单例模式 饿汉式

静态变量

class Singleton {
    private static final Singleton instance = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return instance;
    }
}

静态代码块

class Singleton {
    private Singleton() {
    }

    private static final Singleton instance;

    static {
        instance = new Singleton();
    }

    public static Singleton getInstance() {
        return instance;
    }
}