类存在时关闭模式，与其他类保持打开状态

public class Singleton {
    private static Singleton instance;

    // 私有构造函数，防止外部实例化
    private Singleton() {}

    // 提供一个静态方法获取实例
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    // 提供公共方法供其他类使用
    public void doSomething() {
        System.out.println("Doing something...");
    }
}