Java Interface Interview Questions
Top 10 Questions about Java Interfaces for Interviews
Java interface interview questions are a series of questions that are asked to assess a candidate's understanding and knowledge about Java interfaces. These questions cover various aspects of interfaces in Java, such as their definition, purpose, implementation, usage, and advantages. Interviewers may also ask questions related to inheritance, abstract classes, and multiple interfaces. These questions not only assess a candidate's technical skills but also their problem-solving and critical thinking abilities. It is essential for a candidate to be well-versed with Java interfaces and their various features to crack interface interview questions successfully.
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1 - What is an interface in Java?
An interface in Java is a collection of abstract methods that do not have any implementation. It is used to define a set of methods that a class must implement in order to be considered as implementing the interface. It acts as a contract between the class and the user, specifying the methods that must be implemented and their respective parameters and return types.
2) Can an interface contain variables?
Yes, an interface can contain variables, but they must be declared as final and must have a value assigned to them. These variables act as constants and can be used by the implementing classes.
3) What is the difference between an interface and an abstract class?
An interface can only contain abstract methods and constants, whereas an abstract class can have both abstract and non-abstract methods. Additionally, a class can implement multiple interfaces, but can only inherit from one abstract class.
4) How do you implement an interface in Java?
To implement an interface in Java, a class must use the `implements` keyword followed by the name of the interface. The class must then provide implementations for all the methods declared in the interface.
5) Can an interface extend another interface?
Yes, an interface can extend multiple interfaces using the `extends` keyword. This allows the sub-interface to inherit all the methods and constants from its parent interface.
6) What is marker or tagged interface?
A marker or tagged interface is an interface that does not contain any methods. It acts as a special type of interface to indicate the implementing class has a specific capability or to group related classes together.
7) Can an interface have private methods?
No, an interface in Java cannot have private methods. All the methods declared in an interface are by default public and must be implemented as public in the implementing class.
8) What is a default method in Java interface?
A default method in a Java interface is a method that has a default implementation, but can be overridden by the implementing class if needed. These methods are marked with the `default` keyword and can be called from the implementing class.
9) Why do we use interfaces in Java?
Interfaces are used in Java to achieve abstraction and to provide a contract for classes to implement. They also allow for multiple inheritance, allowing a class to implement multiple interfaces.
10) Can an interface have a constructor?
No, an interface in Java cannot have a constructor as it cannot be instantiated. However, a default constructor is created by the compiler if there are any static variables in the interface.
11) What is the purpose of the functional interface in Java 8?
The purpose of a functional interface in Java 8 is to indicate that the interface only contains one abstract method, also known as a functional method. It allows the interface to be used as a lambda expression, providing more flexibility in programming.
12) Can a class extend multiple interfaces in Java?
No, a class in Java can only inherit from one class, but it can implement multiple interfaces. This is known as multiple inheritance through interfaces.
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13) What is an interface with no methods known as?
An interface with no methods is known as a marker or tagged interface. It is used to indicate the implementing class has a specific capability or to group related classes together.
14) What is the difference between an interface and a class in Java?
An interface can only contain abstract methods and constants, whereas a class can have non-abstract methods and can be instantiated. Additionally, a class can extend only one class, but can implement multiple interfaces.
15) Can we create an instance of an interface in Java?
No, an interface in Java cannot be instantiated as it does not have a constructor. However, a reference variable of an interface type can be created and can refer to an instance of the implementing class.
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