3.10. Nested Types

The classes, interfaces, and enumerated types we have seen so far in this book have all been defined as top-level classes. This means that they are direct members of packages, defined independently of other types. However, type definitions can also be nested within other type definitions. These nested types, commonly known as " inner classes," are a powerful and elegant feature of the Java language. A type can be nested within another type in four ways:

Static member types

A static member type is any type defined as a static member of another type. A static method is called a class method, so, by analogy, we could call this type of nested type a "class type," but this terminology would obviously be confusing. A static member type behaves much like an ordinary top-level type, but its name is part of the namespace, rather than the package, of the containing type. Also, a static member type can access the static members of the class that contains it. Nested interfaces, enumerated types, and annotation types are implicitly static, whether or not the static keyword appears. Any type nested within an interface or annotation is also implicitly static. Static member types may be defined within top-level types or nested to any depth within other static member types. A static member type may not be defined within any other kind of nested type, however.

Nonstatic member classes

A "nonstatic member type" is simply a member type that is not declared static. Since interfaces, enumerated types, and annotations are always implicitly static, however, we usually use the term "nonstatic member class" instead. Nonstatic member classes may be defined within other classes or enumerated types and are analogous to instance methods or fields. An instance of a nonstatic member class is always associated with an instance of the enclosing type, and the code of a nonstatic member class has access to all the fields and methods (both static and non-static) of its enclosing type. Several features of Java syntax exist specifically to work with the enclosing instance of a nonstatic member class.

Local classes

A local class is a class defined within a block of Java code. Interfaces, enumerated types, and annotation types may not be defined locally. Like a local variable, a local class is visible only within the block in which it is defined. Although local classes are not member classes, they are still defined within an enclosing class, so they share many of the features of member classes. Additionally, however, a local class can access any final local variables or parameters that are accessible in the scope of the block that defines the class.

Anonymous classes

An anonymous class is a kind of local class that has no name; it combines the syntax for class definition with the syntax for object instantiation. While a local class definition is a Java statement, an anonymous class definition (and instantiation) is a Java expression, so it can appear as part of a larger expression, such as method invocation. Interfaces, enumerated types, and annotation types cannot be defined anonymously.