import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.Iterator;

import javax.imageio.ImageIO;
import javax.imageio.ImageWriter;
import javax.imageio.stream.ImageOutputStream;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Bilddatei_konvertieren {

	public static void main(String[] args) {

		String path = "/Users/joecze/Desktop/test/";
		String srcName = "001";
		String srcExt = "jpg";
		String destName = "bild";
		String destExt = "png";

		BufferedImage img = null;

		File source = new File(path + srcName + "." + srcExt);
		if (source.canRead() && source.isFile()) {
			try {
				img = ImageIO.read(source);
			} catch (IOException e1) {
				e1.printStackTrace();
			}
		}

		Iterator<ImageWriter> writers = ImageIO
				.getImageWritersByFormatName(destExt);
		ImageWriter writer = (ImageWriter) writers.next();

		File f = new File(path + destName + "." + destExt);
		ImageOutputStream ios;
		try {
			ios = ImageIO.createImageOutputStream(f);
			writer.setOutput(ios);
			writer.write((BufferedImage) img);
			ios.close();
		} catch (IOException e) {
		  e.printStackTrace();
		}

		ImageIcon bild = new ImageIcon(img);
		JLabel label = new JLabel(bild);
		label.setIcon(bild);

		JFrame frame = new JFrame();
		frame.add(label);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.pack();
		frame.setVisible(true);

	}
}