https://docs.oracle.com/javase/tutorial/uiswing/events/actionlistener.html

Action listeners are probably the easiest — and most common — event handlers to implement. You implement an action listener to define what should be done when an user performs certain operation.

An action event occurs, whenever an action is performed by the user. Examples: When the user clicks a button, chooses a menu item, presses Enter in a text field. The result is that an actionPerformed message is sent to all action listeners that are registered on the relevant component.

To write an Action Listener, follow the steps given below:

Declare an event handler class and specify that the class either implements an ActionListener interface or extends a class that implements an ActionListener interface. For example:
public class MyClass implements ActionListener {
Register an instance of the event handler class as a listener on one or more components. For example:
someComponent.addActionListener(instanceOfMyClass);

import javax.swing.*;  
import java.awt.event.*;  


public class TextFieldExample implements ActionListener{  
    JTextField tf1,tf2,tf3;  
    JButton b1,b2;  
    TextFieldExample(){  
        JFrame f= new JFrame();  
        tf1=new JTextField();  
        tf1.setBounds(50,50,150,20);  
        tf2=new JTextField();  
        tf2.setBounds(50,100,150,20);  
        tf3=new JTextField();  
        tf3.setBounds(50,150,150,20);  
        tf3.setEditable(false);   
        b1=new JButton("+");  
        b1.setBounds(50,200,50,50);  
        b2=new JButton("-");  
        b2.setBounds(120,200,50,50);  
        b1.addActionListener(this);  
        b2.addActionListener(this);  
        f.add(tf1);f.add(tf2);f.add(tf3);f.add(b1);f.add(b2);  
        f.setSize(300,300);  
        f.setLayout(null);  
        f.setVisible(true);  
    }         
    public void actionPerformed(ActionEvent e) {  
        String s1=tf1.getText();  
        String s2=tf2.getText();  
        int a=Integer.parseInt(s1);  
        int b=Integer.parseInt(s2);  
        int c=0;  
        if(e.getSource()==b1){  
            c=a+b;  
        }else if(e.getSource()==b2){  
            c=a-b;  
        }  
        String result=String.valueOf(c);  
        tf3.setText(result);  
    }  
   public static void main(String[] args)
    {  
    new TextFieldExample();  
     } 
}