Java addMouseListener 在 Oracle 示例中接受奇怪的参数

Question

以下是使用 Oracle 网站上的 mouseEvent 的示例:

我不希望任何人阅读整个代码。我只对下面代码的特定行感兴趣:

addMouseListener(this);//为什么允许“this”作为参数？

在Oracle网站给出的这行代码中，他们试图为面板添加一个mouseListener。这可以。但是，引用面板引用的 this 被用作 addMouseListener() 方法的参数。 为什么允许这样做？

我检查了addMouseListener API，它只允许MouseListener对象，为什么这里允许面板对象？

addMouseListener API:http://docs.oracle.com/javase/7/docs/api/java/awt/Component.html#addMouseListener(java.awt.event.MouseListener)

import java.awt.GridLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;

import javax.swing.*;

public class MouseEventDemo extends JPanel
        implements MouseListener {
    BlankArea blankArea;
    JTextArea textArea;
    static final String NEWLINE = System.getProperty("line.separator");

    public static void main(String[] args) {
        /* Use an appropriate Look and Feel */
        try {
            //UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
            //UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");
            UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
        } catch (UnsupportedLookAndFeelException ex) {
            ex.printStackTrace();
        } catch (IllegalAccessException ex) {
            ex.printStackTrace();
        } catch (InstantiationException ex) {
            ex.printStackTrace();
        } catch (ClassNotFoundException ex) {
            ex.printStackTrace();
        }
        /* Turn off metal's use of bold fonts */
        UIManager.put("swing.boldMetal", Boolean.FALSE);
        //Schedule a job for the event dispatch thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event dispatch thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("MouseEventDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        JComponent newContentPane = new MouseEventDemo();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public MouseEventDemo() {
        super(new GridLayout(0,1));
        blankArea = new BlankArea(Color.YELLOW);
        add(blankArea);
        textArea = new JTextArea();
        textArea.setEditable(false);
        JScrollPane scrollPane = new JScrollPane(textArea);
        scrollPane.setVerticalScrollBarPolicy(
                JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        scrollPane.setPreferredSize(new Dimension(200, 75));
        add(scrollPane);

        //Register for mouse events on blankArea and the panel.
        blankArea.addMouseListener(this);    
        addMouseListener(this);      //WHY IS "this" ALLOWED AS AN ARGUMENT?
        setPreferredSize(new Dimension(450, 450));
        setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
    }

    void eventOutput(String eventDescription, MouseEvent e) {
        textArea.append(eventDescription + " detected on "
                + e.getComponent().getClass().getName()
                + "." + NEWLINE);
        textArea.setCaretPosition(textArea.getDocument().getLength());
    }

    public void mousePressed(MouseEvent e) {
        eventOutput("Mouse pressed (# of clicks: "
                + e.getClickCount() + ")", e);
    }

    public void mouseReleased(MouseEvent e) {
        eventOutput("Mouse released (# of clicks: "
                + e.getClickCount() + ")", e);
    }

    public void mouseEntered(MouseEvent e) {
        eventOutput("Mouse entered", e);
    }

    public void mouseExited(MouseEvent e) {
        eventOutput("Mouse exited", e);
    }

    public void mouseClicked(MouseEvent e) {
        eventOutput("Mouse clicked (# of clicks: "
                + e.getClickCount() + ")", e);
    }
}

Answer 1

注意类声明:

public class MouseEventDemo extends JPanel
        implements MouseListener {

特别是，请注意 implements 子句。 MouseEventDemo 类的实例是 MouseListener，因此它们是 JPanel.addMouseListener() 的有效参数。