Extending an inner class in java

Anirudh Shukla :

I'm having trouble trying to implement this statement I read in Oracle's Docs about Inheritance when it comes to inner classes.

The statement :

A nested class has access to all the private members of its enclosing class—both fields and methods. Therefore, a public or protected nested class inherited by a subclass has indirect access to all of the private members of the superclass.

In order to test this out i.e. to see if I can achieve the above I created a top level class OC1 which had an inner class IC1 ,then I created another top level class OC2 which extended IC1.

Before I could even start writing a single method , the IDE stopped me at the OC2 class body itself saying

"No enclosing instance of type DataStructure is available due to some intermediate constructor invocation"

I read some other answers and most of them point to either a) Changing the inner class to static Nested Class -- it resolves the error b) The whole scenario is unnecessary and convoluted.

Here is the code:

 public class DataStructure {
    // Create an array
    private final static int SIZE = 15;
    private int[] arrayOfInts = new int[SIZE];

    public DataStructure() {
        // fill the array with ascending integer values
        super();
        for (int i = 0; i < SIZE; i++) {
            arrayOfInts[i] = i;
        }
    }

    //other methods
    //IC1
    protected  class instanceArr{

        private int a = 8;
        private static final int B = 4;
        protected instanceArr(){
        }

        protected void doSomething(){
            System.out.println("arrayOfInts[] is accessible " + arrayOfInts[6]);
        }

    }

    //main method
}

OC2

public class DataStructureChild extends DataStructure.instanceArr{


    public DataStructureChild(){

    }
}

I know that the scenario is not an ideal one but I don't want to change inner class to static nested class - it would defeat my purpose of basically trying to see whether arrayOfInts is accessible without OC1's instance in hand.

Am I misinterpreting this statement ? if not then kindly point me in the correct direction.

PS - this is my first question here - apologies in advance if some guidelines were flouted.

Peter Rader :

Yes, this is a Trap caused by Java's synthetic sugar. You think the inner-non-static-class have the default-no-arguments-constructor but that is wrong. Internally the constructor of IC1 have the OC1 as first argument in the constructor - even if you can not see it.

Thats why the OC2 constructor must use the OC1 as constructor-argument:

public DataStructureChild(DataStructure argument) {
}

Unfortunaltely this is not enougth, you need to get sure the argument is not-null:

public DataStructureChild(DataStructure argument) {
    argument.super();
}

It looks very wierd but it works.

Extending an inner class in java

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