Java extends vs implements: Key Differences, Diamond Problem, and Interface Rules

In Java, the 'extends' keyword is used for class-to-class or interface-to-interface inheritance, while 'implements' is used when a class fulfills an interface contract. Java restricts a class from extending multiple classes to avoid the Diamond Problem, where ambiguity arises if two parent classes define the same method. However, a class can implement multiple interfaces simultaneously, since interfaces define contracts rather than concrete implementations. An interface can also extend multiple other interfaces, enabling flexible multiple inheritance at the contract level. These concepts are foundational to Java's inheritance model and are widely applied in enterprise frameworks such as Spring Boot and Hibernate.