public interface CallBack {
	/*
	 * 响应回调函数
	 */
	public void slove();
}

public class A implements CallBack {
	B b = new B();
	@Override
	/*
	 * (non-Javadoc)
	 * @see CallBack#slove()
	 * 响应回调函数
	 */
	public void slove() {
		System.out.println("the problem is solve!");
	}
	/*
	 * 登记回调函数
	 */
	public void askQuestion(){
		System.out.println("ask b solve the problem!");
		/*
		 * 自己去做其他事
		 */
		new Thread(new Runnable() {
			
			@Override
			public void run() {
				System.out.println("A want to do another thing!");
			}
		}).start();
		/*
		 * ask b to solve this problem
		 */
		this.b.call(this);
	}
	/*
	 * test
	 */
	 public static void main(String[] args)  {
		A a = new A();
		a.askQuestion();
	}
}

public class B {
	/*
	 * 回调函数
	 */
	public void call(CallBack a){
		/*
		 * b help a solve the priblem
		 */
		System.out.println("b help a solve the problem!");
		/*
		 * call back
		 */
		a.slove();
		
	}
}

测试结果