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Interfaces in java

The document discusses Java interfaces. It defines an interface as a reference type that contains abstract methods that classes implement, inheriting the methods. Interfaces can contain constants, default methods, static methods, and nested types in addition to abstract methods. Interfaces are similar to classes but cannot be instantiated and all methods are abstract. Classes implement interfaces to achieve behaviors defined in the interface. The document outlines properties of interfaces like being implicitly abstract and methods being public. It provides examples of declaring interfaces and implementing and extending interfaces.

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JAVA INTERFACES SHIV KUMAR BCA 5TH SEM. 1822562
WHAT IS AN INTERFACE? • An interface is a reference type in Java. It is similar to class. • It is a collection of abstract methods. • A class implements an interface, thereby inheriting the abstract methods of the interface. • Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods. • Writing an interface is similar to writing a class. But a class describes the attributes and behaviours of an object. And an interface contains behaviours that a class implements.
WHY AND WHEN TO USE INTERFACES? • To achieve security - Hide certain details and only show the important details of an object (interface). • Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. • When a class implements an interface, we can think of the class as signing a contract, agreeing to perform the specific behaviours of the interface. If a class does not perform all the behaviours of the interface, the class must declare itself as abstract.
AN INTERFACE IS SIMILAR TO A CLASS IN THE FOLLOWING WAYS − • An interface can contain any number of methods. • An interface is written in a file with a .java extension, with the name of the interface matching the name of the file. • The byte code of an interface appears in a .class file. • Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.
HOWEVER, AN INTERFACE IS DIFFERENT FROM A CLASS IN SEVERAL WAYS, INCLUDING − • We cannot instantiate an interface. • An interface does not contain any constructors. • All of the methods in an interface are abstract. • An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final. • An interface is not extended by a class; it is implemented by a class. • An interface can extend multiple interfaces.
INTERFACES HAVE THE FOLLOWING PROPERTIES − • An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface. • Methods in an interface are implicitly public.
DECLARING INTERFACES • The interface keyword is used to declare an interface. • Following is EXAMPLE of an interface declaration −
IMPLEMENTING INTERFACES • A class uses the implements keyword to implement an interface. • OUTPUT: The pig says: wee wee Zzz
EXTENDING INTERFACES • An interface can extend another interface in the same way that a class can extend another class. • The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface. • The following Sports interface is extended by Hockey and Football interfaces.
EXTENDING MULTIPLE INTERFACES • A Java class can only extend one parent class. Multiple inheritance is not allowed. • Interfaces are not classes, however, and an interface can extend more than one parent interface. • The extends keyword is used once, and the parent interfaces are declared in a comma-separated list. • For example, if the Hockey interface extended both Sports and Event, it would be declared as −
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