Java: Non-static nested classes and instance.super()

I’m having a hard time wrapping my head around non-static nested classes in Java. Consider the following example, which prints “Inner” and then “Child”.

class Outer {
    class Inner {
        Inner() { System.out.println("Inner"); }
    }
}

public class Child extends Outer.Inner {
    Child(Outer o) {
        o.super();
        System.out.println("Child");
    }
    public static void main(String args[]) {
        new Child(new Outer());
    }
}

I understand that instances of Inner always have to be associated with an Outer instance, and that that applies to Child too since it extends Inner. My question is what the o.super() syntax means – why does it call the Inner constructor?

I’ve only seen a plain super(args) used to call the superclass constructor and super.method() to call the superclass version of an overridden method, but never something of the form instance.super().

Answer

It’s called a “qualified superclass constructor invocation”.

Citing from here:

Explicit constructor invocation statements can be divided into two kinds:

  • Alternate constructor invocations begin with the keyword this (possibly prefaced with explicit type arguments). They are used to invoke an alternate constructor of the same class.

  • Superclass constructor invocations begin with either the keyword super (possibly prefaced with explicit type arguments) or a Primary expression. They are used to invoke a constructor of the direct superclass. Superclass constructor invocations may be further subdivided:

  • Unqualified superclass constructor invocations begin with the keyword super (possibly prefaced with explicit type arguments).

  • Qualified superclass constructor invocations begin with a Primary expression . They allow a subclass constructor to explicitly specify the newly created object’s immediately enclosing instance with respect to the direct superclass (§8.1.3). This may be necessary when the superclass is an inner class.

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