How to use Java InputVerifier() Constructor

Below code shows implementation of InputVerifier() constructor in JTextFields to validate inputs. import java.awt.BorderLayout; import ...

Below code shows implementation of InputVerifier() constructor in JTextFields to validate inputs.

import java.awt.BorderLayout;

import javax.swing.InputVerifier;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class InputVerifierExample extends JFrame{
   JTextField field1;
    JTextField field2 ;
    JTextField field3;

    public InputVerifierExample() {
  super("Input Verifier");
  field1=new JTextField();
   field2=new JTextField();
    field3=new JTextField();
  
    InputVerifier verifier = new InputVerifier() {
      public boolean verify(JComponent comp) {
        boolean returnValue;
        JTextField textField = (JTextField) comp;
        try {
          Integer.parseInt(textField.getText());
          returnValue = true;
        } catch (NumberFormatException e) {
          returnValue = false;
        }
        return returnValue;
      }
    };
field1.setInputVerifier(verifier);
  field3.setInputVerifier(verifier);

 add(field1, BorderLayout.NORTH);
 add(field2, BorderLayout.CENTER);
    add(field3, BorderLayout.SOUTH);
  
      setDefaultCloseOperation(2);
    setSize(300,100);
 
      setVisible(true);    
    }
    
   
    public static void main(String[] a){
     new InputVerifierExample();
    }
}