单例模式

要点：使用静态内部类初始化.

public class Singleton {
    public static class SingletonInstanceClass {
        private static final Singleton instance = new Singleton();
    }
    public static Singleton getInstance() {
        return SingletonInstanceClass.instance;
    }
    private Singleton() {
        
    }
}
//静态内部类只有在调用getInstance方法的时候执行一次