What is the purpose of the `interface` keyword in Java?

The purpose of the interface keyword in Java is to define a contract that classes can implement. An interface establishes a set of methods that any implementing class must provide, without specifying how these methods should be executed. This allows for a form of abstraction and promotes a strong separation between the definition of behavior and its implementation.

By using interfaces, Java supports multiple inheritance of type, allowing a class to implement multiple interfaces, thereby enabling polymorphism. This means that any class implementing the same interface can be treated interchangeably, which is particularly useful in designing consistent APIs and enhancing code reusability.

In contrast, the other listed options do not accurately represent the functionality of interfaces. An abstract class can define some methods with implementation, while an interface cannot. Creating a separate namespace for methods does not reflect the fundamental role of interfaces, which center on method declarations. Lastly, indicating that a class is abstract does not correctly encapsulate the purpose of an interface, as while abstract classes may define some methods, interfaces exclusively define method signatures.