Set focus to a field in window/frame (2): FocusTraversalPolicy

The FocusTraversalPolicy controls the sequence of fields gaining focus when you tapping through the fields and also the field that has the focus when the window/frame is first constructed/activated/visible.

Following is a sample code.

import java.awt.Component;
import java.awt.Container;
import java.awt.FocusTraversalPolicy;
import java.awt.GridLayout;
import java.awt.Window;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;

public class FocusTest extends JFrame {
    JTextField f1 = null;
    JTextField f2 = null;
    JButton b1 = null;
    JButton b2 = null;
    public FocusTest() {
        getContentPane().setLayout(new GridLayout(3,2));
        JLabel label1 = new JLabel("Input 1: ");
        JLabel label2 = new JLabel("Input 2: ");
        f1 = new JTextField(15);
        f2 = new JTextField(20);
        b1 = new JButton("button 1");
        b2 = new JButton("button 2");
     
        add(label1);
        add(f1);
        add(label2);
        add(f2);
        add(b1);
        add(b2);

        b1.addActionListener(new ButtonListener());
        b2.addActionListener(new ButtonListener());
     
        ArrayList<Component> tp = new ArrayList<Component>();
        tp.add(f1);
        tp.add(f2);
        tp.add(b1);
        tp.add(b2);
     
        setFocusTraversalPolicyProvider(true);
        setFocusTraversalPolicy(new MyTraversalPolicy(tp));
     
        pack();
        setVisible(true);
    }
 
    private class MyTraversalPolicy extends FocusTraversalPolicy {
        ArrayList<Component> components = null;
        public MyTraversalPolicy(ArrayList<Component> list){
            this.components = list;
        }
     
        public Component getComponentAfter(Container focusCycleRoot, Component focused){
            int idx = components.indexOf(focused);
            if (idx >= components.size()-1){
                idx = -1;
            }
            return components.get(idx+1);
        }
     
        public Component getComponentBefore(Container c, Component focused){
            int idx = components.indexOf(focused);
            if (idx <= 0){
                idx = components.size();
            }
            return components.get(idx-1);
        }
     
        public Component getDefaultComponent(Container c){
            return f2;
        }
     
        public Component getFirstComponent(Container c){
            return components.get(0);
        }
     
        //Set the field of focus 
        //when the window is first activated/contructed/visible
        public Component getInitialComponent(Window w){
            return f2;
        }
     
        public Component getLastComponent(Container c){
            return components.get(components.size()-1);
        }
    }
 
    private class ButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent e){
            if (e.getSource() == b1){
                f1.setText("button 1 is pressed");
                f2.setText("");
            } else if (e.getSource() == b2){
                f2.setText("button 2 is pressed");
                f1.setText("");
            }
        }
    }
 
    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new FocusTest();
            }
        }
        );
    }
}

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Reference:

1. Set focus to a field in java swing when a window/JFrame is first activated/constructed (1): requestFocusInWindow

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