What distinguishes a class from an interface in Java?

In Java, a fundamental distinction between a class and an interface lies in the nature of their respective capabilities concerning method implementations. Classes can contain both method declarations and concrete implementations, allowing them to provide the specific behavior or functionality associated with those methods. This means that within a class, you can write the actual code that defines what a method does.

In contrast, interfaces were designed to specify a contract for what methods a class must implement, but they traditionally do not include any method implementations of their own. This characteristic allows different classes to implement the same interface in various ways, promoting flexibility and multiple inheritance of type. Initially, interfaces could only declare methods without any body, thus, they could not provide any concrete behavior themselves.

While newer updates to Java do allow interfaces to include default methods with implementations, this feature is mainly intended to enhance backward compatibility and support functional programming patterns. The primary distinction remains that classes are typically focused on defining specific behaviors while interfaces define a set of methods to be implemented by any class that chooses to adhere to that contract.

Therefore, the key difference highlighted here—that classes can have implementations whereas interfaces traditionally cannot, is crucial for understanding object-oriented design in Java.