Friday, 27 January 2012

Java interview questions: - Explain different types of inner classes?

Following are the different types of inner classes in  Java interview questions.

Nested top-level classes-
  • If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class
  • Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer. Inner
  • Top-level inner classes implicitly have access only to static variables. There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes -
  • Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables.
  • This means a public member class acts similarly to a nested top-level class
  • The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes-
  • Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.
  • Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes-
  • Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor
  • Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors
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