Modifying value of superclass through subclass, and calling the value of the superclass in another subclass

Question

I'm trying to modify a value in a superclass from a subclass, and call that value from another subclass. For some reason when I do the value becomes null, or resets. I was working on a larger scale project, but ran into this problem and figured it'd be easier for everyone if I tested it in a smaller program. Here's what I've got:

public class Superclass { private int x; protected void setX(int n) { this.x = n; System.out.println(this.x); } protected int getX() { return this.x; } } public class Subclass extends Superclass { public void begin() { setX(55); Subclass2 subclass2 = new Subclass2(); subclass2.begin(); } } public class Subclass2 extends Superclass { public void begin() { System.out.println(getX()); } } public class Main { public static void main(String[] args) { Subclass subclass = new Subclass(); subclass.begin(); } }

The superclass value successfully sets to 55, but when I call it from Subclass 2 it returns 0.

Thanks

Hanchen Jiang · Accepted Answer · 2021-12-26 03:13:19Z

You are instantiating a new object subclass2 here

public void begin() { setX(55); Subclass2 subclass2 = new Subclass2(); subclass2.begin(); }

The x in subclass2 is not the same as the x in subclass. And since x is not initialized in Subclass2, a primitive int by default has a value of 0.

The value of x is 0 when you instantiate either Subclass or Subclass2. The x in subclass became 55 because you called begin() on subclass, which setX(55). During begin(), a totally different object subclass2 was created. subclass2's x is never touched. Therefore, calling begin() on subclass2 prints 0.

But doesn't the X in the superclass stay the same, no matter what I instantiate?
No it doesn't stay the same. A value in a superclass is not a static value. It gets instantiated new every time you call its or one of its children's constructors.
Ah, ok thank you. How would I go about saving the Integer so I could call it then?
Are you asking how you can have a value on the class so it is shared by all instances of the class? If that's the case, what you want is a static variable. Like private static int x; There are plenty of resources to learn about those. Just Google something like "java static variable".

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