Java – Interview Question and Answers on Interface

In this article, we will cover some of the interview questions with their justification on Java interface

These are most frequently asked interview question from OOPS concepts

Note: All justification done w.r.t both version i.e.;

• Till Java 7 version
• Post Java 8 release

Q) What is Interface in Java? OR Explain Interface in Java ?

Till Java 7 version,

• An interface in java is a pure abstract class which means all methods are abstract and variables are constants
• By default, all methods inside interface are public & abstract and variables are public, static & final
• Interface is a mean to achieve full abstraction in Java

Post Java 8 release,

• Interface can contain default and static methods, in addition to abstract methods
• Though it looks to similar to abstract classes but they are actually different in many ways
• Read more about Java 8 default and static methods in Interface in detail

Q) What type of variables can be defined in an interface ?

Be default, any variables declared inside interface is,

• public
• static
• final

Note: even when not declared explicitly inside interface

Which means these implicit modifier make all variables inside interface is CONSTANT

Q) Can we instantiate an interface ?

• No, we cannot instantiate interface
• Since, interface doesn’t have constructor and contains only abstract methods and constants therefore we don’t need to instantiate
• Instead implementing classes provide concrete implementation for these abstract methods and constants can be accessed using <interfaceName>.<variableName>

Post Java 8 release,

• In addition to abstract methods and constants, Java 8 introduced default and static methods
• default methods can be accessed using implementing class’s reference object
• static methods can be accessed using interface name i.e.; <interfaceName>.<staticMethodName>
• Read more about Java 8 default and static methods in Interface in detail

Q) Can we create an object for an interface ?

• No, we cannot create an object of an interface

Q) What happens, if we define concrete method inside Interface ?

Till Java 7 version,

• Compilation error will be thrown stating below reason
• Compile-time error: Abstract methods do not specify a body

Post Java 8 release,

• Concrete method (method with body) are allowed with default or static keyword prefixed, as shown below
• Otherwise, even in Java 8 compilation error will be thrown as seen below screen capture

Q) Can a method inside an interface be declared as final ?

• By default, methods declared inside interface are public & abstract even if we don’t declare it explicitly compiler adds these modifier during compilation time
• Interface allows only public & abstract modifiers in method declaration
• If final keyword added in method declaration then compiler will throw error as seen in the below screen capture
• Compile-time error: Illegal modifier for the interface method display; only public & abstract are permitted

Post Java 8 release,

• Still, final modifier is not allowed in any of the methods in interface i.e.; abstract, default & static methods
• Compile-time error: Illegal modifier for the interface method display; only public, abstract, default, static and strictfp are permitted

Q) What happens, if we don’t initialize variables inside Interface ?

• Compiler throws error stating final variable needs to be initialized
• As variables defined inside interface are by default public, static & final. So, final variable always needs to be initialized where it is declared
• Compile-time error: The blank final field <fieldname> may not have been initialized

• No change even post Java 8 release

Q) Can we declare members as private or protected modifier inside Interface ?

• Variables (members) defined inside interface are by default public, static & final
• Therefore, no other access-modifier allowed except public
• During compilation process, compiler inserts/adds public, static & final keyword for variables
• These are interface variables and are accessed using interface name
• For example, <interfaceName>.<memberName> from any other class
• No change even post Java 8 release too

Q) How can we access variables defined inside Interface ?

• Members defined inside interface can be accessed using interface name from any other class
• For example, <interfaceName>.<memberName> from any other class
• No change even post Java 8 release too

Q) Can we modify variables defined inside Interface ?

• Since, variables defined inside interface are final therefore we cannot change the value of these variables anywhere (simple OOPS concept)
• If we try to change the value, then compiler throws error
• Compile-time error: The final field <interfaceName>.<fieldName> cannot be assigned
• No change even post Java 8 release too

Q) Can we re-assign a value to a field of interface ?

• Re-assigning fields throws compile-time error as these are final by default
• Compile-time error: The final field <interfaceName>.<fieldName> cannot be assigned

Q) What modifiers are allowed for methods in an interface ?

• Till Java 7 version, only public & abstract are permitted
• Post Java 8 release, only public, abstract, default, static and strictfp are permitted

Q) Is it ok to add “abstract” keyword to interface definition ?

• Yes, we can actually add abstract keyword to interface definition (somewhat similar to abstract classes)

Q) Whether class compiles successfully, if we don’t implement any of the abstract methods from Interface ?

• No, compilation error will be thrown
• If a class implements any interface then it must provide definition or concrete implementation for every abstract method

Post Java 8 release,

• Still, implementing class must provide definition or concrete implementation for every abstract method in interface
• Exceptional being default and static methods; it is okay if we don’t override default method
• Read more about Java 8 default and static methods in Interface in detail

Q) What is the best possible solution, if we don’t want to implement few of the abstract methods from Interface ?

• The best solution is to declare the implementing class as abstract; compilation will succeed
• But next inheriting class (i.e.; extending this class) must provide concrete method implementation or declare again as abstract

Q) Can we reduce the visibility of the methods while implementing interface ?

• By default abstract methods declared inside interface are public
• As per overriding rule, access visibility of the methods can be widened further
• So, it is must to declare overriding methods as public; as no other access visibility is more wider than public
• Read more about Java overriding rules here

Q) Can we declare constructor inside interface ?

• Compilation error will be thrown stating “Interfaces cannot have constructors”
• Compile-time error: Interfaces cannot have constructors

Q) Can interface be final ?

• No, interface cannot be final and compilation error will be thrown
• Compile-time error: Illegal modifier for the interface <interfaceName>; only public and abstract are permitted

Q) Can interface extend any class ?

• No, interface cannot extend any class
• Interface can only extend one or more other interfaces

Q) Can an interface implement any other interface ?

• No, interface cannot implement other interface
• Interface can only extend one or more other interfaces

Q) Can an interface extend another interface ?

• Yes, an interface can extend one or more interfaces

Q) What is marker interface or tagged Interface ?

• An interface with no fields or methods is known as marker interface
• Marker interface are used to indicate or provide essential information to JVM or compiler
• Alternatively, it is referred as tagged interface
• java.io.Serializable or java.lang.Cloneable are example of marker or tagged interface
• Marker interface improves readability in comparison with any other alternatives

Q) Can an interface contain another interface as member ?

• Yes, an interface can contain another interface
• This is referred as Nested interface

Q) What is Nested Interface in Java ?

• An interface declaration contained inside another class or interface is known as Nested interface
• During compilation, compiler inserts/adds static keyword to nested interface

DemoInterfaceA.java

package in.bench.resources.itf.example;

public interface DemoInterfaceA {

	String NAME = "BenchResources.Net";

	// Nested interface inside another interface
	interface NextedItfExample {

	}
}

Q) What if the difference between abstract class and interface ?

• Below table shows some of the differences between Abstract Class & Interface
• Read this article for more information with detailed explanation and exmaples

Related Articles:

• Java Constructor with example
• Default constructor
• Parametrized constructor
• Default constructor v/s Parametrized constructor
• Constructor overloading
• Constructor chaining
• Constructor v/s Methods
• Private Constructor
• Static constructor in Java – A big interview question ?

References:

• http://docs.oracle.com/javase/tutorial/java/IandI/interfaceDef.html
• http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
• http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
• http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html#static
• https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html

Happy Coding !!
Happy Learning !!