![]() It can also be declared with an empty body (i.e. That is, when we implement an interface method, it must be declared as public.Ħ. We cannot reduce the visibility of an interface method while overriding. A class that implements an interface, must provide its own implementations of all the methods defined in the interface.ĥ. If we declare a variable in an interface, it must be initialized at the time of declaration.Ĥ. An interface may not be declared with final keyword.ģ. But we can create a reference to an interface that can point to an object of any of its derived types implementing it.Ģ. An interface cannot be instantiated directly. Here are some key points for defining an interface in java that must be kept in mind. A Java class can implement any number of interfaces by using keyword implements.Ĩ. Interface can extend an interface and can also extend multiple interfaces. Therefore, they are just public constants. So, we cannot change their value by implementing class once they are initialized.Ħ. The interface is the only mechanism that allows achieving multiple inheritance in java.ħ. So, there is no need to write abstract or public modifiers before them.Ĥ. The fields (data members) declared in an interface are by default public, static, and final. All the (non-default) methods declared in the interface are by default abstract and public. It can contain three types of methods: abstract, default, and static methods.ģ. It also represents the Is-A relationship.Ģ. Interface provides pure abstraction in java. There are the following features of an interface in Java. We can also declare default methods and static methods inside interfaces.ĭ) From Java 9 onwards, an interface can also declare private methods. From Java 8 onwards, it is also possible to add a default implementation to an interface method.Ĭ) To support lambda functions, Java 8 has added a new feature to interface. ![]() An interface can only declare abstract methods.ī) Java 8 changed this rule. We cannot change their value once they initialized.Ī) Earlier to Java 8, an interface could not define any implementation whatsoever. Therefore, all the variables declared in an interface are considered as public, static, and final by default and acts like constant. Look at the below figure to understand better. Moreover, it also adds public, static, and final keywords before interface variables. Java compiler automatically adds public and abstract keywords before to all interface methods. The variable x declared in MyInterface is like a simple variable. Void m1() // public and abstract keywords invisibly present.Īs you can see in the above example, both methods m1() and m2() defined in interface are declared with no body and do not have public or abstract modifiers present. Int x = 10 // public static final keyword invisibly present. ![]() Let’s understand the declaration of an interface with the help of an example. declare methods that abstract by default.īefore interface keyword, we can specify access modifiers such as public, or default with abstract keyword. It has the following general form: Syntax: It is declared by using the keyword interface followed by interface name. In Java, an interface is declared syntactically much like a class. By using interfaces, we can achieve the functionality of multiple inheritance. It is used to achieve full abstraction in java.ĥ. Programmers use interface to customize features of software differently for different objects.Ĥ. In other words, we can provide interface methods to the third-party vendors for their implementation.įor example, HDFC bank can expose methods or interfaces to various shopping carts.ģ. Using interfaces is the best way to expose our project’s API to some other projects. In industry, architect-level people create interfaces, and then it is given to developers for writing classes by implementing interfaces provided.Ģ. There are mainly five reasons or purposes of using an interface in Java. Since the implementation classes will have all the methods with a body, it is possible to create an instance of implementation classes. A class can implement any number of interfaces in Java.Įvery implementation class can have its own implementation for abstract methods specified in the interface as shown in the below figure. ![]() Once an interface is defined, we can create any number of separate classes and can provide their own implementation for all the abstract methods defined by an interface.Ī class that implements an interface is called implementation class. So, it is not compulsory to write abstract keyword with an interface. It is used to achieve complete abstraction.Įvery interface in java is abstract by default. In other words, an interface is a collection of abstract methods and constants (i.e. An interface in Java is syntactically similar to a class but can have only abstract methods declaration and constants as members.
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