Inner classes or nested classes in Java Program
- 1. Slides prepared by Rose Williams, Binghamton University Chapter 13 Interfaces and Inner Classes
- 2. © 2006 Pearson Addison-Wesley. All rights reserved 13-2 The Serializable Interface • An extreme but commonly used example of an interface is the Serializable interface – It has no method headings and no defined constants: It is completely empty – It is used merely as a type tag that indicates to the system that it may implement file I/O in a particular way
- 3. © 2006 Pearson Addison-Wesley. All rights reserved 13-3 The Cloneable Interface • The Cloneable interface is another unusual example of a Java interface – It does not contain method headings or defined constants – It is used to indicate how the method clone (inherited from the Object class) should be used and redefined
- 4. © 2006 Pearson Addison-Wesley. All rights reserved 13-4 The Cloneable Interface • The method Object.clone() does a bit- by-bit copy of the object's data in storage • If the data is all primitive type data or data of immutable class types (such as String), then this is adequate – This is the simple case • The following is an example of a simple class that has no instance variables of a mutable class type, and no specified base class – So the base class is Object
- 5. © 2006 Pearson Addison-Wesley. All rights reserved 13-5 Implementation of the Method clone: Simple Case
- 6. © 2006 Pearson Addison-Wesley. All rights reserved 13-6 The Cloneable Interface • If the data in the object to be cloned includes instance variables whose type is a mutable class, then the simple implementation of clone would cause a privacy leak • When implementing the Cloneable interface for a class like this: – First invoke the clone method of the base class Object (or whatever the base class is) – Then reset the values of any new instance variables whose types are mutable class types – This is done by making copies of the instance variables by invoking their clone methods
- 7. © 2006 Pearson Addison-Wesley. All rights reserved 13-7 The Cloneable Interface • Note that this will work properly only if the Cloneable interface is implemented properly for the classes to which the instance variables belong – And for the classes to which any of the instance variables of the above classes belong, and so on and so forth • The following shows an example
- 8. © 2006 Pearson Addison-Wesley. All rights reserved 13-8 Implementation of the Method clone: Harder Case
- 9. © 2006 Pearson Addison-Wesley. All rights reserved 13-9 Simple Uses of Inner Classes • Inner (or nested) classes are classes defined within other classes – The class that includes the inner class is called the outer class • There are four categories of inner classes in Java: 1.Inner classes (non-static) 2.Static inner classes 3.Local classes (defined inside a block of Java code) 4.Anonymous classes (defined inside a block of Java code)
- 10. © 2006 Pearson Addison-Wesley. All rights reserved 13-10 Simple Uses of Inner Classes • An inner class definition is a member of the outer class in the same way that the instance variables and methods of the outer class are members – An inner class is local to the outer class definition – The name of an inner class may be reused for something else outside the outer class definition – If the inner class is private, then the inner class cannot be accessed by name outside the definition of the outer class
- 11. © 2006 Pearson Addison-Wesley. All rights reserved 13-11 Simple Uses of Inner Classes • There are two main advantages to inner classes – They can make the outer class more self- contained since they are defined inside a class – Both of their methods have access to each other's private methods and instance variables • Using an inner class as a helping class is one of the most useful applications of inner classes – If used as a helping class, an inner class should be marked private
- 12. © 2006 Pearson Addison-Wesley. All rights reserved 13-12 Tip: Inner and Outer Classes Have Access to Each Other's Private Members • Within the definition of a method of an inner class: – It is legal to reference a private instance variable of the outer class – It is legal to invoke a private method of the outer class • Within the definition of a method of the outer class – It is legal to reference a private instance variable of the inner class on an object of the inner class – It is legal to invoke a (nonstatic) method of the inner class as long as an object of the inner class is used as a calling object • Within the definition of the inner or outer classes, the modifiers public and private are equivalent
- 13. © 2006 Pearson Addison-Wesley. All rights reserved 13-13 Class with an Inner Class
- 14. © 2006 Pearson Addison-Wesley. All rights reserved 13-14 Class with an Inner Class
- 15. © 2006 Pearson Addison-Wesley. All rights reserved 13-15 Class with an Inner Class
- 16. © 2006 Pearson Addison-Wesley. All rights reserved 13-16 The .class File for an Inner Class • Compiling any class in Java produces a .class file named ClassName.class • Compiling a class with one (or more) inner classes causes both (or more) classes to be compiled, and produces two (or more) .class files – Such as ClassName.class and ClassName$InnerClassName.class