Java Interfaces Quick Kit: 20 flashcards
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1. What is an **interface** in Java?
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A blueprint of methods that a class promises to implement, defining *what* a class can do, not *how*.
2. What do interfaces declare?
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Method **signatures** (names, parameters, return types) without bodies (prior to Java 8+).
3. Which keyword is used for a class to adopt an interface?
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`implements`
4. What must a class do when it implements an interface?
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Provide bodies for all its abstract methods.
5. Can a Java class implement multiple interfaces?
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Yes, a class can implement multiple interfaces.
6. How does implementing multiple interfaces relate to multiple inheritance in Java?
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It allows Java to achieve a form of multiple inheritance for behavior, as Java classes cannot extend multiple classes.
7. What are interface variables implicitly?
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`public static final` (constants).
8. What are interface methods implicitly (before Java 8)?
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`public abstract`.
9. What types of methods can interfaces have since Java 8?
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`default` and `static` methods with bodies.
10. What types of methods can interfaces have since Java 9?
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`private` helper methods, in addition to `default` and `static`.
11. Can you instantiate an interface directly?
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No, you cannot. You create objects of classes that implement the interface.
12. In the example `interface Animal { void sound(); }`, what kind of method is `sound()`?
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An abstract method.
13. In the example `interface Animal { default void sleep() { ... } }`, what kind of method is `sleep()`?
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A default method (introduced in Java 8+).
14. What does `Animal a = new Dog();` demonstrate?
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**Polymorphism**, where the variable type is the interface and the object is the concrete class.
15. Can an interface have constructors?
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No, interfaces cannot have constructors.
16. Can an abstract class have constructors?
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Yes, abstract classes can have constructors.
17. What kind of fields can an interface have?
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Only constants (`public static final`).
18. What kind of fields can an abstract class have?
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Any type of fields (constants, instance variables, etc.).
19. When should you generally use an interface over an abstract class?
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When defining a shared capability or contract that multiple unrelated classes might implement.
20. When should you generally use an abstract class over an interface?
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When defining a shared partial implementation for related classes that share a common base type.
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