Simple Drag and Drop

Introduction

We are going to implement a simple drag and drop function. The program will display a red block on the screen and you may drag and drop it to anywhere inside the window.

Mouse Event

For a simple implementation, we just need to implement two functions. One is known as mousePressed and the other is known as mouseDragged. These two functions are defined in two different interfaces. Hence we need to implement both interfaces. class DragDropTest implements java.awt.event.MouseListener, java.awt.event.MouseMotionListener { } Though we are only interested in two functions, we still need to provide empty implementation for every function defined in MouseListener and MouseMotionListener @Override // MouseListener public void mouseExited(MouseEvent e) {} @Override // MouseListener public void mouseEntered(MouseEvent e) {} @Override // MouseListener public void mouseReleased(MouseEvent e) {} @Override // MouseListener public void mouseClicked(MouseEvent e) {} @Override // MouseMotionListener public void mouseMoved(MouseEvent e) {}

Basic Flow of Drag and Drop

1. Remember the mouse position when the mouse is pressed. This is the starting point.
2. When the mouseDragged event is received, calculate a displacement vector from the starting point to the current mouse position.
3. Add the displacement vector to the position of the object.

The following is the implementation of step 1. All we have to do is just remembering the starting position when mouse is pressed. @Override // MouseListener public void mousePressed(MouseEvent e) { startingPoint.x = e.getX(); startingPoint.y = e.getY(); }

The following is the implementation of step 2 and 3. @Override // MouseMotionListener public void mouseDragged(MouseEvent e) { // compute displacement vector int dx=e.getX()-startingPoint.x; int dy=e.getY()-startingPoint.y; // get position of object int x=redPanel.getX(); int y=redPanel.getY(); // add the displacement vector to the object x += dx; y += dy; redPanel.setLocation(x,y); }

Demonstration

For a full demonstration, compile and run the following source code.
/****************************************************************************** * File : DragDropTest.java * Author : Ivan Macteki @ http://java.macteki.com/ * Description : * A simple test for drag and drop * Tested with : OpenJDK 1.8 * Version : 1.0 * Compile and run : * javac DragDropTest.java * java DragDropTest * Documentation : * http://java.macteki.com/2015/02/simple-drag-and-drop.html ******************************************************************************/ import javax.swing.SwingUtilities; import javax.swing.JFrame; import javax.swing.JPanel; import java.awt.event.MouseEvent; import java.awt.Point; import javax.swing.BorderFactory; class DragDropTest implements java.awt.event.MouseListener, java.awt.event.MouseMotionListener { JPanel redPanel; Point startingPoint = new Point(); void init() { JFrame frame=new JFrame("Drag Drop Test - by Macteki"); JPanel panel=new JPanel(); panel.setPreferredSize(new java.awt.Dimension(400,400)); panel.setBackground(java.awt.Color.CYAN); panel.setBorder(BorderFactory.createLoweredBevelBorder()); frame.setLocation(50,100); frame.add(panel); frame.pack(); panel.setLayout(null); redPanel=new JPanel(); redPanel.setBackground(java.awt.Color.RED); redPanel.setBorder(BorderFactory.createRaisedBevelBorder()); redPanel.setBounds(50,50,50,50); panel.add(redPanel); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); redPanel.addMouseListener(this); redPanel.addMouseMotionListener(this); } @Override // MouseListener public void mouseExited(MouseEvent e) {} @Override // MouseListener public void mouseEntered(MouseEvent e) {} @Override // MouseListener public void mouseReleased(MouseEvent e) {} @Override // MouseListener public void mousePressed(MouseEvent e) { startingPoint.x = e.getX(); startingPoint.y = e.getY(); } @Override // MouseListener public void mouseClicked(MouseEvent e) {} @Override // MouseMotionListener public void mouseMoved(MouseEvent e) {} @Override // MouseMotionListener public void mouseDragged(MouseEvent e) { // compute displacement vector int dx=e.getX()-startingPoint.x; int dy=e.getY()-startingPoint.y; // get position of object int x=redPanel.getX(); int y=redPanel.getY(); // add the displacement vector to the object x += dx; y += dy; redPanel.setLocation(x,y); } public static void main(String[] args) throws Exception { SwingUtilities.invokeLater( ()->new DragDropTest().init() ); } } // end DragDropTest.java