由于某种原因，我的java类不会继承公共void。 有人可以帮我吗？

Question

I'm pretty much new to Java development and I'm trying to build a simple android game, for this, I need to inherit some methods from other classes. Only, for some reason it won't. Am I missing something here?

Here is code from the public Java class "Addition"

public class Addition {
    Random random = new Random();

    int Answer;
    int PartOne;
    int PartTwo;
    int FalseAnswer;

    //CALL IN THIS FIRST!

    public void Calculate() {
        Answer = random.nextInt(999) + 2;
        PartTwo = random.nextInt(Answer--) + 1;
        PartOne = Answer - PartTwo;
        FalseAnswer = random.nextInt(Math.round(Answer * 1.2f)) + Math.round(Answer * 0.8f);
    }

//some more stuff.

Here is the class subtraction, that I want to base upon Addition. Here is the code that I have there:

    public class Subtraction{
    Addition addition = new Addition();
    addition.Calculate();
}

Don't mind the formatting, it is just to demonstrate. It is giving me an error, saying Calculate does not exist or cannot be found (neither can the other methods in the class).

I have also already tried inherting directly:

public class Subtraction extends Addition{
    Calculate();
}

Didn't work either. The only thing it let's me do is this:

public class Subtraction extends Addition{
    @Override
    public void Calculate() {
        super.Calculate();
    }
}

Which is not what I want. I'm sure I'm missing something obvious, but can someone please point it out? Thank you in advance.

Answer 1

public class Subtraction extends Addition{

}

is all you need. Don't write Calculate(); in that class, you've already inherited it.

Answer 2

You get the error because you can't call methods like that outside of any class method. That' why this would work

public class Subtraction extends Addition{
    @Override
    public void Calculate() {
        super.Calculate(); // calls the 'calculate' method of the 'Addition' class
    }
}

& this won't:

public class Subtraction extends Addition{
    Calculate(); // Outside of any method, invalid call
}

Answer 3

If you aren't trying to inherit from the Addition class using:

public class Subtraction extends Addition
{
    @Override
    public void Calculate()
    {
        super.Calculate(); // calls the 'calculate' method of the 'Addition' class
    }
}

Then you need to put

Addition addition = new Addition();
addition.Calculate();

into a method that can be called. Do it like this:

public class Subtraction
{
    public void Calculate()
    {
        Addition addition = new Addition();
        addition.Calculate();
    }
}

You have to do it this way because the method calls have to be in an executable context (in a method or constructor).