Topic MCQs — Programming, OOP & Data Structures

40 original MCQs | Mix: Java, Python, OOP, DS | Answers at end

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Q1. Which OOP pillar hides internal data using access control?

• A) Inheritance
• B) Encapsulation
• C) Polymorphism
• D) Recursion

Q2. Runtime polymorphism in Java is mainly achieved by:

• A) Method overloading only
• B) Method overriding
• C) Operator overloading (as in C++)
• D) Multiple inheritance of classes

Q3. A class is best described as:

• A) A runtime instance in memory
• B) A blueprint for creating objects
• C) Always equal to one object
• D) A JVM process

Q4. A Java constructor:

• A) Must return int
• B) Has the same name as the class and no return type
• C) Cannot be overloaded
• D) Is called only by the garbage collector

Q5. Java source is compiled mainly into:

• A) Native machine code only
• B) Bytecode executed by the JVM
• C) Python bytecode
• D) HTML only

Q6. Which access modifier allows access only within the same class?

• A) public
• B) protected
• C) private
• D) default

Q7. A static method in Java:

• A) Belongs to each object separately with different copies always
• B) Belongs to the class and is shared
• C) Cannot be called at all
• D) Must override an abstract method

Q8. A Java class can:

• A) Extend multiple classes
• B) Implement multiple interfaces
• C) Neither extend nor implement
• D) Only extend interfaces

Q9. Which is true?

• A) Abstract classes can be instantiated directly
• B) Interfaces cannot be implemented
• C) Abstract classes may contain constructors
• D) Interfaces must always have instance fields

Q10. The finally block in Java:

• A) Runs only if an exception occurs
• B) Typically executes whether or not an exception is thrown
• C) Replaces the need for catch always
• D) Is used only for inheritance

Q11. Which statement about String in Java is correct?

• A) Strings are mutable by default
• B) Strings are immutable
• C) StringBuilder is immutable
• D) == always compares string content safely

Q12. Preferred content comparison for two Java String objects:

• A) ==
• B) .equals()
• C) =
• D) compare keyword only

Q13. In Python, which type is immutable?

• A) list
• B) dict
• C) tuple
• D) set

Q14. Python dict keys must be:

• A) Always lists
• B) Hashable (typically immutable)
• C) Only floating numbers
• D) Duplicated freely as keys

Q15. Python uses indentation primarily to:

• A) Improve color themes only
• B) Define code blocks
• C) Declare variable types
• D) Link C libraries

Q16. In def f(*args):, args inside the function is typically a:

• A) dict
• B) tuple of positional arguments
• C) set
• D) file handle

Q17. Opening a file with mode 'w' in Python will:

• A) Always fail if file exists
• B) Truncate/create for writing
• C) Open read-only
• D) Append only without create

Q18. Stack follows:

• A) FIFO
• B) LIFO
• C) Random access only
• D) Priority by key only

Q19. Queue follows:

• A) LIFO
• B) FIFO
• C) Sorted order always
• D) Binary heap order only

Q20. Which structure is best for undo operations?

• A) Queue
• B) Stack
• C) Only hash table
• D) Only graph

Q21. In a singly linked list, random access by index is typically:

• A) O(1)
• B) O(log n)
• C) O(n)
• D) O(1) with hashing always

Q22. In a Binary Search Tree (typical ordering):

• A) Left child > node > right child
• B) Left child < node < right child
• C) All children equal to root
• D) No ordering rule

Q23. Binary search requires the array to be:

• A) Unsorted
• B) Sorted
• C) Circular only
• D) Linked as a graph

Q24. Worst-case time of binary search on a sorted array of n elements:

• A) O(n)
• B) O(log n)
• C) O(n²)
• D) O(1)

Q25. Average time complexity of merge sort:

• A) O(n)
• B) O(n log n)
• C) O(n²)
• D) O(2ⁿ)

Q26. Worst-case time of quicksort (naive pivot):

• A) O(n log n)
• B) O(n²)
• C) O(log n)
• D) O(1)

Q27. Selection sort’s time complexity is typically:

• A) O(n) best and worst
• B) O(n²) in common cases
• C) O(n log n) always
• D) O(log n)

Q28. Which sorting algorithm is generally stable and O(n log n) worst case?

