下面是sun的教程中的一段applet,再instanceof那一句中。使用了Receiver。但是这个applet中从来没有声明过这个类啊，编译器又怎么知道Reciver到底是什么呢？Receiver在另一个文件中声明的。


/* 
 * 1.1 version.
 */

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Enumeration;

public class Sender extends Applet 
                    implements ActionListener {
    private String myName;
    private TextField nameField;
    private TextArea status;
    private String newline;

    public void init() {
        GridBagLayout gridBag = new GridBagLayout();
        GridBagConstraints c = new GridBagConstraints();

        setLayout(gridBag);

        Label receiverLabel = new Label("Receiver name:", 
                                        Label.RIGHT);
        gridBag.setConstraints(receiverLabel, c);
        add(receiverLabel);

        nameField = new TextField(getParameter("RECEIVERNAME"),
                                               10);
        c.fill = GridBagConstraints.HORIZONTAL;
        gridBag.setConstraints(nameField, c);
        add(nameField);
        nameField.addActionListener(this);

        Button button = new Button("Send message");
        c.gridwidth = GridBagConstraints.REMAINDER; //end row
        c.anchor = GridBagConstraints.WEST; //stick to the 
                                            //text field
        c.fill = GridBagConstraints.NONE; //keep the button 
                                          //small
        gridBag.setConstraints(button, c);
        add(button);
        button.addActionListener(this);

        status = new TextArea(5, 60);
        status.setEditable(false);
        c.anchor = GridBagConstraints.CENTER; //reset to the default
        c.fill = GridBagConstraints.BOTH; //make this big
        c.weightx = 1.0;
        c.weighty = 1.0;
        gridBag.setConstraints(status, c);
        add(status);

        myName = getParameter("NAME");
        Label senderLabel = new Label("(My name is " + myName + ".)",
                                      Label.CENTER);
        c.weightx = 0.0;
        c.weighty = 0.0;
        gridBag.setConstraints(senderLabel, c);
        add(senderLabel);

newline = System.getProperty("line.separator");
    }

    public void actionPerformed(ActionEvent event) {
        Applet receiver = null;
        String receiverName = nameField.getText(); //Get name to 
                                                   //search for.
        receiver = getAppletContext().getApplet(receiverName);
        if (receiver != null) {
            //Use the instanceof operator to make sure the applet
            //we found is a Receiver object.
            if (!(receiver instanceof Receiver)) {  //问题就在这里
                status.append("Found applet named "
                              + receiverName + ", "
                              + "but it's not a Receiver object."
      + newline);
            } else {
                status.append("Found applet named "
                              + receiverName + newline
                              + "  Sending message to it."
      + newline);
                //Cast the receiver to be a Receiver object
                //(instead of just an Applet object) so that the
                //compiler will let us call a Receiver method.
                ((Receiver)receiver).processRequestFrom(myName);
            }
        } else {
            status.append("Couldn't find any applet named "
                          + receiverName + "." + newline);
        }
    }

    public Insets getInsets() {
        return new Insets(3,3,3,3);
    }

    public void paint(Graphics g) {
        g.drawRect(0, 0, 
                   getSize().width - 1, getSize().height - 1);
    }

    public String getAppletInfo() {
        return "Sender by Kathy Walrath";
    }
}

只要你有Receiver.class并且在classpath设置过的目录里，编译器当然就可以找到啦。

因为它们都在同一个包(包为空)里,同一个包的内容是不用引入的