Inheritance

1
Inheritance

• Inheritance is the primary concept separating
  object-oriented programming from other
  programming paradigms.

2
Inheritance

• Inheritance defines a relationship among classes.
• A class C2 may inherit from (or extend, or derive
  from) another class C1.
• C1 parent (super-class)
• C2 child (sub-class)
• We say that C1 is the parent (super-class) of
  C2.
• Or, C2 is the child (sub-class) of C1.

3
Inheritance (cont.)

• Using the inheritance mechanism, a sub-class will
  inherit functionalities of its super-class.
• Of course, a sub-class may also define its own
  functionalities.
• So, a sub-class will have
• its own fields the fields inherited from its
  super-class
• its own methods the methods inherited from its
  super-class.
• All of the members of a super-class will be
  inherited by its sub-class, but only certain
  members will be directly accessible by their
  names in that sub-class.
• public and protected members of a class are
  accessible by name from the sub-classes of that
  class.
• private members are not accessible by name from
  sub-classes.
• package members are accessible by name from
  sub-classes in the same package.

4
extends keyword

• extends clause specifies the super-class of a
  class.
• ClassModifiers class SubClassName extends
  SuperClassName
• ClassMemberDeclarations
• class C2 extends C1 ...
• sub-class super-class
• (child) (parent)
• C2 is derived from C1.

5
Accessibility Example (1)

• package fruit
• public class Fruit
• public int weight
• int color
• protected String name
• private String scientificName
• package fruit
• public class Apple extends Fruit
• public String toString()
• return weight , color , name ,
  scientificName
• This code wont compile. Why?

6
Accessibility Example (2)

• package fruit
• public class Fruit
• public int weight
• int color
• protected String name
• private String scientificName
• package fruit.apple
• public class Apple extends Fruit
• public String toString()
• return weight , color , name
• This code wont compile. Why?

7
Accessibility Example (3)

• package fruit
• public class Fruit
• public int weight
• int color
• protected String name
• private String scientificName
• package fruit.apple
• public class Apple extends Fruit
• public String toString()
• return weight , name
• Will this code compile? Why?

8
Another Inheritance Example

• Assume the following class definitions
• public class Fruit
• private String name
• public String getName()
• return name
• public class Apple extends Fruit
• And the following usage
• Apple a new Apple()
• System.out.println(a.getName()) // Apple
  inherits Fruits // getName method

9
Object class

• If we do not use an extends clause in a class
  declaration, the super-class of that declared
  class is the class Object.
• In Java, every class (except the Object class)
  has a super-class.
• class C1 ...
• is equivalent to
• class C1 extends Object ...
• This means that Object class will be on the top
  of the class hierarchy. In other words, the class
  hierarchy in Java is a tree, and its root node is
  the Object class.

10
Constructors of Sub-Classes

• The initialization of the fields of a sub-class
  consists of two phases
• The initialization of the inherited fields
• The initialization of the fields that are
  declared in that sub-class.
• One of the constructors of the super-class must
  be invoked to initialize the fields inherited
  from the super-class.
• One of the constructors of the super-class must
  be invoked explicitly Otherwise the no-argument
  version of the constructor of the super-class
  will be automatically invoked.
• This invocation must be the first statement of
  the constructor of the sub-class. (Or one of the
  constructors of the sub-class must invoke another
  version of the constructor of that sub-class).
• How do we call super-class constructors?
  reserved word super

11
Constructors of Sub-Classes (Example)

• class C1
• private int x
• public C1() x 0
• public C1(int x) this.x x
• class C2 extends C1
• private int y
• public C2(int x, int y)
• super(x) the constructor of the super-class
  is explicitly invoked
• this.y y
• public C2(int y)
• this(1,y) Another version of the constructor
  of this class is invoked
• public C2()
• y 2 No-argument version of the constructor
  of the super-class
• is implicitly invoked. super()

12
A Parcel Class

• public class Parcel
• protected double weight
• protected double volume
• protected boolean isAirFreight
• public Parcel(double w, double v, boolean b)
• weight w volume v isAirFreight b
• // shippingCost method uses weight, volume, and
  isAirFreight to // calculate cost of shipping
  this parcel
• public double shippingCost() ...

13
A RectangularParcel Class

• public class RectangularParcel extends Parcel
• private double parcelLength, parcelWidth,
  parcelHeight
• private final int weightPerCubicMeter 677
• public RectangularParcel(double len, double w,
  double h,
• boolean b)
• super(0, 0, b)
• parcelLength len
• parcelWidth w
• parcelHeight h
• volume parcelLength parcelWidth
  parcelHeight
• weight volume weightPerCubicMeter
• ...

Call superclass constructor with weight 0,
volume 0, and boolean value b that was passed
in the constructor call for RectangularParcel
set weight and volume to 0 since we can now
calculate them instead of asking for them in the
constructor.
14
A CylindricalParcel Class

• public class CylindricalParcel extends Parcel
• private double parcelRadius, parcelLength
• public CylindricalParcel(double len, double r)
• super(0, 0, false)
• parcelLength len
• parcelRadius r
• volume parcelLength Math.PI
  Math.pow(parcelRadius, 2)
• weight volume 600
• ...

15
UML Notation for Inheritance
16
Constructors (cont.)

• What happens when no constructor is defined for a
  class?
• ? No-argument constructor will be provided
  implicitly for that class
• class C2 extends C1
• // no constructor is defined
• public C2() // this constructor is
• super() // automatically provided by
  Java.
• If super-class does not have no-argument version
  of the constructor, a compilation error occurs.
• If another constructor of the class is present,
  no-argument version of the constructor will not
  be provided automatically!

17
Order of Initialization Step

1. The fields of the super-class are initialized
   using default values.
2. One of the constructors of the super-class is
   executed.
3. The fields of the extended class (sub-class) are
   initialized using the default values.
4. One of the constructors of the extended class
   (sub-class) is executed.

18
Order of Initialization Step (cont.)

• class C1
• private int x 1 // executed first
• public C1()
• x 2 // executed second
• class C2 extends C1
• private int y 3 // executed third
• public C2()
• super()
• y 4 // executed fourth

19
Class Hierarchies

• Multiple classes can be derived from a single
  parent.
• Inheritance relations develop into a class
  hierarchy.
• C1
• C2 C3 C4
• C5 C6 C7
• Parent classes should carry most general
  properties.
• Child classes should carry most specific
  properties.

Notice All subclasses (except Object) have
exactly one parent. There is no multiple
inheritance in Java.
20
Object Class

• class C1 ...
• class C2 extends C1 ...
• class C3 extends C1 ...
• class C4 extends C2 ...
• class C5 extends C2 ...
• class C6 ...
• class C7 extends C6 ... Object
• C1 C6
• C2 C3 C7
• C4 C5