• A) Quicksort (typical in-place)
• B) Selection sort
• C) Merge sort
• D) Heap sort (typical)

Q29. Recursion requires:

• A) No base case ever
• B) A base case to terminate
• C) Only global variables
• D) A queue instead of stack always

Q30. Time complexity of a simple loop from 1 to n is:

• A) O(1)
• B) O(log n)
• C) O(n)
• D) O(n!)

Q31. Accessing a[i] in an array is typically:

• A) O(n)
• B) O(1)
• C) O(n log n)
• D) O(2ⁿ)

Q32. BFS on a graph commonly uses a:

• A) Stack only
• B) Queue
• C) Only BST
• D) Only StringBuilder

Q33. DFS on a graph commonly uses a:

• A) Queue only
• B) Stack (or recursion call stack)
• C) Only hash set without structure
• D) Only circular array always

Q34. Method overloading is an example of:

• A) Runtime polymorphism only
• B) Compile-time polymorphism
• C) Encapsulation only
• D) Abstraction only

Q35. Which Python structure does not allow duplicate elements?

• A) list
• B) tuple
• C) set
• D) list of lists only

Q36. Adjacency list representation of a graph is often preferred when the graph is:

• A) Extremely dense always
• B) Sparse
• C) Only a single node
• D) Impossible to traverse

Q37. Checked exceptions in Java:

• A) Need not be declared or handled
• B) Must be handled or declared
• C) Are the same as Errors only
• D) Occur only in Python

Q38. StringBuilder is preferred over String concatenation in loops mainly because:

• A) It is immutable
• B) It is mutable and avoids many temporary String objects
• C) It cannot append
• D) It runs outside the JVM

Q39. Height of a skewed BST with n nodes is:

• A) O(1)
• B) O(log n)
• C) O(n)
• D) O(n²)

Q40. Big-O order from slowest-growing to fastest-growing:

• A) O(n²), O(n), O(1)
• B) O(1), O(log n), O(n), O(n log n), O(n²)
• C) O(n!), O(1), O(n)
• D) O(2ⁿ), O(log n), O(1)

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Answer Key

Q	Ans	Brief explanation
1	B	Encapsulation + access modifiers hide data.
2	B	Overriding → dynamic dispatch at runtime.
3	B	Class = blueprint/template.
4	B	Constructor named like class; no return type.
5	B	javac → bytecode for JVM.
6	C	private = same class only.
7	B	static members are class-level.
8	B	Multiple interfaces OK; single class extend.
9	C	Abstract classes may have constructors.
10	B	finally usually runs after try/catch.
11	B	String objects are immutable.
12	B	equals compares content.
13	C	tuple immutable; list/dict/set mutable.
14	B	Keys must be hashable.
15	B	Indentation defines blocks.
16	B	*args collected as tuple.
17	B	'w' truncates or creates for write.
18	B	Stack = LIFO.
19	B	Queue = FIFO.
20	B	Undo maps naturally to stack.
21	C	Must walk nodes → O(n).
22	B	Standard BST ordering.
23	B	Binary search needs sorted data.
24	B	Halving each step → O(log n).
25	B	Merge sort Θ(n log n) typical.
26	B	Bad pivots → O(n²).
27	B	Selection is O(n²).
28	C	Merge is stable O(n log n) worst.
29	B	Base case stops recursion.
30	C	Linear scan/loop → O(n).
31	B	Direct indexing → O(1).
32	B	BFS uses queue.
33	B	DFS uses stack/recursion.
34	B	Overloading resolved at compile time.
35	C	set stores unique elements.
36	B	Sparse graphs → adjacency lists efficient.
37	B	Checked exceptions: handle or declare.
38	B	Mutable buffer is efficient in loops.
39	C	Skewed tree height ≈ n.
40	B	Classic asymptotic growth order.

Count: 40 MCQs