Advanced Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 281

super(); // invokes Inner1() constructor
^
Because the superclass of Inner2 is itself an inner class, an obscure language rule comes into play.
As you know, the instantiation of an inner class, such as Inner1 , requires an enclosing instance to
be supplied to the constructor. Normally, it is supplied implicitly, but it can also be supplied
explicitly with a superclass constructor invocation of the form expression.super(args) [JLS
8.8.7].
If the enclosing instance is supplied implicitly, the compiler generates the expression: It uses the
this reference for the innermost enclosing class of which the superclass is a member. This is,
admittedly, quite a mouthful, but it is what the compiler does. In this case, the superclass is Inner1 .
Because the current class, Inner2 , extends Outer indirectly, it has Inner1 as an inherited member.
Therefore, the qualifying expression for the superclass constructor is simply this . The compiler
supplies an enclosing instance, rewriting super to this.super . Had we done this ourselves, the
compilation error would have made even more sense:
Outer.java:12: cannot reference this before
supertype constructor has been called
this.super();
^
Now the problem is clear: The default Inner2 constructor attempts to reference this before the
superclass constructor has been called, which is illegal [JLS 8.8.7.1]. The brute-force way to fix this
problem is to provide the reasonable enclosing instance explicitly:
public class Outer {
class Inner1 extends Outer { }
class Inner2 extends Inner1 {
public Inner2() {
Outer.this .super();
}