Inheritance

1
Inheritance
2
Overloading

• Overloading refers to the ability to allow
  different methods or constructors of a class to
  share the same name.
• The legality of overloading depends on the
  signature of the methods and constructors.

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Rule

• Two methods or constructors in a class can share
  the name if they have different signatures.

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Extending Classes

• Inheritance defines a relationship between
  classes.
• superclass, subclass
• All public and protected members of the
  superclass are accessible in the subclass.

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Example - Conceptual
Account
Checking Account
Savings Account
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Example - Source

• public class Account
• protected float minimumbalance100.0f
• public class SavingsAccount extends Account
• private float minimumbalance250.0f
• public class CheckingAccount extends Account

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Protected

• public makes a member accessible by everyone.
• private makes a member only accessible by
  instances of the class.
• protected makes a member accessible by instances
  of the class and instances of any subclass.

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Inheriting Methods

• Subclasses inherit superclass methods declared as
  public or protected (or not specified).
• Subclasses dont inherit a superclass method if
  the subclass defines a method using the same
  name. The subclass method is said to override
  the one in the parent.
• Subclasses dont inherit private methods.
• Subclasses dont inherit constructors.

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Constructors

• The initialization of extended classes requires
• the initialization of the fields inherited from
  the super class
• the initialization of the fields declared in the
  extended class.

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Example (1)

• public class CheckingAccount extends Account
• CheckingAccount ()
• minimumbalance250.0f
• public class SavingsAccount extends Account
• SavingsAccount ()

• public class Account
• float minimumbalance
• Account()
• minimumbalance100.0f

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Example (2)

• public class TrivialApplication
• public static void main(String args)
• SavingsAccount myAcctnew SavingsAccount()
• System.out.println( myAcct.minimumbalance )

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Default

• If no constructor is defined in the extended
  class, the default constructor is provided by
  default.
• public class Extended extends Super
• public Extended ()
• super()

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Abstract Class

• An abstract class can be created using the
  modifier abstract in the declaration. No
  instances of an abstract class can be created.

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Example

• abstract class Student
• protected final static int num_tests3
• protected String name
• protected int test
• protected String courseGrade
• public Student()
• this(No Name)
• public Student(String name)
• this.namename
• testnew intnum_tests
• courseGrade

• class GraduateStudent extends Student
• class UndergraduateStudent extends Student

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Polymorphism

• In its simplest form, polymorphism allows a
  variable to refer to objects from different
  classes.
• Student s
• snew GraduateStudent()
• snew UndergraduateStudent()

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Rule of Assignment

• The type of the expression on the right-hand
  side of an assignment must be a subtype of the
  type of the variable on the left-hand side.

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Example

• Student s1, s2
• s1new Undergraduate()
• s2new Graduate()
• Graduate s3
• s3s2//compilation error
• s3(Graduate)s2

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Casting

• Casting a variable to a subtype is called
  downcasting. Java allows explicit casting of any
  reference type to any other reference type at
  compile time.
• The validity of an explicit cast is checked at
  run time. If it is invalid, a ClassCastException
  will be thrown.

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Example

• Student s1, s2
• s1new Undergraduate()
• s2new Graduate()
• Graduate s3
• s3(Graduate)s1//run time error, exception
  thrown

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Proper Form

• Use instanceof before downcasting.
• if (student1 instanceof Graduate)
• else
• Catch the ClassCastException
• try
• Graduate g(Graduate) s1
• catch (ClassCastException e)

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Overriding Methods

• Overriding refers to methods of different classes
  that have an inheritance relationship.
• Methods have the exact same signature, name, and
  return type.