Inner class basics. (Marcus Green's mock exam 2)

[amol kat]

From Marcus Green's mock exam 2:
Which of the following statements are true?
1) Adding more classes via import statements will cause a performance overhead, only import classes you actually use.
2) Under no circumstances can a class be defined with the private modifier
3) A inner class may under some circumstances be defined with the protected modifier
4) An interface cannot be instantiated
I said 4.
Marcus Green says 3,4
Cant inner class be ALWAYS defined with the protected modifier??
Thnx in advance,
Amol

[Jamal Hasanov]

Hi,
Not always:
A inner class may under some circumstances be defined with the protected modifier
Inner class can also be declared as public,private,default access modifier.
Jamal Hasanov
www.j-think.com

[Anthony Villanueva]

Cant inner class be ALWAYS defined with the protected modifier??

Not if it's local or anonymous.

[bani kaali]

A inner class may under some circumstances be defined with the protected modifier

can any one explain this with an example?
Thanks
bani

[Corey McGlone]

Originally posted by bani kaali:
can any one explain this with an example?

Here ya go...a protected inner class:

class Outer
{
   protected class Inner
   {
      public void doIt() {System.out.println("doing it");}
   }
}
 
public class Outer2 extends Outer
{
   public static void main(String[] args)
   {
      Outer2.Inner i = new Outer2().new Inner();
      i.doIt();
   }
}
 
// Output
 
doing it

I hope that helps,
Corey

[Francisco A Guimaraes]

Not if it's local or anonymous.

what�s a local inner class?
Francisco

[Anthony Villanueva]

A local inner class is a class declared within the scope of a method. Whether it is considered static or not (as far as access considerations are concerned) depends on whether the enclosing method is static or not as well. Also, only local variables declared as final are accessible by the local class.
Here's a simple example:

 
class Test 
{
private int x = 7;
private static int y = 8;
        
public static void main(String []yep)
{       
new Test().someMethod();
}
public void someMethod()
{
int a = 1;
final int b = 2;
class InnerClass
{
void sayHi()
{
System.out.println("Hi!");
System.out.println("x: " + x);
System.out.println("y: " + y); // will NOT compile
System.out.println("a: " + a); // will NOT compile
System.out.println("b: " + b);
}
}
InnerClass inner = new InnerClass();
inner.sayHi(); 
}
}

[ June 25, 2002: Message edited by: Anthony Villanueva ]

[Francisco A Guimaraes]

Just one thing Anthony...y is accessible, look:

class Test
{
private int x = 7;
private static int y = 8;
public static void main(String []yep)
{
new Test().someMethod();
}
public void someMethod()
{
int a = 1;
final int b = 2;
class InnerClass
{
void sayHi()
{
System.out.println("Hi!");
System.out.println("x: " + x);
System.out.println("y: " + y); // this is right
//System.out.println("a: " + a);  will NOT compile
System.out.println("b: " + b);
}
}
InnerClass inner = new InnerClass();
inner.sayHi();
}
}

will produce this output:
Hi!
x: 7
y: 8
b: 2
Francisco

[amol kat]

I thought "local inner class" is called a "local class" and inner class is always that top level thing which if is static is called a top-level nested class and if not static is called an inner class.

[Corey McGlone]

That's how I usually think of them, as well. However, sometimes, you'll see the term "inner class" used to include inner classes, local classes, and anonymous classes. In that respect, anonymous classes and local classes are just special types of inner classes.
Of course, if the class is a static member of a class, it is not an inner class at all. In that case, it is a top-level nested class.
Corey

[Anthony Villanueva]

Just one thing Anthony...y is accessible

Ayiyi.. so it is! My mind is going...
Okay, a local class in a non-static context can access both static and non-static instance variables of the immediately enclosing class. However, a local class in a static context can only access the static variables of the enclosing class.

class Test 
{
private int x = 7;
private static int y = 8;
        
public static void main(String []yep)
{       
new Test().someMethod();
}
public static void someMethod()
{
int a = 1;
final int b = 2;
class InnerClass
{
void sayHi()
{
System.out.println("Hi!");
System.out.println("x: " + x); // this better NOT compile
System.out.println("y: " + y); 
System.out.println("a: " + a); // will NOT compile
System.out.println("b: " + b);
}
}
InnerClass inner = new InnerClass();
inner.sayHi(); 
}
}