Java-您可以从超类继承并拥有一个静态构造函数来引用基类而不是父类吗？

Question

Take a look at the following code

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

class Child extends Parent{
    public Child(){
        System.out.println("Child initialized.");
    }
}

If I wanted the code to create a static instance in each subclass as well, would I have to copy over the code for getInstance , or does this work?

Answer 1

If I'm not misunderstanding, you want to create a singleton for each subclass and you do not want to repeat the code. If you pass the class of the child to getInstance as parameter that can be possible:

class Parent{
    public static HashMap<Class<? extends Parent>, Parent> instance 
                  = new HashMap<Class<? extends Parent>, Parent>();
    protected Parent(){}
    public static <T> T getInstance(Class<? extends Parent> cls) 
                    throws InstantiationException, IllegalAccessException{
        if (instance.get(cls) == null) {
            instance.put(cls, cls.newInstance());
        }
        return (T)instance.get(cls);
    }
}

class Child extends Parent{
    protected Child(){

    }
}

Child child = Child.getInstance(Child.class);