JQPlus Question on Inner Classes

[Himanshu Jhamb]

Question ID :953582884020
Which of these statements are true? Select 3
a) Non-static inner classes cannot have static members
b) Objects of top level nested classes can be created without an instance of the outer class
c) Member variables in any nested class cannot be declared final
d) Anonymous classes cannot have constructors
e) Anonymous classes cannot be static
My answer - b) & d)
I don't understand why JQPlus also choses e) as the right answer.
I have read inner classes from Khalid Mughal & its very clearly given that we can have Anonymous Inner classes that are static. He has even given examples demonstrating the same... Can someone help, please !!!

[Rajeev Nair]

I think option a) is true right?. Non static inner class cannot have static members unless they are compile time constants like public static final.

[Himanshu Jhamb]

Well !
If its a compile time constant...
i.e. static final int a = 0;
... then it is static, also... right?
So, option a) is false.
- Himanshu

[Tina Ang]

I agree with Himanshu that a) is false.
I think answer e) is correct. According to JLS 15.9.5 "An anonymous class is never abstract. An anonymous class is always an inner class; it is never static. An anonymous class is always implicitly final."

Tina
JLS 15.9.5

[Himanshu Jhamb]

Well!
In that case, can we get Mr. Khalid Mughal to comment on this? Val, anyway, we can manage that ?
I think I would stick with what JLS says.
Waiting for your comments, Val.
thanks
- Himanshu

[Valentin Crettaz]

The best way to get the correct answers is to write a little piece of code for each option and see how it behaves.
a. FALSE

public class Test{
class Inner{
final static int i=0;
}
}

Compiles fine.
b. TRUE

public class Test{
static class Inner{
}
public static void main(String[] args){
Inner i = new Test.Inner();
}
}

compiles fine
c. FALSE

public class Test{
static class Inner{
final int i=0;
}
class Inner2 {
final long j=0;
}
}

compiles fine
d. TRUE

public class Test{
public static void main(String[] args){
Runnable r = new Runnable(){
Runnable(){
System.out.println("in constructor");
}
};
}
}

Does not compile. The error is:

Test.java:4: invalid method declaration; return type required
                        Runnable(){
                        ^
1 error

Which means that the compiler treats Runnable() as a method and not a constructor.
e. This last one is tricky and the JLS and KM seems to be of different opinions. JLS says that anonymous classes cannot be declared static and KM says that an anonymous classes declared within a static context is implicitely static.
Well, it is true that an anonymous class cannot be declared static. Moreover, an anonymous class is a local class, it is always created within the context of another method or initializer and there is no point in declaring a local method static even if the context is static. An anonymous class just serves the purpose of declaring and creating a instance of a class "on the fly". The result is always an instance of a class that can usually be returned. That object can be used anywhere in the program. It can be assigned to a static member or an instance member (see code below). Moreover an object is never static, it is the reference that points to it that can be static but not the object itself.

public class Test{
static Runnable r1;
Runnable r2;
public static void main(String[] args){
Test t = new Test();
t.doSomething();
}
public void doSomething(){
r1 = getRunnable();
r2 = getRunnable();
}
public static Runnable getRunnable(){
return new Runnable(){
public void run(){}
};
}
}

[Himanshu Jhamb]

Thanks Val ... for all the trouble you take to explain stuff
Indeed, very helpful.
- Himanshu