这两段代码有什么区别？

Question

I cannot understand why one of the following bits of code compiles and the other one doesn't.

The one that doesn't compile (Compiler says the method KeyBidings() needs a return type):

public KeyBidings(){
    Action rightAction = new AbstractAction(){
        public void actionPreformed(ActionEvent e){
            x+=10;
            drawPanel.repaint();
        }
    };
    Action leftAction = new AbstractAction(){
        public void actionPreformed(ActionEvent e){
            x-=10;
            drawPanel.repaint();
        }
    };

        InputMap inputMap = drawPanel.getInputMap(JPanel.WHEN_IN_FOCUSED_WINDOW);
        ActionMap actionMap = drawPanel.getActionMap();

    inputMap.put(KeyStroke.getKeyStroke("RIGHT"), "rightAction");
    actionMap.put("rightAction", rightAction);
    inputMap.put(KeyStroke.getKeyStroke("LEFT"), "leftAction");
    actionMap.put("leftAction", leftAction);

    add(drawPanel);
    pack();
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setSize(640, 480);
    setTitle("Game");
    setLocationRelativeTo(null);
    setVisible(true);
}

And the one that compiles just fine:

public KeyBidings(){
    Action rightAction = new AbstractAction(){
        public void actionPerformed(ActionEvent e) {
            x +=10;
            drawPanel.repaint();
        }
    };

        InputMap inputMap = drawPanel.getInputMap(JPanel.WHEN_IN_FOCUSED_WINDOW);
        ActionMap actionMap = drawPanel.getActionMap();

    inputMap.put(KeyStroke.getKeyStroke("RIGHT"), "rightAction");
    actionMap.put("rightAction", rightAction);

    add(drawPanel);

    pack();
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setLocationRelativeTo(null);
    setVisible(true);
}

EDIT: I didn't know the difference between a constructor and a method, but now i have another problem: https://gyazo.com/cd3c21a8562589451814903febaf89fe

What's the problem here? I've included the source codes for both classes below.

Source Code 1: http://pastebin.com/vwNtJZEG Source Code 2: http://pastebin.com/nL4SbtkM

Answer 1

The second one is the constructor of a class named KeyBidings, whereas the first one is a method, with a missing return type, of some other class.

Read the tutorial about constructors .

Note that the compiler doesn't say that the method may not be public, as your title says. It says that it must have a return type. That's quite different.