2025 年 Java 面试高频手写代码题（附核心实现）

一、设计模式类

1. ‌单例模式（双重校验锁）‌

   public class Singleton {
       private static volatile Singleton instance;
       private Singleton() {}
       public static Singleton getInstance() {
           if (instance == null) {
               synchronized (Singleton.class) {
                   if (instance == null) instance = new Singleton();
               }
           }
           return instance;
       }
   }
   

   ‌考察点‌：volatile 禁止指令重排、双重检查优化性能、线程安全‌

2. ‌生产者-消费者模式（BlockingQueue 实现）‌

   public class ProducerConsumer {
       private final BlockingQueue<Integer> queue = new LinkedBlockingQueue<>(10);
       public void produce(int data) throws InterruptedException { queue.put(data); }
       public int consume() throws InterruptedException { return queue.take(); }
   }
   

   ‌考察点‌：线程间通信、阻塞队列的使用‌。

二、多线程与并发

1. ‌两个线程交替打印 1-100（synchronized + wait/notify）‌

   public class AlternatePrint {
       private static int num = 1;
       private static final Object lock = new Object();