What text input component last had the focus?

Suppose I have a Java application that has more than one component in which you can enter text. Now suppose this application also has a dialog that lets you insert a single character (like the dialog in Word that comes up when you select Insert from the Edit menu) into those components. You want it to insert the character into whichever text component last had the focus.

But how do you know which text component last had the focus?

I could keep track of this manually, by having each text component report to the application whenever it gets the focus and then have the application insert the new character into whichever component that last had the focus.

But this must be a common problem (consider Paste buttons in tool bars-开发者_运维问答--how does it know where to paste it into?). Is there something already built in to Swing that lets you get a handle to the last text component that had the focus? Or do I need to write this myself?

Is there something already built in to Swing that lets you get a handle to the last text component that had the focus?

You create an Action that extends TextAction. The TextAction class has a method that allows you to obtain the last text component that had focus.

Edit:

You can create your own Action and do whatever you want. The Action can then be added to any JMenuItem or JButton. For example:

class SelectAll extends TextAction
{
    public SelectAll()
    {
        super("Select All");
    }

    public void actionPerformed(ActionEvent e)
    {
        JTextComponent component = getFocusedComponent();
        component.selectAll();
    }
}

If you just want to insert a character at the caret position of the text field then you can probably just do

component.replaceSelection(...);

Edit 2:

I don't understand what the confusion is with this answer. Here is a simple example:

1. select some text
2. use the mouse to click on the check box
3. tab or use the mouse to click on the "Cut" button

It doesn't matter that the text field doesn't currently have focus when the Action is invoked. The TextAction tracks the last text component that had focus.

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

public class TextActionTest extends JFrame
{
    JTextField textField = new JTextField("Select Me");
    JTabbedPane tabbedPane;

    public TextActionTest()
    {
        add(textField, BorderLayout.NORTH);
        add(new JCheckBox("Click Me!"));
        add(new JButton(new CutAction()), BorderLayout.SOUTH);
    }

    public static void main(String[] args)
    {
        TextActionTest frame = new TextActionTest();
        frame.setDefaultCloseOperation( EXIT_ON_CLOSE );
        frame.pack();
        frame.setLocationRelativeTo( null );
        frame.setVisible(true);
     }

    class CutAction extends TextAction
    {
        public CutAction()
        {
            super("Click to Cut Text");
        }

        public void actionPerformed(ActionEvent e)
        {
            JTextComponent component = getFocusedComponent();
//          JTextComponent component = getTextComponent(e);
            component.cut();
        }
    }
}

Just like suggested by @lesmana (+1 for that). Here you have an example that shows that on focusLost the focus listener returns the previously focused component.

import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Focusing
{
    public static void main(String[] args)
    {
        JPanel p = new JPanel();
        JTextField tf1 = new JTextField(6);
        tf1.setName("tf1");
        p.add(tf1);
        JTextField tf2 = new JTextField(6);
        tf2.setName("tf2");
        p.add(tf2);
        FocusListener fl = new FocusListener()
        {
            @Override
            public void focusGained(FocusEvent e)
            {
                System.out.println("focusGained e.getSource().c=" + ((JComponent) e.getSource()).getName());
            }

            @Override
            public void focusLost(FocusEvent e)
            {
                System.out.println("focusLost e.getSource().c=" + ((JComponent) e.getSource()).getName());
            }
        };
        tf1.addFocusListener(fl);
        tf2.addFocusListener(fl);
        JPanel contentPane = new JPanel();
        contentPane.add(p);
        JFrame f = new JFrame();
        f.setContentPane(contentPane);
        f.setSize(800, 600);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
}

All the best, Boro.

I've never done this directly, but you could look into the FocusEvents and the Focus Subsystem.

Hopefully there is something in the Focus Subsystem that would fire events that you could listen for.

You can register a FocusListener to every text component. The FocusEvent object has a reference to the last component which had focus.