How to use "Interfaces" in Java

Java uses "Interface" to implement multiple inheritance.

An interface is just as Java Class, but it only has static constants and abstract method.

Syntax for declaring interface

interface {
//methods
}

To use a interface in your class , append the keyword "implements" after your class name followed by the interface name

Ex

class Dog implements Pet

interface RidableAnimal extends Animal, Vehicle

What is an Interface and why it is required?

Assignment: To learn interfaces

Step 1) Copy following code into an editor.

interface Pet{
  public void test();
}
class Dog implements Pet{
   public void test(){
     System.out.println("Interface Method Implemented");
  }
   public static void main(String args[]){
     Pet p = new Dog();
     p.test();
  }
}

Step 2) Save , Compile & Run the code. Observe the Output.

Java interface Example

To understand the concept of Java Interface better, let see an example. The class "Media Player". Media Player has two subclasses, CD player, and DVD player. Each having its unique implementation method to play music.

Another class "Combo drive" is inheriting both CD and DVD (see image below). Which play method should it inherit? This may cause serious design issues. And hence, Java does not allow multiple inheritance.

Now let's go quickly back to our previous example of Dog.

Suppose you have a requirement where class "dog" inheriting class "animal" and "Pet" (see image below). But you cannot extend two classes in Java. So what would you do? The solution is Interface.

The rule book for interface says,

• An interface is 100% abstract class and has only abstract methods.
• Class can implement any number of interfaces.

Class Dog can extend to class "Animal" and implement interface as "Pet".

Must know facts about Interfaces

Let's discover some more facts about "Interface and Class."

• Difference between Class and Interface –
  • In class, you can instantiate variable and create an object. In an interface, you can't.
  • The interface cannot contain concrete(with implementation) methods.
  • The access specifiers used with classes are private, protected and public. While in Interface only one specifier is used- Public.
• A Java class can implement multiple Java Interfaces. It is necessary that the class must implement all the methods declared in the interfaces.
• Class should override all the abstract methods declared in the interface
• The interface allows sending a message to an object without concerning which classes it belongs.
• Class needs to provide functionality for the methods declared in the interface.
• All methods in an interface are implicitly public and abstract
• An interface cannot be instantiated
• An interface reference can point to objects of its implementing classes
• An interface can extend from one or many interfaces. Class can extend only one class but implement any number of interfaces
• An interface cannot implement another Interface. It has to extend another interface if needed.
• An interface which is declared inside another interface is referred as nested interface
• At the time of declaration, interface variable must be initialized. Otherwise, the compiler will throw an error.
• The class cannot implement two interfaces in java that have methods with same name but different return type.

Point to Ponder

While designing, how do you choose between Abstract & Interface ?

• Use an abstract class when a template needs to be defined for a group of subclasses
• Use an interface when a role needs to be defined for other classes, regardless of the inheritance tree of these classes

Summary:

• The class which implements the interface needs to provide functionality for the methods declared in the interface
• All methods in an interface are implicitly public and abstract
• An interface cannot be instantiated
• An interface reference can point to objects of its implementing classes
• An interface can extend from one or many interfaces. A class can extend only one class but implement any number of interfaces