How to call the superclass implementation of an overridden method from inside the superclass in Java?

Question

Suppose I have two classes SuperClass and SubClass which extends SuperClass, such as below:

public abstract class SuperClass {
    private Object field1;

    protected SuperClass(Object obj) {
        setField1(obj);
    }

    public void setField1(Object obj) {
        // perform some check on obj and
        // then set field1 such that field1==obj
    }
}


public class SubClass extends SuperClass {
    public SubClass(Object obj) {
        super(obj);
    }

    @Override
    public void setField1(Object obj) {
        super.setField1(obj);
        // do some work that is necessary only when
        // field1 is set through SubClass.setField1()
    }
}

What I need is, when I create an object of SubClass, I need method SuperClass.setField1() to be called from inside SuperClass constructor, not SubClass.setField1(). But with the above code, method SubClass.setField1() is called which causes a NullPointerException to be thrown, since the work done in SubClass.setField1() is performed on a field that's yet null.

Is there any way to explicitly call SuperClass.setField1()‍‍‍ from inside SuperClass constructor?

Relevant: https://stackoverflow.com/questions/4595512/java-calling-a-super-method-which-calls-an-overridden-method#4595762 — kabanus, Jul 29 '18 at 11:51
Possible duplicate of [Java Polymorphism How to call to super class method for subclass object](https://stackoverflow.com/questions/14907424/java-polymorphism-how-to-call-to-super-class-method-for-subclass-object) — ernest_k, Jul 29 '18 at 11:53
@zhh Because the additional work in overridden method is performed on fields that are declared in `SubClass` and are not instantiated yet, since the `super` command must be called first of all, in `SubClass` constructor. — Hedayat Mahdipour, Jul 29 '18 at 11:57
From which line NullPointerException is coming? Can you please paste the whole code how you are invoking the constructor and which parameters you are passing to it? — Rahul Vedpathak, Jul 29 '18 at 11:59
I think we can find a solution as requested, but actually your problem arises from calling a polymorphous method from a constructor. This is discouraged in Java (because such strange effects can occur). Use e.g. a static factory to work around this - and you will not have to solve this problem. Do you need help for the static factory? — CoronA, Jul 29 '18 at 12:02
[Calling overrideable methods from inside a constructor is well-known to be dangerous](https://stackoverflow.com/q/3404301/3788176). Just don't do it. — Andy Turner, Jul 29 '18 at 12:16
@CoronA I'm familiar with static factories. But I'm not sure if this can be applied to my example. In my actual program, the SuperClass corresponds to an abstract class called `MilitaryUnit` which has a field `graphic` (corresponding to `field1`). And a subclass called `MovableUnit` which introduces the additional field `animation` that applies to `graphic` to move it. In class `MovableUnit` when `super()` is called, there's no animation yet, but it calls method `setGraphic()`. Every time a new `graphic` is set, it must be introduced to `animation` in method `MovableUnit.setGraphic()`. — Hedayat Mahdipour, Jul 29 '18 at 12:19
@CoronA I'm not sure if I could explain the situation clearly. — Hedayat Mahdipour, Jul 29 '18 at 12:20
It would be best if you add some code the reproduces the problem. I understand why a static factory is not applicable, but yet I do not understand why you call `setField` in your constructor. Just copy and paste the contents of `setField`, not every copy and paste is bad. — CoronA, Jul 29 '18 at 12:27

Lino · Accepted Answer · 2018-07-29T18:07:52.757

You could move said method body to a private method and let the default method (the one which may be overridden by the subclass) delegate to the former. See this example

public abstract class SuperClass {

    private Object field1;

    protected SuperClass(Object obj){
        // call the safe implementation
        setField1Safe(obj);
    }


    public void setField1(Object obj){
        // just delegates
        setField1Safe(obj);
    }

    private void setField1Safe(Object obj){
        // perform some check on obj and
        // then set field1 such that field1==obj
    }
}

public class SubClass extends SuperClass{
     public SubClass(Object obj){
         super(obj);
     }

     @Override
     public void setField1(Object obj){
         super.setField1(obj);
         // do some work that is necessary only when
         // field1 is set through SubClass.setField1()
     }
}

That way the sub class can still override setField1 but if you really depend on the implementation then you can call the private setField1Safe method.

if he is gonna make the method private then there will not be any method overriding .he is trying override the methods — JAMSHAID, Jul 29 '18 at 12:32
I think Lino should rewrite his example and adjust it to the given one. I supect that the method `doStuff` is meant to be inherited while `doStuffButPrivate` is called from the constructor. — CoronA, Jul 29 '18 at 14:38
@CoronA that's what I intended to tell with this, I've updated to reflect the changes OP should made in his code — Lino, Jul 29 '18 at 18:08

How to call the superclass implementation of an overridden method from inside the superclass in Java?

1 Answers1