About Question com.enthuware.ets.scjp.v6.2.189 :

[TwistedLizard]

Option 2:
Non-static inner classes cannot contain static members.

Commentary:
They can if the static variable is also made final.

Is this only for primitive member variables? Attempting to declare a final reference variable in a non-static inner class results in a compiler error, in this case at least:

Code: Select all

import java.util.Date;

class Test{
  class Inner{
    final static Date date = new Date();    //illegal static declaration
  }
  public static void main(String[] args){}
}

[admin]

JLS Section 8.1.3 clearly states:

It is a compile-time error if an inner class declares a member that is explicitly or
implicitly static , unless the member is a constant variable (§4.12.4).

And as per 4.12.4,

A constant variable is a final variable of primitive type or type String that is initialized with a constant expression (§15.28)

So for example, static final String s = ""; compiles but static final String s = new String(); doesn't.

That's explains why your code fails to compile.

[TwistedLizard]

ok. Thanks for your help.

[nkaragulov]

Since Java 16, Inner classes can declare non-final static members as well.
To create a class variable for an inner class, the programmer must place the desired variable in an enclosing class.
It is helpful at this point to abuse the terminology somewhat, and say, loosely, that the static keyword always marks a "top-level" construct (variable, method or class), which is never subject to an enclosing instance.

To create a class variable for an inner class, the programmer must place the desired variable in an enclosing class.

does it mean the following?

Code: Select all

class Outer {
  Inner inner = new Inner();
  class Inner {}
}

or

Code: Select all

class Outer {
  static int i;
  class Inner {}
}

[admin]

class Outer {
Inner inner = new Inner();
class Inner {}
}

[nkaragulov]

class Outer {
Inner inner = new Inner();
class Inner {}
}

So what is the relation of the explanation to static members?
What does it mean in the context of the whole paragraph?

[admin]

I am sorry, I misspoke. This statement is now meaningless and must be removed. It was actually take from here: https://jcp.org/aboutJava/communityproc ... lasses.pdf

But it is no more valid because an inner class (i.e. a non-static nested class) can have static members even if they are not final. For example:

Code: Select all

class Outer {
  class Inner {
    static int i;
  }
}

In the above example, static int i is a member of Inner.