Chapter 10

Question 1

What is an abstract class in Java?

Answer 1

A class that cannot be instantiated and is used as a base for subclasses. It often includes abstract methods with no body.

Question 2

Can an abstract class be instantiated?

Answer 2

No, you cannot create objects of an abstract class.

Question 3

How do you declare an abstract class?

Answer 3

Using the abstract keyword in the class declaration, e.g., public abstract class Product {}.

Question 4

Can abstract classes have non-abstract methods?

Answer 4

Yes, abstract classes can include methods with full implementations.

Question 5

What happens if a subclass doesn’t implement all but some abstract methods from its abstract parent?

Answer 5

Then the subclass must also be declared abstract.

Question 6

Can abstract methods be static or final?

Answer 6

No, abstract methods cannot be static or final.

Question 7

How are abstract classes represented in UML?

Answer 7

They are shown in italics.

Question 8

What is a Java interface?

Answer 8

A collection of abstract methods and constants that define a contract a class must implement.

Question 9

Can you instantiate an interface?

Answer 9

No, interfaces cannot be instantiated.

Question 10

What keyword is used to create an interface?

Answer 10

interface, e.g., public interface Doable {}

Question 11

How do you implement an interface in a class?

Answer 11

Using the implements keyword in the class declaration.

Question 12

Are interface methods public by default?

Answer 12

Yes, all methods in an interface are public by default.

Question 13

Can a class implement multiple interfaces?

Answer 13

Yes, Java supports multiple interface implementation.

Question 14

What are the default modifiers for fields in an interface?

Answer 14

All fields are public static final by default.

Question 15

What is the purpose of the Comparable interface?

Answer 15

It defines a method compareTo to allow object comparison for sorting.

Question 16

What does compareTo return when two objects are equal?

Answer 16

It returns 0.

Question 17

What does compareTo return if the calling object is less than the other?

Answer 17

It returns a negative number.

Question 18

What does compareTo return if the calling object is greater than the other?

Answer 18

It returns a positive number.

Question 19

What Java method can sort a list of Comparable objects?

Answer 19

Collections.sort().

Question 20

When should you use an abstract class?

Answer 20

When classes share code or have protected/private fields or partial implementations.

Question 21

When should you use an interface?

Answer 21

When unrelated classes need to implement the same method signatures or you want multiple inheritance.

Question 22

What are differences between abstract classes and interfaces?

Answer 22

Abstract classes can have fields and non-abstract methods; interfaces only have public static final fields and abstract methods.

Question 23

What is polymorphism?

Answer 23

The ability of a reference variable to change behavior depending on the object it refers to.

Question 24

What are the two types of polymorphism in Java?

Answer 24

Compile-time (method overloading) and runtime (method overriding).

Question 25

What is a polymorphic reference?

Answer 25

A reference that can refer to different object types over time.

Question 26

How does Java determine which method to call in runtime polymorphism?

Answer 26

Based on the actual object type, not the reference type.

Question 27

What is a widening conversion?

Answer 27

Assigning a subclass object to a superclass reference.

Question 28

What is a narrowing conversion?

Answer 28

Casting a superclass object to a subclass reference.

Question 29

How are abstract classes and methods shown in UML?

Answer 29

They are shown in italics.

Question 30

On what type is an overridden method called?

Answer 30

The overridden method is called based on the object's actual type at runtime, not the reference type.

Question 31

What does the JVM do in runtime polymorphism?

Answer 31

It dynamically determines which method to call based on the actual object type.

Question 32

What is a widening conversion in Java?

Answer 32

Assigning a subclass object to a superclass reference; it's safe and implicit.

Question 33

What is a narrowing conversion in Java?

Answer 33

Assigning a superclass object to a subclass reference using an explicit cast; potentially unsafe.

Question 34

What happens when you use an interface reference to call methods?

Answer 34

The method called depends on the actual object the interface reference points to.

Question 35

What are the core Java collection interfaces?

Answer 35

`List`, `Map`, `Set`, `Queue` and their subtypes like `ArrayList`, `HashMap`, `TreeSet`, etc.

Question 36

How do you sort custom objects like Human by name length?

Answer 36

Implement `Comparable` and define `compareTo` to compare name lengths.

Question 37

What does `Collections.sort()` require?

Answer 37

The list elements must implement the `Comparable` interface.