CS115

Question 1

What is encapsulation in Java?

Answer 1

Hiding data by making attributes private and exposing them via public getter and setter methods.

Question 2

What is inheritance in Java?

Answer 2

Allows a subclass to acquire the properties and methods of a superclass using the
‘extends’ keyword

Question 3

What is polymorphism?

Answer 3

Polymorphism allows different classes to be treated as instances of the same superclass, with
method behavior determined at runtime.

Question 4

What does the ‘super’ keyword do?

Answer 4

It refers to the superclass of the current object and is used to access superclass methods or
constructors.

Question 5

What does the ‘this’ keyword refer to?

Answer 5

‘this’ refers to the current instance of the class.

Question 6

What is method overloading?

Answer 6

Defining multiple methods in the same class with the same name but different parameter lists.

Question 7

What is method overriding?

Answer 7

Redefining a superclass method in a subclass using the same method signature.

Question 8

What is the purpose of the ‘static’ keyword?

Answer 8

It defines a method or variable that belongs to the class rather than an instance of the class

Question 9

What is a constructor in Java?

Answer 9

A special method used to initialize objects; it has the same name as the class and no return
type.

Question 10

What is the default access modifier in Java?

Answer 10

Package-private (accessible only within the same package).

Question 11

: What are the operations of a Stack?

Answer 11

push, pop, peek

Question 12

What are the operations of a Queue?

Answer 12

enqueue (offer), dequeue (poll), peek

Question 13

What is a HashMap?

Answer 13

A data structure that stores key-value pairs and allows fast lookup using a hash function.

Question 14

What is a HashSet?

Answer 14

A data structure that stores unique elements and uses hashing for fast access.

Question 15

What is a binary search tree?

Answer 15

A binary tree where the left child is less than or equal to the node, and the right child is greater.

Question 16

What is inorder traversal?

Answer 16

Visit left subtree, node, then right subtree.

Question 17

What is preorder traversal?

Answer 17

Visit node, then left and right subtrees.

Question 18

What is postorder traversal?

Answer 18

Visit left and right subtrees, then the node.

Question 19

How do you handle exceptions in Java?

Answer 19

Using try-catch blocks; optionally a finally block for cleanup.

Question 19

What is the difference between checked and unchecked exceptions?

Answer 19

Checked exceptions are checked at compile time (e.g., IOException); unchecked are checked
at runtime (e.g., NullPointerException).

Question 20

What is an enum in Java?

Answer 20

A special data type that enables a variable to be a set of predefined constants.

Question 21

What is an interface in Java?

Answer 21

A contract that specifies method declarations without implementations; classes must
implement them.

Question 22

How is an abstract class different from an interface?

Answer 22

Abstract classes can have implemented methods and constructors, whereas interfaces cannot
(except default and static methods).

Question 23

What are generics in Java?

Answer 23

They allow classes and methods to operate on objects of various types while providing
compile-time type safety

Question 24

How do you define a generic class?

Answer 24

public class ClassName { ... }

Question 25

How should class names be written in Java?

Answer 25

With an initial capital letter, using camel case.

Question 26

How should variables and method names be written?

Answer 26

With an initial lowercase letter, using camel case.

Question 27

How are constants written?

Answer 27

In all uppercase letters, with underscores separating words.

Question 28

Why must hashCode() be overridden when equals() is overridden?

Answer 28

Because equal objects must return the same hash code for correct behavior in hash-based collections.

Question 29

What is a hash collision?

Answer 29

A situation where two different objects have the same hash code.

Question 30

What is the defining property of a max heap?

Answer 30

Each parent node has a value greater than or equal to its children, and the root is the maximum value.

Question 31

What is bucket chaining?

Answer 31

A method to handle hash collisions by storing multiple entries in the same array index using a linked list.

Question 32

: What is a priority queue?

Answer 32

A data structure where elements are served based on priority, not insertion order.

Question 33

: When should you use an abstract class instead of an interface?

