COP3252 Advanced Java Programming

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COP3252Advanced Java Programming

• 14-Feb-08
• Lecture Set 10
• Static Class Members, Final Modifier, Inheritance

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Static Class Members

• Each object has its own copy of all the instance
  variables of the class.
• Sometimes, only a single copy of a particular
  variable should be shared by all objects created
  from a class.
• A static field (or class variable) is used in
  this case.
• Represents classwise information
• all objects in the class share the same piece of
  information

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Keyword static

• Used to declare a static variable
• When to use static?
• Sometimes, you want to know how many objects from
  a particular class exist
• You can create a static count variable in the
  class definition --- and have that variable
  incremented/decremented upon object
  initialization/deinitialization.

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Static

• Static variables have class scope
• Static class members exist even when no objects
  of the class exist
• They are available as soon as the class is loaded
  into memory at execution time.
• To access a public static member when no objects
  exist, prefix the class name and a dot (.) to the
  to the static member (such as Math.PI).
• To access private static members when no object
  exists, there must be a public static method
  provided and the method must be called by
  qualifying its name with the class name and a dot.

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Static Example Code

• See Employee.java and EmployeeTest.java

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Static Methods

• A method declared static cannot acces non-static
  class members because a static method can be
  called even when no objects of the class have
  been instantiated.
• Additionally, the this reference can not be
  used in a static method (same reason as above).

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Static import

• Enables you to refer to imported static members
  as if they were declared in the class that uses
  them.
• This makes the class name and dot no longer
  required.
• Syntax static import packageName.Classname.stati
  cMemberName

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The final modifier

• Final is the same as the old constant modifier
  in C, Pascal, etc
• A class or a member of a class (variable, method,
  etc) can be declared final.
• Once a value is set for this variable, it can not
  be changed in the program.

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The final modifier

• Final is used to help protect data integrity.
• Example
• final int universalAnswer 42
• You can create a blank final variable
• Final variable that is not initialized when it is
  created.
• You can assign a value to the variable later, but
  it can not be changed once it is assigned

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Blank final variable

• //Create the variable, but dont initialize it
• Final String name
• //Give the variable a value that can not be
  changed
• name new String(This is a string)

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Inheritance

• Technique that allows one class to be derived
  from another.
• New class is created by absorbing an existing
  classs members and adding new or modified
  capabilities.

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Superclasses and Subclasses

• Superclass existing class
• Called a base class in C
• Also known as a parent class
• Subclass new class
• Inherits from the superclass
• Called a derived class in C
• Also known as a child class
• Subclasses can become superclasses for future
  subclasses.

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Subclasses

• Usually more specific that the super class
• Adds its own methods and fields
• Exhibits the behaviors of the superclass, but
  with additional behaviors specific to the
  subclass.
• Sometimes referred to as specialization.

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Direct and Indirect superclasses

• Direct superclass from which the subclass
  directly inherits
• Indirect any class above the direct superclass
  in the class hierarchy.
• Every class in Java directly or indirectly
  extends from the class Object.

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Single vs Multiple Inheritance

• Singe class is derived from one direct
  superclass
• Multiple class is derived from more than one
  direct superclass
• Not supported in Java (but supported in C)
• Note Lecture on interfaces (later in the
  course) will explain how we can still obtain the
  idea of multiple inheritance in Java.

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The keyword super()

• When you create a child class, the child class
  constructor caninvoke the base class constructor
• Do this with the keyword super -- acts as the
  call to the superclass constructor
• The call to super() must be the first line of the
  derived class constructor
• If explicit call to parent constructor not made,
  the subclass' constructor will automatically
  invoke super(). (the default constructor of the
  superclass, if there is one)
• Can also use super to invoke a method from the
  parent class (from inside the derived class).
• Super.method(parameters)

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Inheritance Example

• Recall the SuperString class you were asked to
  write for Assignment 3
• See MegaSuperString.java and MegaSuperStringTest.
  java for example of inheritance
• MegaSuperString inherits from SuperString