public class MyObject {
  synchronized public void methodA() {
    //do something....
  }

public class MyObject {

  public void methodA() {
    synchronized(this){
      //do something....
    }
  }
 

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();
  }
}

public class MyObject {

  private String userName = "b";
  private String passWord = "bb";
  
  synchronized public void methodA(String userName, String passWord) {
    this.userName = userName;
    try{
      Thread.sleep(5000);
    }catch(InterruptedException e){
      
    }
    this.passWord = passWord;
  }

  synchronized public void methodB() {
    System.out.println("userName" + userName + ": " + "passWord" + passWord);
  }
}