Extending a second Inner Class

[Milton Ochoa]

Hi everyone,

Today I was studying about Inner Class, and I trying a few combinations with the classes. Then a found this:

public class TopLevelClass{
 
class InnerClass extends TopLevelClass{
 
class Inner_InnerClass extends InnerClass { // but if we drop-out the "extends InnerClass" then this will compile correctly
}
}
 
}

this is the output that the compiler shows:

javac c:\test\TopLevelClass.java
c:\test\TopLevelClass.java:5: cannot reference this before supertype constructor
 has been called
                class Inner_InnerClass extends InnerClass {
                ^
1 error

This is very rare, why the second inner class cannot extend from the first class?

And if someone can explain me the compile error.

Thank you

[Campbell Ritchie]

What sort of constructor do you have in the superclass? Are you required to call in with a super(x, y, z); statement?

[Milton Ochoa]

Hi Campbell.

I dont understand what are you question me!

can you explain more please!

[Peter Chase]

The inner inner class implicitly has a reference to an instance of the inner and outer class.

Also, the inner inner class [default] constructor needs to call the inner class [default] constructor, as its superclass constructor.

Somehow, in a way that I haven't exactly worked out, the compiler needs to access the instance of the inner class before the inner class constructor has been called, as the superclass constructor.

To be honest, I don't think there is a huge amount to be gained by deep analysis of what is going on here (which is why I haven't bothered!). I don't believe there would ever be a good reason to have an inner class that extends its enclosing class. So just Don't Do That Then.

[Brian Cole]

Note that if either of the nested classes is declared to be static, or either is declared with a different superclass, then it compiles without error. But where's the fun in that.

It will also compile if we use a qualified superclass constructor invocation:

public class TopLevelClass{
 
class InnerClass extends TopLevelClass {
 
class Inner_InnerClass extends InnerClass {
public Inner_InnerClass() {
new TopLevelClass().super();
}
}
}
 
}

Or we could write it this way:

public class TopLevelClass{
 
class InnerClass extends TopLevelClass {
 
class Inner_InnerClass extends InnerClass {
public Inner_InnerClass(TopLevelClass primary) {
primary.super();
}
}
}
 
}

I mostly have a handle on why the original doesn't compile, but I'm having trouble putting it in words. (Which means I must not completely understand it.)

(cf. JLS �8.1.3)
[ January 31, 2008: Message edited by: Brian Cole ]