public class MyObject {
 synchronized public void methodA() {
  //do something....
 }
 synchronized public void methodB() {
  //do some other thing
 }
}
public class ThreadA extends Thread {
 private MyObject object;
 //省略构造方法
 @Override
  public void run() {
  super.run();
  object.methodA();
 }
}
public class ThreadB extends Thread {
 private MyObject object;
 //省略构造方法
 @Override
  public void run() {
  super.run();
  object.methodB();
 }
}
public class Run {
 public static void main(String[] args) {
  MyObject object = new MyObject();
  //线程A与线程B 持有的是同一个对象:object
  ThreadA a = new ThreadA(object);
  ThreadB b = new ThreadB(object);
  a.start();
  b.start();
 }
}

import java.util.ArrayList;
import java.util.List;
public class MyList {
 private List<String> list = new ArrayList<String>();
 public void add() {
  list.add("elements");
 }
 public int size() {
  return list.size();
 }
}
import mylist.MyList;
public class ThreadA extends Thread {
 private MyList list;
 public ThreadA(MyList list) {
  super();
  this.list = list;
 }
 @Override
  public void run() {
  try {
   for (int i = 0; i < 10; i++) {
    list.add();
    System.out.println("添加了" + (i + 1) + "个元素");
    Thread.sleep(1000);
   }
  }
  catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}
import mylist.MyList;
public class ThreadB extends Thread {
 private MyList list;
 public ThreadB(MyList list) {
  super();
  this.list = list;
 }
 @Override
  public void run() {
  try {
   while (true) {
    if (list.size() == 5) {
     System.out.println("==5, 线程b准备退出了");
     throw new InterruptedException();
    }
   }
  }
  catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}
import mylist.MyList;
import extthread.ThreadA;
import extthread.ThreadB;
public class Test {
 public static void main(String[] args) {
  MyList service = new MyList();
  ThreadA a = new ThreadA(service);
  a.setName("A");
  a.start();
  ThreadB b = new ThreadB(service);
  b.setName("B");
  b.start();
 }
}

import java.util.ArrayList;
import java.util.List;
public class MyList {
 private static List<String> list = new ArrayList<String>();
 public static void add() {
  list.add("anyString");
 }
 public static int size() {
  return list.size();
 }
}
public class ThreadA extends Thread {
 private Object lock;
 public ThreadA(Object lock) {
  super();
  this.lock = lock;
 }
 @Override
  public void run() {
  try {
   synchronized (lock) {
    if (MyList.size() != 5) {
     System.out.println("wait begin "
         + System.currentTimeMillis());
     lock.wait();
     System.out.println("wait end "
         + System.currentTimeMillis());
    }
   }
  }
  catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}
public class ThreadB extends Thread {
 private Object lock;
 public ThreadB(Object lock) {
  super();
  this.lock = lock;
 }
 @Override
  public void run() {
  try {
   synchronized (lock) {
    for (int i = 0; i < 10; i++) {
     MyList.add();
     if (MyList.size() == 5) {
      lock.notify();
      System.out.println("已经发出了通知");
     }
     System.out.println("添加了" + (i + 1) + "个元素!");
     Thread.sleep(1000);
    }
   }
  }
  catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}
public class Run {
 public static void main(String[] args) {
  try {
   Object lock = new Object();
   ThreadA a = new ThreadA(lock);
   a.start();
   Thread.sleep(50);
   ThreadB b = new ThreadB(lock);
   b.start();
  }
  catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}