Why is a static class illegal in Java?

Question

I'm developing an Android application but have hit a bit of a brick wall, I keep getting the error:

Illegal modifier for the class FavsPopupFragment; only public, abstract & final are permitted 

This happened after following this answer to another SO question. Here is the code that I have:

package com.package.name;  /* Imports were here */  public static class FavsPopupFragment extends SherlockDialogFragment {      static FavsPopupFragment newInstance() {         FavsPopupFragment frag = new FavsPopupFragment();         return frag;     } } 

The error appears on the class name. I don't understand why this won't work, please help. Thank you.

Answer 1

You can't create a top level static class; that's what the compiler is trying to tell you. Also have a look at the answer here as to why this is the case. The gist is:

What the static boils down to is that an instance of the class can stand on its own. Or, the other way around: a non-static inner class (= instance inner class) cannot exist without an instance of the outer class. Since a top-level class does not have an outer class, it can't be anything but static.

Because all top-level classes are static, having the static keyword in a top-level class definition is pointless.

Answer 2

As the previous answers stated, you can't use the static keyword in top level classes. But i wonder, why did you want it to be static?

Let me show you how a static / non static inner class is used in an example:

public class A {     public class B{}      public static class C{}      public static void foo()     {         B b = new B(); //incorrect          A a = new A();         A.B b = a.new B(); //correct          C c = new C(); //correct     }     public void bar()     {         B b = new B();         C c = new C(); // both are correct     } } 

And from a completely different class:

public class D {     public void foo()     {         A.B b = new A.B() //incorrect          A a = new A()         A.B b = a.new B() //correct          A.C c = new A.C() //correct     } }