More Features of Java

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More Features of Java

• Inheritance
• Interfaces
• Nested Classes
• Packages

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Inheritance

• extends keyword to indicate a class is a
  subclass of another (general to specific)
• square extends rectangle
• Can only be one superclass for each subclass
  (single inheritance)
• Object class is most general of all classes
• State and behavior is inherited from all ancestors

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What is Inherited

• All public and protected superclass members
• Unspecified superclass members
• Dont inherit overridden members
• Dont inherit private members

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Hiding Variables and Overriding methods

• Hide variables in superclass by redeclaring in
  subclass (careful)
• Refer to superclass variable with super keyword
• Override Redefine method in subclass
• Return type, method name, number and type of
  arguments must be the same

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• Can have a different throws clause
• Can provide more access but not less
• Dont have to completely override method.Just
  add functionality. Simply class overridden
  method using super keyword
• Cant override final or static methods
• Must override abstract methods

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Object

• Methods to override
• clone must implement Cloneable interface to
  create objects from other objects of same type
• equals Compare to objects for equality
• finalize Clean up before garbage collection
• toString Returns string representation of
  object.
• getClass returns runtime representation of
  object (Class object)

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Etc...

• final classes and methods for security and design
  goals.
• Abstract classes and methods for conceptual
  constructs

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Interfaces

• To define a set of behaviors that can be
  implemented by any class
• A name collection of method definitions without
  implementations plus constant declarations
• Protocols is another name.
• Unlike Abstract Class, doesnt force class
  relationship on users

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More on Interfaces

• Classes can implement multiple interfaces
• Not multiple inheritance
• Inherits only constants from interface
• Cannot inherit method implementations
• Interface hierarchy independent of class
• Interfaces cannot grow

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Packages

• A package is a collection of related classes and
  interfaces that provides access protection and
  namespace management.
• package statement at top of source code
• Refer to by long name (file pathname)
• import package or package member
• class path is list of directories to search for
  class files

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Nested Classes

• Defining a class as a member of another class
• Has access to all of enclosing class members
• Static nested classes (class)
• Inner classes (instance)

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Inner Classes

• Anonymous Classes