Java Interview Questions on Interface

Q1.What is an interface in Java and how is it different from a class?

Ans: An interface is a collection of abstract methods, which means they do not have a body. You can only implement them in a class. A class that implements an interface requires concrete implementation for all the interface’s methods. A class can have both abstract and concrete methods and can also have fields and constructors. It can also inherit from a single superclass, in which case it can implement multiple interfaces.

Some of the differences between an interface and a class are:

• An interface cannot be instantiated, while a class can.

• An interface can only have public and abstract methods, while a class can have any access modifier and any type of methods.

• An interface can have static or default methods, which provide a common behavior for all implementing classes, while a class cannot.

• An interface can extend multiple interfaces, while a class can only extend one class.

• An interface can be used as a type for polymorphism, while a class can be used as a type for inheritance.

Q2.How do you define methods in an interface?

In an interface, you can define methods by using the interface keyword, followed by the method name, return type and parameters. It is unnecessary to define the method body. The class that implements the interface automatically provides the method body. All methods that you define in an interface are implicitly public and abstract. There is no requirement to manually state that while defining a method.

For example, you can define an interface named Animal with two methods: eat and sleep.

Q3.How can you implement multiple interfaces in a single class?

Q4.What are the benefits of using interfaces in Java?

Q5.Can an interface extend another interface in Java?

Q6.Can an interface have static or default methods in Java?

Q7.Can an interface have constructors or fields in Java?

Q8.Can an interface be declared as final or abstract in Java?

Q9.How do you achieve multiple inheritance in Java using interfaces?

Q10.What is a functional interface in Java and how can you use it with lambda expressions?

Q11.What is the difference between a marker interface and a tagged interface in Java?

Q12.What are some of the common marker interfaces in Java?

Q13.How do you create your own marker interface in Java?

Q14.What is the difference between Comparable and Comparator interfaces in Java?

Q15.How do you sort a list of objects using Comparable or Comparator interfaces in Java?

Q16.What is the difference between Iterable and Iterator interfaces in Java?

Q17.How do you use the enhanced for loop with Iterable and Iterator interfaces in Java?

Q18.What is the difference between Collection and Collections interfaces in Java?

Q19.What are some of the subinterfaces of Collection interface in Java?

Q20.What are some of the methods of Collections interface in Java?