Understanding Abstract Classes and Interfaces

In Java, which keyword is used to declare an abstract class?

• `interface`

• `abstract`

• `virtual`

• `extends`

Which of the following statements correctly describes an abstract class?

• An abstract class can be instantiated directly using the `new` keyword

• An abstract class cannot contain any concrete (non-abstract) methods

• An abstract class may contain both abstract and non-abstract methods

• An abstract class must implement all methods of an interface it references

Which of the following is a correct characteristic of an interface in Java?

• An interface can have instance variables with any access modifier

• All methods in an interface are implicitly `public` and `abstract` (prior to Java 8)

• An interface can be instantiated directly

• An interface can extend an abstract class

In Java, a class that implements an interface must:

• Declare itself as abstract

• Provide implementations for all abstract methods of the interface, unless the class is abstract

• Use the `extends` keyword instead of `implements`

• Override only the methods it chooses to implement

Which of the following correctly identifies a key difference between an abstract class and an interface in Java?

• An abstract class supports multiple inheritance, while an interface does not

• An interface can have constructors, while an abstract class cannot

• A class can implement multiple interfaces but can only extend one abstract class

• An abstract class cannot have any implemented methods, while an interface can

Which of the following is a valid use case for an abstract class?

• When you want to define a contract that unrelated classes can implement

• When you want to share common code and state among closely related classes

• When you need a class to support multiple inheritance

• When you want to define only method signatures without any implementation

Consider the following Java code: ```java interface Drawable { void draw(); } class Circle implements Drawable { public void draw() { System.out.println("Drawing Circle"); } } ``` What will happen if the `draw()` method is removed from the `Circle` class?

• The code will compile and run without errors

• A runtime exception will be thrown

• A compile-time error will occur because `Circle` does not implement all methods of `Drawable`

• The `Circle` class will automatically inherit the `draw()` method from `Drawable`

Which of the following statements about interface variables in Java is correct?

• Interface variables are `public`, `static`, and `final` by default

• Interface variables can be `private` and non-static

• Interface variables must be initialized in the implementing class

• Interface variables can hold different values in different implementing classes

Which of the following best describes when to use an interface over an abstract class?

• When you need to provide a default implementation for some methods

• When you want to define a type that can be adopted by unrelated classes across different hierarchies

• When you need to maintain state using instance variables

• When you want to restrict the number of classes that can use the type

In Java, which of the following is true about a class that extends an abstract class?

• It must be declared abstract itself

• It must override all abstract methods of the parent abstract class, unless it is also abstract

• It cannot override non-abstract methods of the parent abstract class

• It can only extend one interface in addition to the abstract class