CSMJava ProgrammingI Spring,2005

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Inheritance Lesson - 5
CSM-Java Programming-I
Spring,2005
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• Objectives
• Review of last class
• Inheritance
• super
• Method Overriding
• Abstract Classes

CSM-Java Programming-I Lesson-1
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Inheritance

• When a class B acquires the properties of another
  class A, then class B is said to have inherited
  class A.
• Here, class A is a superclass and class B is a
  subclass.
• A subclass inherits all the instance variables
  and methods defined by the superclass and adds
  its own, unique elsements.
• The keyword extends is used to inherit a class.
• Eg class subclass extends superclass

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Inheritance

• Every class in Java is an extended class, whether
  it is declared with an extends keyword or not.
• If a class does not explicitly extend from
  another class, it implicitly extends the Object
  class.

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Example Inheritance

• public class BankAccount
• int acctNum
• String name
• public BankAccount(int bAcctNum, String name)
• acctNum bAcctNum
• .
• public double getBalance ()

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Example Inheritance

• public class CheckingAccount extends BankAccount
• double interest
• public CheckingAccount(int aNum, String aName,
  double aInt)
• super (aNum, aName)
• interest aInt
• public double calculateInterest()
• .

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super

• Whenever a subclass class super(), it is calling
  the constructor of its immediate superclass.
• super() must be the first statement executed
  inside a subclass constructor.
• super.member always uses the superclasss member
  (member could be a method or an instance variable)

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Example super

• public class BankAccount
• protected double getBalance()
• return 1000.00
• public class SavingsAccount extends BankAccount
• protected double getBalance()
• return 1010.00

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super

• protected void printBalance()
• BankAccount ba this //superclass variable
  //can reference //subclass object.
• System.out.println(super.getBalance())
• System.out.println(this.getBalance())
• System.out.println(ba.getBalance())
• Output
• 1000.00
• 1010.00
• 1010.00

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Method Overriding

• When a method in a subclass has the same name and
  type signature and a method in its superclass,
  then the method in the subclass is said to
  override the method in the subclass.
• Only non-static methods can be overridden.
• Both signature and return type must be the same
  as the superclass.
• The throws clause of an overriding method can
  have fewer types listed than the method in the
  superclass, or more specific types or both.

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Example Method Overriding

• public class Parent
• public void hello()
• System.out.println(Hello from parent)
• public class Child extends Parent
• public void hello()
• System.out.println(Hello from Child)

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Method Overriding

• An extended class can change the access of a
  superclasss methods, but only if it provides
  more access.
• A method declared protected in the super class
  can be redeclared protected or public, but not
  private.
• Fields cannot be overridden they can only be
  hidden.
• To access the hidden fields use the super keyword.

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final and Inheritance

• A method declared final cannot be overridden
• A class can also be declared final. Such a class
  cannot be extended.
• final class Security
• //
• A final classs methods are implicitly final.
• static and private methods cannot be overridden.

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Abstract Classes

• A superclass that only defines a generalized
  form that will be shared by all its subclasses,
  leaving the implementation details to its
  subclasses is said to an abstract class.
• A concrete class has concrete methods, including
  implementations of any abstract methods inherited
  from its superclasses.
• Any class with abstract methods should be
  declared abstract.

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Example Abstract Class

• abstract class Shape
• abstract double area()
• public void display () // concrete method
• // Do something
• class Square extends Shape
• double area()
• // Do something

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Abstract Classes

• An abstract class cannot have objects because it
  is not complete, but it can have references.
• Static methods and constructors cannot be
  declared abstract.
• Any subclass of an abstract class must either
  implement all the abstract methods in the
  superclass or be itself declared abstract.
• A concrete method can be overriden to become
  abstract.
• It is illegal to declare a class both final and
  abstract.

CSM-Java Programming-I Lesson-1