Java Interfaces Quick Kit

Question What is an **interface** in Java? tap to flip ↻ Answer A blueprint of methods that a class promises to implement, defining *what* a class can do, not *how*. Question What do interfaces declare? tap to flip ↻ Answer Method **signatures** (names, parameters, return types) without bodies (prior to Java 8+). Question Which keyword is used for a class to adopt an interface? tap to flip ↻ Answer `implements` Question What must a class do when it implements an interface? tap to flip ↻ Answer Provide bodies for all its abstract methods. Question Can a Java class implement multiple interfaces? tap to flip ↻ Answer Yes, a class can implement multiple interfaces. Question How does implementing multiple interfaces relate to multiple inheritance in Java? tap to flip ↻ Answer It allows Java to achieve a form of multiple inheritance for behavior, as Java classes cannot extend multiple classes. Question What are interface variables implicitly? tap to flip ↻ Answer `public static final` (constants). Question What are interface methods implicitly (before Java 8)? tap to flip ↻ Answer `public abstract`. Question What types of methods can interfaces have since Java 8? tap to flip ↻ Answer `default` and `static` methods with bodies. Question What types of methods can interfaces have since Java 9? tap to flip ↻ Answer `private` helper methods, in addition to `default` and `static`. Question Can you instantiate an interface directly? tap to flip ↻ Answer No, you cannot. You create objects of classes that implement the interface. Question In the example `interface Animal { void sound(); }`, what kind of method is `sound()`? tap to flip ↻ Answer An abstract method. Question In the example `interface Animal { default void sleep() { ... } }`, what kind of method is `sleep()`? tap to flip ↻ Answer A default method (introduced in Java 8+). Question What does `Animal a = new Dog();` demonstrate? tap to flip ↻ Answer **Polymorphism**, where the variable type is the interface and the object is the concrete class. Question Can an interface have constructors? tap to flip ↻ Answer No, interfaces cannot have constructors. Question Can an abstract class have constructors? tap to flip ↻ Answer Yes, abstract classes can have constructors. Question What kind of fields can an interface have? tap to flip ↻ Answer Only constants (`public static final`). Question What kind of fields can an abstract class have? tap to flip ↻ Answer Any type of fields (constants, instance variables, etc.). Question When should you generally use an interface over an abstract class? tap to flip ↻ Answer When defining a shared capability or contract that multiple unrelated classes might implement. Question When should you generally use an abstract class over an interface? tap to flip ↻ Answer When defining a shared partial implementation for related classes that share a common base type.