Java：使用接口从父类调用子类方法

Question

I don't know if this question is valid or I am doing something wrong with defining the parent class structure.

But the following are the formation of the classes and interface are.

public interface Test {
    public void print();
    public void write();
}

class Parent implements Test{
    @Override
        public void print() {
            System.out.print("This is parent class");
    }

    @Override
        public void write() {
            System.out.print("This is write method in parent class");
    }
 }

class Child extends Parent{
    @Override
    public void print(){
        System.out.print("This is child class);
    }
}

Expected output when I call method using the interface

Test test = new Parent();
test.print();

It should call print method from the Child class.

And when I call method using interface

Test test = new Parent();
test.write();

It should call write method from the Parent class.

So right now it's not happening, in both the cases it's calling the method from the Parent class.

So any suggestion or answer are much appreciated.

Answer 1

By using:

Test test = new Parent();
test.write();

your test is of type Parent and is unaware of Child . Hence your output indicates both methods on the Parent class are called.

Try:

Test test = new Child();
test.print();   // Will call Child::print()
test.write();   // Will call Parent::write()

and you should achieve what you want.

NB For this to work, you must add write() to your Test interface, thus:

public interface Test {
    public void print();
    public void write(); // This is required for it to be accessible via the interface
}

Answer 2

You may need to cast it to the Child class. Maybe put a check before it like this:

if (test instanceof Child) {
    ((Child) test).print();
}

Answer 3

The output makes sense because you are creating (instanciating) an objet of type Parent .

Naturally, your write() and print() will follow the Parent implementation and therefore display :

This is write method in parent class

and

This is parent class

You have to create Child instance to use in same fashion it's write() and print() implementations :

Test test = new Child();
test.write();
test.print();

this piece of code will display what you expected. Since write() has no Child implementation it would display the Parent message