Chap3 inheritance

Inheritance basics
 Java supports inheritance by allowing one
class to incorporate another class into its
declaration
 Done using a keyword “extend”
 Subclass adds to (extends ) the superclass
 A class that is inherited is called a superclass
 The class that does the inheriting is called
subclass
 Hence subclass is a specialized version of
superclass
 It inherits all of the variables and methods

 the triangle class includes all TwoDObject and
adds the field style and a method area() and
method showStyle().
 The triangle ‘s style is stored in style.
 this can be any string that describes the
triangle such as “filled” “outlined”,
“transparent”, “isosceles” or “rounded”.
 The area() method computes and returns the
area of the triangle, showStyle() displays the
triangle style.

 The general form of a class declaration that inherits a
superclass is shown here:
class subclass_name extends superclass_name{
//body of the class
}
 Can mention one superclass for any subclass that you
create.
 Java does not support the inheritance of multiple
super classes into a single subclass
 Allows to create a hierarchy of inheritance in which a
subclass becomes a super class of another subclass.
 No class is superclass of itself.

Advantage of inheritance
 Once you have created a superclass that
defines the attributes common set of objects, it
can be used to create any number of more
specific subclass

Member access and inheritance
 A subclass includes all of the members of its
superclass it cannot access those members of the
superclass that have been declared private
class TwoDshape{
private double width;
private double height;
…..
double area(){
return width*height /2;
}
}

 Program will not compile because the
reference to width and height inside the area()
method causes an access violation.
 Since width and height members are private in
nature, they are accessible only by other
members of their own class.
 Private members are not accessible outside
the class, even subclasses.
 Java uses accessor methods to provide
access to the private members of the class.

Constructors and inheritance
 The constructor for the superclass constructs
the superclass portion of the object,
constructor for the subclass constructs the
subclass part.
 The superclass has no knowledge of access to
any element in a subclass., hence their
construction must be separate.
 The superclass portion of the object is
constructed automatically using a default
constructor.

 Here triangle’s constructor initializes the
members of the TwoD class that it inherits
along with its own style field.
 When both the superclass and the subclass
defines as constructor, the process is more
complicated because both superclass and
subclass constructor must be executed.

Using super to call Superclass
constructors
 A subclass can call a constructor defined by its
superclass by the use of the following form of
super:
 Super(parameter_list);
 Here parameter_list specifies any parameter
needed by the constructor in the superclass
 super() must be the first statement executed
inside a subclass constructor
 Eg: defines a constructor that initializes width
and height varaible in the TwoDShape
program.

 Triangle() calls super() with parameters w and h.
 This causes the TwoDshape() constructor to be
called, which initialize width and height using
these values.
 Triangle no longer initializes these values itself.
 TwoDshape can add functionality about which
existing subclasses have no knowledge, thus
preventing existing code from breaking.
 Any form of constructor defined by the superclass
can be called by super()
 The constructor executed will be one that matches

 When a subclass calls super(), it is calling the
constructor of its immediate superclass.
 Super() always refers to the superclass
immediately above the calling class.
 It is true even in a multilevel hierarchy
 super() must always be the first statement
executed inside a constructor.

Using super to access superclass
members
 Using super we can refer to the members of
the superclass.
 General form is as follows:
 super.member
 Here the member can be either a method or
an instance variable.
 Used when the members of the subclass hide
members by the same in the superclass.

Creating a multilevel hierarchy
 Assume that there are three classes A,B and C
where C is a subclass of B, which in turn is
subclass of A.
 Here class C will inherit all of the traits found in
subclasses.

When are constructors called?
 Constructors are called in the order of
derivation from superclass to subclass.
 Super() must be the first statement executed in
subclass constructor.
 If super() is not used then the default
constructor will be executed.

