Trying to understand inheritance in Java

Question

I feel I have a decent understanding of inheritance in Java but am struggling with the following situation:

public class SuperClass {

    private String name;
    private int age;

    public SuperClass(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void sharedMethod() {
        System.out.println("I'm a shared method in the SuperClass");
    }

}


public class SubClass extends SuperClass {

    public SubClass(String name, int age) {
        super(name, age);
    }

    public void sharedMethod() {
        System.out.println("I'm a shared method in the SubClass");
    }

    public void subMethod() {
        System.out.println("I'm subMethod");
    }

}


public class Main {

    public static void main(String args[]) {

        SuperClass test = new SubClass("Laura", 30);

        test.sharedMethod();

        test.subMethod(); // causes an error
    }
}

I am primarily struggling to understand what is actually being created here when using SuperClass test = new SubClass("Laura", 30); (which only seems to work when SubClass extends SuperClass - how's that possible without Casting?).

The reason I don't understand it is because I can call the sharedMethod which prints out the text from the SubClass method (so presumably SubClass is overriding the SuperClass method in this instance), but yet I can't call the subMethod from the SubClass at all. Sorry if I didn't articulate it very well, I just can't seem to wrap my head around what is being referenced, what methods can be used, why you would do this, why the SubClass overrides a method if the method is shared, but can't be called otherwise.

Answer 1

By SuperClass test = new SubClass("Laura", 30), you create a SubClass instance, but reference it by SuperClass.

At compile stage, the compiler found that SuperClass does not have method subMethod, so you get compile error.

At runtime, JVM found that test is actually a SubClass, so the sharedMethod from SubClass:

public void sharedMethod() {
    System.out.println("I'm a shared method in the SubClass");
}

will be executed.