repaint just the affected part of the component

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.Rectangle;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;

import javax.swing.JComponent;
import javax.swing.JFrame;

public class ClippedDragImage extends DragImage {
  int oldX, oldY;

  public ClippedDragImage(Image i) {
    super(i);
  }

  public void mouseDragged(MouseEvent e) {
    imageX = e.getX();
    imageY = e.getY();
    Rectangle r = getAffectedArea(oldX, oldY, imageX, imageY, imageWidth, imageHeight);
    repaint(r); // repaint just the affected part of the component
    oldX = imageX;
    oldY = imageY;
  }

  private Rectangle getAffectedArea(int oldx, int oldy, int newx, int newy, int width, int height) {
    int x = Math.min(oldx, newx);
    int y = Math.min(oldy, newy);
    int w = (Math.max(oldx, newx) + width) - x;
    int h = (Math.max(oldy, newy) + height) - y;
    return new Rectangle(x, y, w, h);
  }

  public static void main(String[] args) {
    String imageFile = "A.jpg";
    Image image = Toolkit.getDefaultToolkit().getImage(
        ClippedDragImage.class.getResource(imageFile));
    image = image.getScaledInstance(imageWidth, imageHeight, Image.SCALE_DEFAULT);
    JFrame frame = new JFrame("ClippedDragImage");
    frame.getContentPane().add(new ClippedDragImage(image));
    frame.setSize(300, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
  }
}

class DragImage extends JComponent implements MouseMotionListener {
  static int imageWidth = 60, imageHeight = 60;

  int grid = 10;

  int imageX, imageY;

  Image image;

  public DragImage(Image i) {
    image = i;
    addMouseMotionListener(this);
  }

  public void mouseDragged(MouseEvent e) {
    imageX = e.getX();
    imageY = e.getY();
    repaint();
  }

  public void mouseMoved(MouseEvent e) {
  }

  public void paint(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;

    g2.drawImage(image, imageX, imageY, this);
  }

}

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repaint just the affected part of the component : Buffer Paint « 2D Graphics GUI « Java

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