Question

I am not sure if my problem is platform specific, but I think it is not.
Because my expirience is based on the Windows specific java.awt.Toolkit and the Windows-Clipboard.

The following example class shows the problem i am faced with.
NOTE: Before running the program, make sure you have no Image in you system clipboard.

If there is no Image in the system clipboard the Program put a new screenshot to it.

Then I get the Clipboard data two times!

All 3 Images are equal! – the original Screenshot and every Image I get from the clipboard.
which is ok.

But now running the program a second time.
NOTE: There is the old screenshot in the clipboard!

The Program generates a new screenshot and get the old one from the clipboard two times.

No Images is equal to any! – The first (new screenshot) should be not equal, it is okay

But every next Image I get is not equal.

Q1: If every next Image I get is not equal, why it was equal at the first time?
Q2: The bigger Question: How can I compare a java.awt.Image to get every next Image equal.

I am looking for an easy and fast comparison of two Images or an easy way to figure out that the clipboard doesn’t has changed.

public class Example {

    public static void main( String[] args ) throws Exception {

        final Toolkit   toolkit   = Toolkit.getDefaultToolkit();
        final Clipboard clipboard = toolkit.getSystemClipboard();
        final Image     origImage = new Robot().createScreenCapture( new Rectangle( toolkit.getScreenSize() ) );

        if( !clipboard.isDataFlavorAvailable( DataFlavor.imageFlavor )
            || clipboard.getData( DataFlavor.imageFlavor ) == null ) {
            clipboard.setContents( new ImageSelection( origImage ), null );
        }

        Image clipImage1 = (Image)clipboard.getData( DataFlavor.imageFlavor );
        Image clipImage2 = (Image)clipboard.getData( DataFlavor.imageFlavor );

        System.out.println(origImage.hashCode());
        System.out.println(clipImage1.hashCode());
        System.out.println(clipImage2.hashCode());
        System.out.println(clipImage1.equals( clipImage2 ));


    }

    public static class ImageSelection implements Transferable {
        private Image image;
        public ImageSelection(Image image) {
            this.image = image;
        }
        @Override
        public DataFlavor[] getTransferDataFlavors() {
            return new DataFlavor[]{DataFlavor.imageFlavor};
        }
        @Override
        public boolean isDataFlavorSupported(DataFlavor flavor) {
            return DataFlavor.imageFlavor.equals(flavor);
        }
        @Override
        public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
            if (!DataFlavor.imageFlavor.equals(flavor)) {
                throw new UnsupportedFlavorException(flavor);
            }
            return image;
        }
    }    
}

Answer 1

Q1: The object references were the same in the first case.

Q2: here is a way to check the data is equal in the screen shots:

  //createScreenCapture() returns BufferedImage which is more useful for what you are doing.

  static boolean bufferedImageEquals( BufferedImage b1, BufferedImage b2 ) {
    if ( b1 == b2 ) {return true;} // true if both are null
    if ( b1 == null || b2 == null ) { return false; }
    if ( b1.getWidth() != b2.getWidth() ) { return false; }
    if ( b1.getHeight() != b2.getHeight() ) { return false; }
    for ( int i = 0; i < b1.getWidth(); i++) {
     for ( int j = 0; j < b1.getHeight(); j++ ) {
       if ( b1.getRGB(i,j) != b2.getRGB(i,j) ) { 
           return false;
       }
      }
    }
    return true;
  }