Superclass references and
subclass objects
 Java is strongly typed language
 type compatibility is strictly enforced
 A reference variable for one class type cannot
normally refer to an object of another class type.
class X{
int a;
x(int i)
{
a= i ;
}
}

class Y{
int a;
Y(int i){
a=i;
}
}
class incompatibletypes{
X objx1=new X(10);
X objx2;
Y y=new Y(15);
objx2=x; // OK, both of same type
objx2=y; //error , not of same type
}

 Class x and y are physically same, it is not
possible to assign an X reference to Y object
they have different types.
 An object can only refer a object of its type.
 A reference variable of superclass can be
assigned a reference to an object of any
subclass derived from that superclass.
 A superclass object can refer to a subclass
object.

class X{
int a;
X (int i)
{
a=i;
}
}
class Y extends X{
int b;
Y(int i, int j){
super(j);
b= i;
}
class SupSub{
public static void main(String
args[]){
X x1=new X(10);
X x2;
Y y=new Y(5,6);
x2=x1;
System.out.println(“x2.a:”+x2.a);
x2=y;
System.out.println(“x2.a:”+x2.a);
X2.a=19; //OK
X2.b=27; //error, x does not have a
and b member
}
}

Method overriding
 When a method in a subclass has the same
return type and signature as a method in its
superclass, then the method in the subclass is
said to be override the method in superclass.
 When an overridden method is called from
within a subclass , it will always refer to the
version of that method defined by the
subclass.
 The version defined by the superclass will be
hidden.

 If the superclass version of an overridden
method has to be accessed then, you can do
so by using super().
 Method overriding occurs only when the
signatures of the two methods are identical.
 If they are not then, the two methods are
simply overloaded.

Overridden methods support
polyorphsim
 Dynamic method dispatch
 Mechanism by which call to an overridden
method is resolved at run time rather than
compile time
 Dynamic method dispatch is important
because this how java implements run-time
polymorphism.
 Is superclass reference variable can refer to a
subclass objects.
 Java determines which version of that method
to execute based upon the type of object being
referred to at a time the call occurs.

 When different types of objects are referred to,
different versions of overridden method will be
called.
 It is the type of the objects being referred to that
determines which version of an overridden
method will be executed.
 If a superclass contains a method that is
overridden by a subclass, then when different
types of objects are referred to through a
superclass reference variable , different versions
of the method are executed.

Why overridden methods?
 It allows to support polymorphism in JAVA
 Polymorphism allows a general class to specify
methods that will be common to all of its
derivatives, while allowing subclasses to define
the specific implementation of some or all of those
methods.
 It helps to implement “one interface, multiple
methods”
 By combining inheritance with the overridden
methods, a superclass can define the general
form of methods that will be used by all of its
subclasses.

Using Abstract Classes
 A class that determines the nature of methods
that subclasses must implement but does not
itself provide any implementation of one or
more of these methods, such a class is called
abstract class
 Defined by specifying the abstract type
modifier.
 It contains no body and is there not
implemented by the superclass
 The subclass must override it
 abstract type name(parameter_list);

 No body is present, the abstract modifier can
be used only on normal methods.
 Cannot be applied to static methods or to
constructors.
 A class that contains one or more abstract
method should be declared as abstract class
with the abstract modifier.
 Since abstract class does not define a
complete implementation, there can be no
objects of an abstract class.

 Attempting to create an object of an abstract
class, will lead to a compile time error.
 When a subclass inherits an abstract class, it
should implement all of the abstract method in
superclass
 If it does not, then the subclass must be
specified as abstract.
 Hence abstract is inherited until complete
implementation of the methods are achieved.

Using final
 To prevent a method from overridden, specify
that method with a keyword “final” at the start
of its declaration
 Methods declared as final cannot be
overridden
 example

Final prevents inheritance
 By having a class declared as final, inheritance can be
prevented
 By declaring a class as final, implicitly it declares al
the methods as final too.
 Using abstract and final together is illegal because,
abstract depends on the subclass for its
implementations and final avoids inheritance.
final class A{
//….
}
class B extends A{ //error cant have subclass of A
//….
}

Using final with data members
 final can be applied on data members of the
class.
 If we have members of class as final, the value
cannot be changed throughout the lifetime of
the program..
 Initial values can be given to the variable
 Used to create a named constant.

Object class
 Java defines a class called Object that is an
implicit superclass of all other classes
 All other classes are subclasses of Object
 This means that a reference variable of type
Object can refer to an object of any other
class.
 Object defines the following methods, which
are available for all objects

Method purpose
Object clone() Creates a new object that is same as the object
being cloned.
boolean equals(Object
obj)
Determines whether one object is equal to
another.
void finalize() Called before unused object to be recycled
int hashCode() Returns the hash code associated with the
invoking object
void notify() Resumes execution of a thread waiting on the
invoking object
void wait() Waits on another thread of execution
String toString() Returns a string that describes the object

Chap3 inheritance

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Chap3 inheritance