Java Class Design and OOP Flashcards

Which of the following correctly defines an enum with a constructor in Java?

Answer: enum Color { RED(1), GREEN(2), BLUE(3); private int val; Color(int val){ this.val=val; } }

Enum constructors are implicitly private and are called with arguments per constant; fields hold per-constant data.

What is the result when an abstract class implements an interface but does not implement all of its methods?

Answer: The unimplemented methods become abstract in the abstract class

An abstract class is not required to implement all interface methods; unimplemented methods remain abstract and must be implemented by the first concrete subclass.

Which of the following is an example of the 'is-a' relationship best implemented with inheritance?

Answer: Dog is-a Animal

Inheritance models 'is-a' relationships; a Dog IS an Animal, making inheritance the appropriate design.

When using `@Override`, what happens if the method signature does not match any method in the superclass or interface?

Answer: A compilation error occurs

`@Override` causes a compile-time error if the annotated method does not actually override a superclass or interface method.

What is the output of this code? ```java interface Flyable { default String move() { return "fly"; } } interface Swimmable { default String move() { return "swim"; } } class Duck implements Flyable, Swimmable { public String move() { return Flyable.super.move(); } } System.out.println(new Duck().move()); ```

Answer: fly

Duck resolves the ambiguity by explicitly calling `Flyable.super.move()`, which returns "fly".

Which statement about sealed classes (Java 17+) is correct?

Answer: Sealed classes restrict which classes can extend them using the permits clause

A sealed class uses the `permits` clause to explicitly list which classes are allowed to extend it.

What does it mean for a class to be 'immutable' in Java?

Answer: Its state cannot be changed after construction

An immutable class's state cannot be modified after its object is created — typically achieved with final fields and no setters.