Does static class contain only static method in java?

Question

I have found code in which static class contains methods that are not declared static. Compiler doesn't display any warning. It seems confusing, doesn't it?

Answer 1

There is no concept of static class in java (not even static inner class). If you see class is static and it is in working condition then it must be inner class(also called as nested class) which is declared as static. And there is no restriction to have only static methods in static inner classes.

The significance of declaring an inner class static is to be able to create instances of the nested class independent of the class in which it is nested. If static is not mentioned then every instance of the nested class will be associated with an instance of the class in which it is nested.

This question has more details. Java inner class and static nested class

Answer 2

The answers here are good, but I guess I should add an example. Consider the following class definition

class Foo{
    class Bar{
        int x;
    }
}

To use the class Bar you'll have to create an instance of class Foo first.

Foo foo = new Foo();
Foo.Bar bar = foo.new Bar();
bar.x = 10;

But if you have used the static keyword like the following

class Foo{
    static class Bar{
        int x;
    }
}

You could have skipped a step

Foo.Bar bar = new Foo.Bar();
bar.x = 10;

If you'll try to statically use Foo.Bar directly when the inner class is not static, then you'll get the following error -

error: an enclosing instance that contains Foo.Bar is required
        Foo.Bar bar = new Foo.Bar();
                      ^
1 error

I think this example should clear it.

Answer 3

A static class is a nested class which has no implicit reference to the outer class. A static class can have static methods or instance methods. Note: an inner class can't have static methods.

Answer 4

There's no such thing as a "static class" in Java (at least not in the way you seem to understand it). In C# there is a "static class" that can not be instantiated and can't have non-static members. That concept doesn't exist explicitly in Java (you can get the same effect by making all constructors private, however).

What you're seeing is probably a static nested class like this:

class Foo {
  static class Bar {
  }
}

This only means that a Bar instance does not have a reference to an outer Foo instance (as it would be the case if Bar did not have the static modifier).

Bar can have any kind of members it could have if it were a top-level class.

Answer 5

The static keyword is totally different for attributes/methods and classes.

For attributes and methods, it will tell the compiler to have only one instance of the attribute or method that can be referenced without a living instance of the class.

The static class can only be used for inner class, to indicate that the class can be referenced without an existing instance of the enclosing class. It is therefore useful for inner enumeration that should be accessed from outside.

But a static class can reference whatever it wants, from its own body.

Answer 6

Only inner classes can be declared static. A static class has no pointer to its outer class and can therefore only refer to static fields and methods of the outer class. A static class may however itself contain non-static methods.