Show tooltip when JComboBox key is pressed

 Code that shows tooltip when key occurs by key listener in JComboBox

package test.swing;

import javax.swing.*;
import java.awt.*;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.MouseEvent;

// key event tooltip
public class MainTest {
    public static void main(String[] args) {
        MainUI mainUI = new MainUI();
        mainUI.setVisible(true);
    }
}

class TestComboBox extends JComboBox {
    JTextField mTf = null;
    public TestComboBox() {
        super();
        mTf = (JTextField) editor.getEditorComponent();
        mTf.setToolTipText("TextField : ");
        mTf.addKeyListener(new KeyHandler());
    }

    class KeyHandler extends KeyAdapter {
        @Override
        public void keyReleased(KeyEvent e) {
            // set tooltip
            mTf.setToolTipText("TextField : " + mTf.getText());
            // show tooltip
            ToolTipManager.sharedInstance().mouseMoved(new MouseEvent(mTf, 0, 0, 0,0, 0, 0, false));
            super.keyReleased(e);
        }
    }
}

class MainUI extends JFrame {
    MainUI() {
        setPreferredSize(new Dimension(400, 300));
        setLayout(new FlowLayout());
        TestComboBox combo = new TestComboBox();
        combo.setToolTipText("ComboBox");
        combo.setEditable(true);
        add(combo);
        pack();
    }
}

java swing dynamic tooltip, multi-line tooltip

Implemented with getToolTipText method override

https://docs.oracle.com/javase/8/docs/api/javax/swing/JComponent.html#getToolTipText-java.awt.event.MouseEvent-

package test.swing;

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

// dynamic tooltip
public class MainTest {
    public static void main(String[] args) {
        MainUI mainUI = new MainUI();
        mainUI.setVisible(true);
    }
}

class ToolTipButton extends JButton {
    boolean mIsNum = true;

    public ToolTipButton(String text) {
        super(text);
    }

    // 1. use setToolTipText
    @Override
    public String getToolTipText(MouseEvent event) {
        if (mIsNum) {
            setToolTipText("0123456789");
        }
        else {
            setToolTipText("ABCDEFG");
        }
        mIsNum = !mIsNum;
        return super.getToolTipText(event);
    }

    // 2. return text directly
    @Override
    public String getToolTipText(MouseEvent event) {
        String text = "";
        if (mIsNum) {
            text = "0123456789";
        }
        else {
            text = "ABCDEFG";
        }
        mIsNum = !mIsNum;
        return text;
    }
}

class MainUI extends JFrame {

    MainUI() {
        setPreferredSize(new Dimension(400, 300));
        setLayout(new FlowLayout());
        JButton button = new ToolTipButton("TEST");
        button.setToolTipText("");
        button.setPreferredSize(new Dimension(100, 100));
        add(button);
        pack();
    }
}

Multi-line tooltip

setToolTipText("<html>a<br>b</html>");