添加监听器
tf.addActionLisener(lg);
tf.text=tf.getText();
}
};

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