What is the main difference between an inner class and a static nested class in Java? Does design / implementation play a role in choosing one of these?
28 Answers
From the Java Tutorial:
Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.
Static nested classes are accessed using the enclosing class name:
OuterClass.StaticNestedClass
For example, to create an object for the static nested class, use this syntax:
OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();
Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:
class OuterClass {
...
class InnerClass {
...
}
}
An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.
To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:
OuterClass outerObject = new OuterClass()
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
see: Java Tutorial - Nested Classes
For completeness note that there is also such a thing as an inner class without an enclosing instance:
class A {
int t() { return 1; }
static A a = new A() { int t() { return 2; } };
}
Here, new A() { ... } is an inner class defined in a static context and does not have an enclosing instance.
The Java tutorial says:
Terminology: Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.
In common parlance, the terms "nested" and "inner" are used interchangeably by most programmers, but I'll use the correct term "nested class" which covers both inner and static.
Classes can be nested ad infinitum, e.g. class A can contain class B which contains class C which contains class D, etc. However, more than one level of class nesting is rare, as it is generally bad design.
There are three reasons you might create a nested class:
- organization: sometimes it seems most sensible to sort a class into the namespace of another class, especially when it won't be used in any other context
- access: nested classes have special access to the variables/fields of their containing classes (precisely which variables/fields depends on the kind of nested class, whether inner or static).
- convenience: having to create a new file for every new type is bothersome, again, especially when the type will only be used in one context
There are four kinds of nested class in Java. In brief, they are:
- static class: declared as a static member of another class
- inner class: declared as an instance member of another class
- local inner class: declared inside an instance method of another class
- anonymous inner class: like a local inner class, but written as an expression which returns a one-off object
Let me elaborate in more details.
Static Classes
Static classes are the easiest kind to understand because they have nothing to do with instances of the containing class.
A static class is a class declared as a static member of another class. Just like other static members, such a class is really just a hanger on that uses the containing class as its namespace, e.g. the class Goat declared as a static member of class Rhino in the package pizza is known by the name pizza.Rhino.Goat.
package pizza;
public class Rhino {
...
public static class Goat {
...
}
}
Frankly, static classes are a pretty worthless feature because classes are already divided into namespaces by packages. The only real conceivable reason to create a static class is that such a class has access to its containing class's private static members, but I find this to be a pretty lame justification for the static class feature to exist.