Answer 33

Use an abstract class when you want to share code across related classes; interfaces are for unrelated classes with common behaviors.

Question 34

Can a class implement multiple interfaces in Java?

Answer 34

Yes, Java supports multiple interface implementations but only single class inheritance.

Question 35

When is LinkedList better than ArrayList?

Answer 35

LinkedList is better for frequent insertions and deletions; ArrayList is better for fast access by index.

Question 36

: What is the time complexity of inserting at the beginning of a LinkedList?

Answer 36

: O(1), because it only involves changing references.

Question 37

Why is input validation important?

Answer 37

To prevent invalid or malicious input that could cause security vulnerabilities or program crashes.

Question 38

: What is the purpose of version control?

Answer 38

To manage changes to source code over time, especially in collaborative environments.

Question 39

: What is the difference between authentication and authorisation?

Answer 39

Authentication verifies identity; authorisation determines access rights.

Question 40

What is the role of JavaFX in Java?

Answer 40

It is a library for building graphical user interfaces (GUIs) in Java applications.

Question 41

: What is the difference between a full and balanced binary tree?

Answer 41

A full binary tree has every node with 0 or 2 children; a balanced tree has roughly equal height in left and right subtrees

Question 42

: In a graph-based model of a network, what does an edge represent?

Answer 42

A connection or relationship between two nodes (vertices).

Question 43

How do you read from a file in Java using Scanner?

Answer 43

Create a File object, then pass it to Scanner: Scanner input = new Scanner(new File("filename.txt"));

Question 44

How do you write to a file in Java using PrintWriter?

Answer 44

Create a PrintWriter object and use print/println: PrintWriter out = new PrintWriter("output.txt");

Question 45

What must you always do after writing to a file in Java?

Answer 45

Call close() on the PrintWriter to ensure data is saved.

Question 46

What is the base case in recursion?

Answer 46

The condition under which the recursive method stops calling itself

Question 47

What is the recursive case in recursion?

Answer 47

The part of the method that calls itself with a smaller or simpler input.

Question 48

What is synthesis in a recursive method?

Answer 48

Combining results from recursive calls to build the final answer.

Question 49

: What is reference aliasing in Java?

Answer 49

When two variables refer to the same object in memory; changes via one affect the other.

Question 50

What does 'null' mean in Java

Answer 50

It indicates that a reference does not point to any object.

Question 51

What are the core components of a JavaFX application?

Answer 51

Stage, Scene, and Nodes like Button, Label, TextField

Question 52

How do you start a JavaFX application?

Answer 52

Extend Application class and override start(Stage primaryStage) method.

Question 53

What happens if a static method tries to access a non-static variable?

Answer 53

A compilation error occurs. Static methods cannot directly access non-static instance variables.

Question 54

: What is the result of assigning one object reference to another?

Answer 54

Both references point to the same object. Changes via one reference affect the other.

Question 55

What will happen if a recursive method does not have a base case?

Answer 55

It will lead to a StackOverflowError due to infinite recursive calls.

Question 56

What happens if you override equals() but not hashCode()?

Answer 56

Objects that are equal might not behave correctly in hash-based collections like HashMap or HashSet.

Question 57

Why must main() be static in Java?

Answer 57

Because it is the entry point of the program and must be accessible without creating an instance of the class.

Question 58

What does the 'finally' block do in a try-catch-finally structure?

Answer 58

It executes regardless of whether an exception is thrown or caught, typically used for cleanup.

Question 59

Can an abstract class have constructors?

Answer 59

Yes. Abstract classes can have constructors to initialize fields used by subclasses.

Question 60

What happens if you declare a method abstract but don't make the class abstract?

Answer 60

A compilation error occurs. Any class with abstract methods must be declared abstract.

Question 61

How does Java handle duplicate keys in a HashMap?

Answer 61

The new value replaces the existing value associated with that key.

Question 62

What does 'null' mean when used with object references?

Answer 62

It means the reference does not currently point to any object.