The snippet below assumes that there exist the following variables (coloured green in the code):

• an input stream to read from
• a desired width variable (specifying the height as -1 tells the toolkit to preserve the original aspect ratio)
• and an output stream to write the encoded image to.

Sample code:

import java.awt.Image;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;

// ...

BufferedImage sourceImage = ImageIO.read(inputStream);
Image thumbnail = sourceImage.getScaledInstance(width, -1, Image.SCALE_SMOOTH);
BufferedImage bufferedThumbnail = new BufferedImage(thumbnail.getWidth(null),
                                                    thumbnail.getHeight(null),
                                                    BufferedImage.TYPE_INT_RGB);
bufferedThumbnail.getGraphics().drawImage(thumbnail, 0, 0, null);
ImageIO.write(bufferedThumbnail, "jpeg", outputStream);