Question

Which of the below is true about interfaces? Pick one of the choices interface can contain constructors a class can implement just one interface an interface can declare public and protected method only an interface cannot have instance variables

Answer 1

Answer:

An interface cannot have instance variables is true about interfaces.

Explanation:

An interface is similar to a class, but the main difference is that it can have only declaration and the implementation of the functions and procedures will be given by the class which is implementing the interface.  

So the problem of multiple inheritances is solved by “Interface”. That is a class can implement more than one interface. Since interface does not do any function except for declaration it cannot have any constructor and also an instance.

Answer 2

Answer:

Concept:

• In the Java programming language, an interface is referred to as an abstract type that specifies a class's behaviour. In Java, a class's blueprint is called an interface.

• An interface is similar to a class, but the key distinction is that it can just contain a declaration; the class that implements the interface will provide the methods and procedures.

Explanation:

• Java uses the interface as a tool to achieve abstraction. The Java interface can only include abstract methods where method bodies are not allowed.
• In Java, it is used to achieve multiple inheritance and abstraction. In other words, interfaces are capable of containing abstract methods and variables. There can be no method body.
• The IS-A relationship is also represented by the Java interface.
• Static constants and abstract methods are components of a Java interface.

•    Interface cannot have constructors. It is possible to have only public, static, final variables and public, abstract, methods in interfaces.

• A class may simultaneously implement multiple interfaces. A class can implement numerous interfaces but can only extend one other class. Similar to how a class can extend another class, an interface can extend another interface.

• The protected members can typically be accessed within the same class or a class that inherits it. However, we shall implement the interface; we do not inherit it. As a result, an interface's members cannot be protected.        

•  By default, variables declared in an interface are public, static, and final. Therefore, interfaces can be used to create constants.  Instance variables for interface are not declared because they are public, static, and final (by default).          

Hence, the statement "only an interface cannot have instance variables"   is true.                          

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