In Java, an interface is a reference type that is used to specify a set of methods that a class must implement. In other words, an interface defines a contract that a class must adhere to. However, an interface does not provide any implementation for the methods it defines—that is the responsibility of the class that implements the interface. Interfaces are often used in Java to achieve polymorphism, which is the ability to execute different code depending on the type of object being dealt with.   For example, a method could be written that takes an Object as a parameter. This method could then be invoked with different types of objects, and each time, different code would be executed depending on the actual type of object that was passed in. By using interfaces, Java developers can create flexible and reusable code that can be adapted to work with new types of objects without having to make any changes to the code itself.