Interfaces are the reference types in Java. They are used in Java to define contracts among classes. Any class that implements that interface has to adhere to the contract that the interface defines.

For example, we have an interface car as follows that consists of three abstract methods:

public interface Car { 
   void shape(); 
   void price(); 
   void color(); 
} 

Any class that implements this interface has to implement all the abstract methods of this interface unless it is an abstract class. Interfaces can only be implemented or extended by other interfaces, they cannot be instantiated.

Prior to Java 8, interfaces consisted only of abstract methods and final variables. In Java 8, interfaces may contain default and static methods as well.