OCP11 – Creating nested classes (I)

DEBUN_OCJP11

A nested class is a class that is defined within another class. We have four different nested classes types:

  1. Inner class: A non-static type defined at the member level of a class.
  2. Static nested class: A static type defined at the member level of a class.
  3. Local class: A class defined within a method body.
  4. Anonymous class: A special case of a local class that does not have a name.

By convention we use the term inner or nested class to apply to other Java types, including enums and interfaces.

Interfaces and enums can be declared as both inner classes and static nested classes but not as local or anonymous classes.

We will explain each one of these nested classes in a different post and we will use this post to show which are the syntax rules permitted in Java for nested classes in three tables.

Modifiers in Nested Classes

Permitted modifiers Inner class static nested class Local class Anonymous class
Access modifiers All All None None
abstract Yes Yes Yes No
final Yes Yes Yes No

Members in Nested Classes

Permitted modifiers Inner class static nested class Local class Anonymous class
Instance methods Yes Yes Yes Yes
Instance variables Yes Yes Yes Yes
static methods No Yes No No
static variables Yes (if final) Yes Yes (if final) Yes (if final)

Nested classes access rules

Permitted modifiers Inner class static nested class Local class Anonymous class
Can extend any class or implement any number of interfaces Yes Yes Yes No – must have exactly one superclass or one interface
Can access instance members of enclosing class without a reference Yes No Yes (if declared in an instance method) Yes (if declared in an instance method)
Can access local variables of enclosing method N/A N/A Yes (if final or effectively final) Yes (if final or effectively final)