Introduction

Interface is nothing but 100% pure abstract class, it has static constraints and abstract methods, In Java interface is the mechanism to achieve abstraction and multiple inheritance. interface also represents the IS-A relationship, it cannot be instantiated just like the abstract class.

Since Java 8, we can have default and static method in an interface

Since Java 9, we can have private methods also in an interface.

Why should we use interface

There are mainly three reasons to use interface.

1. It is used to achieve abstraction
2. We can support the functionality of multiple inheritance
3. It can be used to achieve loose coupling

Interface declaration and implementation

we can declare an interface by using interface keyword and implement by using implements keyword

If a class implements an interface compulsory we have to provide implementation for every method of that interface otherwise we have to declare class as abstract, violation leads to compile time error. whenever we are implementing an interface method compulsory it should be declare as public otherwise we will get compile time error.

extends vs implement

• A class can extend only one class at a time
• A class implement any number of interface at a time
• An interface can extend any number of interfaces at a time

Interface method

whether we are declaring or not, every interface method is by-default, public and abstract

interface Itf {

void m1()

}

interface method specifies requirements but not implementation, hence the following method declarations are equal inside interface.

void m1();

public void m1();

abstract void m1();

public abstract void m1();

as every interface method is by-default, public and abstract the following modifiers are not applicable for interface method. private, protected, <default>, final, static, strictfp, synchronized, native

Interface variable

An interface can contain variable the main purpose of these variable is to specify constant at requirement level. every interface variable is always public, static and final whether we are declaring or not

interface Itf {

int x = 11;

}

public - To make this variable available for every implementation class.

static - without existing object also implementation class can access this variable

final - implementation class can access this variable but can't modify

For the interface variable we must perform initialization at the time of declaration only otherwise we will get compile time error.

Interface naming conflicts -

method naming conflict

case 1: If two interface contains method with same signature and same return type in the implementation class we can provide implementation for only one method

case 2: If two interface contains method with same name but different arguments then in the implementation class we have to provide implementation for both methods and these methods are considered as overloaded methods.

case 3: If two interface contains method with same signature but different return types then it is impossible to implement bot interface at a time, we can't write any java class which implements both interface simultaneously.

It is possible a java class can implement any number of interface simultaneously, except two interface contain method with same signature but different return types.

Marker interface

If an interface don't contain any method and by implementing that interface if our objects will get ability such type of interface known as marker interface, tag interface, ability interface.

eg. serializable, clonable, RandomAccess, Single thread mode - these interfaces marked from some ability.

• By implementing serializable interface we can send object across the network and we can save state of object to a file.
• By implementing clonable interface our object will be in a position to provide exactly duplicate object.

Q. Marker interface don't contain any method then how the object gets special ability -

A. JVM is responsible to provide required ability to marker interface

Q. Why JVM provides required ability to marker interface -

A. To reduce complexity of the programming

Q. Is it possible to create our own marker interface -

A. Yes, but customization of JVM is required

Adapter class

Adapter class is a simple java class that implements an interface, an interface only with empty implementation.

• If we implement an interface directly then compulsory we have to provide implementation for every method of that interface.
• If we extends adapter class instead of implementation interface directly then we have to provide implementation of only for required method not for all method, this approach reduce length of the code and improves readability.

Abstract class vs Concrete class vs interface

• If we don't know anything about implementation just we have requirements specification, then we should go for interface eg. Servlet
• If we are talking about implementation but not completely (partially implementation) then we should go for abstract class. eg. Generic Servlet, HttpServlet
• If we are talking about complete implementation and ready to provide service then we should go for concrete class. eg. programmer level Servlet

Difference between class and interface

Interface :

• If we don't know anything about implementation just we have requirements specification, then we should go for interface eg. Servlet
• Every method present inside interface is by default public and abstract
• The following modifiers are not allowed for interface methods strictfp, protected, static, native, private, final, synchronized
• Every variable present inside interface is public, static and final by default
• For the interface variable compulsory we have to perform initialization at the time of declaration
• Inside interface we can't take instance and static blocks
• Inside interface we can't take constructor.

abstract :

• If we are talking about implementation but not completely (partially implementation) then we should go for abstract class. eg. Generic Servlet, HttpServlet
• Every method present inside abstract class need not to be public and abstract, we can take concrete method also.
• There are no restrictions for abstract class method modifier, we can use any modifiers
• Abstract class variable need not to be public, static and final
• There are no restriction for abstract class variable modifier
• Inside abstract class we can take static block and instance blocks
• Inside abstract class we can take constructor

Q. Inside abstract class we can take constructor but we can't create object of abstract class, what is the need -

A. abstract class constructor will be executed whenever we create child class object to perform initialization of parent class instance variable at parent level only, and this constructor meant for child object creation only.