Swing Hack: Window Snapping

While working on another project I came up with a silly idea. How could I force windows to remain completely on screen and to snap to the screen edges? A simple form of window snapping. Since you can receive an event every time the window is moved it's easy to create a Component Listener to do it.

import java.awt.*;
import java.awt.event.*;

public class WindowSnapper extends ComponentAdapter {

    public WindowSnapper(int snap_distance) {
        this.sd = snap_distance;

    private boolean locked = false;
    private int sd = 50;

    public void componentMoved(ComponentEvent evt) {
        if(locked) return;
        Dimension size = Toolkit.getDefaultToolkit().getScreenSize();
        int nx = evt.getComponent().getX();
        int ny = evt.getComponent().getY();
        if(nx < 0) {
            nx = 0;
        if(ny < 0) {
            ny = 0;
        if(nx < 0+sd) {
            nx = 0;
        if(nx > size.getWidth()-evt.getComponent().getWidth()-sd) {
            nx = (int)size.getWidth()-evt.getComponent().getWidth();

        if(ny > size.getHeight()-evt.getComponent().getHeight()-sd) {
            ny = (int)size.getHeight()-evt.getComponent().getHeight();
// make sure we don't get into a recursive loop when the
// set location generates more events
        locked = true;
        locked = false;

The code is nicely encapsulated as a listener. You can simply add it to whatever window you want to snap and the listener takes care of the rest. ex:

my_window.addComponentListener(new WindowSnapper(50));

Unfortunately, since you cannot intercept the move event, only listen to it read only, you will end up with a somewhat ugly flashing effect as the window flips between where the cursor wants it to be and where the snapping wants it to be.

Talk to me about it on Twitter

Posted August 22nd, 2003

Tagged: java swing-hacks java.net