大家帮我看看这个类里为什么会定义接口啊？它起什么作用啊

package org.apache.mina.example.chat.client;import org.apache.mina.core.service.IoHandler;
import org.apache.mina.core.service.IoHandlerAdapter;
import org.apache.mina.core.session.IoSession;
import org.apache.mina.example.chat.ChatCommand;/**
 * {@link IoHandler} implementation of the client side of the simple chat protocol.
 *
 * @author The Apache MINA Project ([email protected])
 */
public class SwingChatClientHandler extends IoHandlerAdapter {    public interface Callback {
        void connected();        void loggedIn();        void loggedOut();        void disconnected();        void messageReceived(String message);        void error(String message);
    }    private final Callback callback;    public SwingChatClientHandler(Callback callback) {
        this.callback = callback;
    }    @Override
    public void sessionOpened(IoSession session) throws Exception {
        callback.connected();
    }    @Override
    public void messageReceived(IoSession session, Object message)
            throws Exception {
        String theMessage = (String) message;
        String[] result = theMessage.split(" ", 3);
        String status = result[1];
        String theCommand = result[0];
        ChatCommand command = ChatCommand.valueOf(theCommand);        if ("OK".equals(status)) {            switch (command.toInt()) {            case ChatCommand.BROADCAST:
                if (result.length == 3) {
                    callback.messageReceived(result[2]);
                }
                break;
            case ChatCommand.LOGIN:
                callback.loggedIn();
                break;            case ChatCommand.QUIT:
                callback.loggedOut();
                break;
            }        } else {
            if (result.length == 3) {
                callback.error(result[2]);
            }
        }
    }    @Override
    public void sessionClosed(IoSession session) throws Exception {
        callback.disconnected();
    }}