import java.awt.AWTEvent;
import java.awt.Container;
import java.awt.EventQueue;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class EventQueuePanel extends JPanel implements ActionListener {
  EventQueuePanel() {
    JButton button = new JButton("Draw line");
    add(button);
    button.addActionListener(this);
  }

  public void actionPerformed(ActionEvent evt) {
    Graphics g = getGraphics();

    displayPrompt(g, "Click to chooose the first point");
    Point p = getClick();
    g.drawOval(p.x - 2, p.y - 2, 4, 4);
    displayPrompt(g, "Click to choose the second point");
    Point q = getClick();
    g.drawOval(q.x - 2, q.y - 2, 4, 4);
    g.drawLine(p.x, p.y, q.x, q.y);
    displayPrompt(g, "Done! Press button the start again.");
    g.dispose();
  }

  public void displayPrompt(Graphics g, String s) {
    y += 20;
    g.drawString(s, 0, y);
  }

  public Point getClick() {
    EventQueue eq = Toolkit.getDefaultToolkit().getSystemEventQueue();
    while (true) {
      try {
        AWTEvent evt = eq.getNextEvent();
        if (evt.getID() == MouseEvent.MOUSE_PRESSED) {
          MouseEvent mevt = (MouseEvent) evt;
          Point p = mevt.getPoint();
          Point top = getRootPane().getLocation();
          p.x -= top.x;
          p.y -= top.y;
          return p;
        }
      } catch (InterruptedException e) {
      }
    }
  }

  private int y = 60;
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setTitle("EventQueueTest");
    frame.setSize(300, 200);
    frame.addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        System.exit(0);
      }
    });

    Container contentPane = frame.getContentPane();
    contentPane.add(new EventQueuePanel());

    frame.show();
  }

}

------解决方案--------------------
Queue data structure  
Java code
public class Queue { 
 private int maxSize; 
  
 private long[] queArray; 
  
 private int front; 
  
 private int rear; 
  
 private int nItems; 
  
 public Queue(int s) { 
   maxSize = s; 
   queArray = new long[maxSize]; 
   front = 0; 
   rear = -1; 
   nItems = 0; 
 } 
  
 //   put item at end of a queue 
 public void insert(long j) { 
   if (rear == maxSize - 1) // deal with wraparound 
   rear = -1; 
   queArray[++rear] = j; // increment rear and insert 
   nItems++;  
 } 
  
 //   take item from front of queue 
 public long remove() { 
   long temp = queArray[front++]; // get value and incr front 
   if (front == maxSize) // deal with wraparound 
   front = 0; 
   nItems--; // one less item 
   return temp; 
 } 
  
 public long peekFront() { 
   return queArray[front]; 
 } 
  
 public boolean isEmpty() { 
   return (nItems == 0); 
 }