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12.5 INTERFACE IN JAVA
              Java does not support multiple inheritance. In order to implement the same, we use the concept of interface. Interface
              in Java is a blue print of a class as it specifies what to do, but does not specify how to do. It is a mechanism to achieve
              abstraction by hiding certain features and revealing only some important details.

              The following are the properties of an Interface.
              •  Interface in Java is declared by the keyword Interface
              •  The methods are all public abstract methods. However, Java 8 edition also allows default and static methods in interface.

              •  The data members are public, static final by default.
              •  A derived class can implement multiple interfaces. However, it must implement all it’s abstract methods.
              •  Interface cannot be instantiated.
              •  An interface has no constructors as it cannot create objects.

              Implements Keyword
              The abstract methods of interface are implemented by the derived class using the keyword implements.

              Let us do a simple programs to understand this concept well.


                Program 10     The interfaces Circle and Square are defined as follows:
                               Interface name                 :  Square
                               Data Member
                               int s                          :  Stores the length of the side
                               Member Methods
                               void sqArea()                  :  Calculates and prints the area of square
                               Interface name                 :  Circle
                               Data Member
                               double r                       :  Stores radius
                               Member Methods
                               void cirArea()                 :   Calculates and prints the area of circle
                               A derived class Area is defined to calculate and print the areas of square and circle as follows:
                               Class  name                    :  Area
                               Member Methods
                               void cirArea()                 :  Calculates and prints the area of circle
                               void sqArea()                  :  Calculates and prints the area of square

                 1       interface Square
                 2       { public static final int s=15;

                 3         public void sqArea();
                 4       }

                 5       interface Circle
                 6       { public static final double r=7.5;

                 7         public void cirArea();
                 8       }

                 9       class Area implements Square,Circle // multiple inheritance



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