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I'm studying at the moment and I've been given the simple (I thought so) task. I have to make this piece of code to work (I can't modify it):

 JButton b = new JButton("Myszą ciśnij"); b.addMouseListener ( (MousePressListener) e -> System.out.println("ok"));

I believe MousePressListener should be FunctionalInterface but then, it can't extend MouseListener. Is there any way to work around this issue, or am I approaching it from bad side?

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    See stackoverflow.com/questions/25299653/… and stackoverflow.com/questions/21833537/… Commented Oct 27, 2015 at 13:52
  • Why not make a random class Foo that is not a functional interface, make an interface Bar that extends Foo that has exactly one abstract method and add the @FunctionalInterface annotation to Bar? If Bar is not a functional interface in Java's eyes it will not compile. Commented Oct 27, 2015 at 13:59

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addMouseListener expects a MouseListener. MouseListener has multiple abstract methods. Therefore no lambda expression can be of type MouseListener.

I don't know what MousePressListener, but if it's an interface that extends MouseListener, it will still inherit multiple abstract methods from MouseListener, so it can't be a functional interface.

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Actually, it can. Managed to do that by making "mouseClicked" abstract, and "mouseEntered", "mouseExited", "mousePressed" and "mouseReleased" default. Thank you for help, though :)

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