简单单例模式

public class  {

    private static Apple apple = null;

    private (){}

    
    public static synchronized Apple getApple(){
        if(apple == null) {
            apple = new Apple(); //需要时再创建Apple实例，需要加synchronized，防止并发操作时多个线程同时创建多个实例
        }
        return apple;
    }
}

public class  {

    private static Apple apple = new Apple(); //提前创建Apple实例

    private (){}

    public static Apple getApple(){ //不需要加synchronized，也能应对并发操作
        return apple;
    } 
}