除了在定义的"工厂类"中,如何防止对象被实例化？

public class MyClass {
    private MyClass() {
        // 构造函数为私有
    }
}

public enum MyEnum {
    INSTANCE;

    private final MyClass myClass;

    MyEnum() {
        myClass = new MyClass();
    }

    public MyClass getInstance() {
        return myClass;
    }
}

public class MyFactory {
    private MyFactory() {
        // 构造函数为私有
    }

    public static class MyClass {
        // 内部类
    }
}

public class MyClass {
    private MyClass() {
        // 构造函数为私有
    }

    public static class Holder {
        public static final MyClass INSTANCE = new MyClass();
    }
}

public interface MyInterface {
    // 接口定义
}

public class MyClass implements MyInterface {
    private MyClass() {
        // 构造函数为私有
    }
}