Java MCQs with answers for placement preparation

Review Java types, control flow, methods and object-oriented concepts, then practise questions with explanations. Build language foundations for technical placement tests and Java interview preparation.

100 practice questionsPage updated 24 Sept 2026Review metadata pendingPractice questions

Practise this concept

Java is a platform-independent, object-oriented programming language known for its 'write once, run anywhere' capability. It powers enterprise applications, Android development, and web services. Understanding Java's core concepts—from OOP principles to advanced features like generics and concurrency—is essential for technical interviews and competitive programming.

Key formulas and rules

7 rules

String Character Count: str.length() returns total UTF-16 code units in the String instance

Array Dimension: arr.length is an immutable public field denoting array capacity

Collection Size: list.size() and set.size() return the current element count in the data structure

Standard Loop Pattern: for (int i = 0; i < n; i++) iterates indices from $0$ to $n - 1$

Enhanced For-Each: for (T item : collection) iterates over elements implementing Iterable<T>

Math Utilities: Math.max(a, b), Math.min(a, b), and Math.sqrt(x) evaluate basic scalar functions

Type Conversions: Integer.parseInt(s) parses a string to a primitive int; String.valueOf(x) converts primitives to string

Key concepts

Object-Oriented Programming (OOP)

Java follows four OOP pillars:

1. Encapsulation: Bundling data and methods, controlling access via private fields with public getters/setters.

2. Inheritance: Creating new classes from existing ones using 'extends', promoting code reuse.

3. Polymorphism: Same method name, different behaviors—achieved through method overloading (compile-time) and overriding (runtime).

4. Abstraction: Hiding implementation details via abstract classes and interfaces.

Inheritance and Access Modifiers

Java supports single inheritance (one parent class). Access modifiers control visibility:

public: Accessible everywhere
private: Only within the class
protected: Within package and subclasses
default (no modifier): Within package only

The 'super' keyword calls parent class methods/constructors. 'this' refers to current instance.

Exception Handling

Java uses try-catch-finally blocks for error handling:

Checked exceptions: Must be handled (IOException, SQLException)
Unchecked exceptions: Runtime errors (NullPointerException, ArrayIndexOutOfBoundsException)

Syntax: try { riskyCode } catch (ExceptionType e) { handle } finally { always executes }

Custom exceptions extend Exception or RuntimeException.

Collections Framework

Core interfaces and implementations:

List: ArrayList (dynamic array), LinkedList (doubly-linked), Vector (thread-safe)
Set: HashSet (unique elements), TreeSet (sorted), LinkedHashSet (insertion order)
Map: HashMap (key-value), TreeMap (sorted by key), LinkedHashMap

ArrayList offers O(1) get but O(n) insert/delete middle. HashMap provides O(1) average for put/get.

Generics

Generics enable type-safe collections and methods:

Class<T>: Type parameter for generic class
<?>: Unbounded wildcard
<? extends T>: Upper bounded (T or subclasses)
<? super T>: Lower bounded (T or superclasses)

Example: ArrayList<String> list = new ArrayList<>(); prevents adding non-String objects at compile time.

Threads and Concurrency

Java provides multithreading via:

Extending Thread class
Implementing Runnable interface (preferred)

Key methods: start(), run(), sleep(ms), join(), yield()

Synchronization prevents race conditions:
synchronized methods
synchronized blocks
ReentrantLock for finer control

The volatile keyword ensures visibility of changes across threads.

Check the idea while it is fresh.Open a focused exercise without losing this article.Practise this concept

Worked examples

Example 1

What is the output?

class Test {
public static void main(String[] args) {
String $s 1 = n e w$ String("hello");
String $s 2 = n e w$ String("hello");
System.out.println(s1 == s2);
System.out.println(s1.equals(s2));
}
}

Solution

Output:
false
true

Explanation:
Step 1: s1 == s2 compares references (memory addresses).
Step 2: new String() creates new objects, so s1 and s2 point to different heap locations.
Step 3: Therefore, == returns false.
Step 4: s1.equals(s2) compares content character by character.
Step 5: Both contain 'hello', so equals() returns true.

Example 2

What is the output?

class A {
int $x = 10$ ;
void show() { System.out.println(x); }
}
class B extends A {
int $x = 20$ ;
void display() { System.out.println(super.x + " " + x); }
public static void main(String[] args) {
B $o bj = n e w$ B();
obj.display();
}
}

Solution

Output:
10 20

Explanation:
Step 1: Class B inherits from class A, so B has access to A's members.
Step 2: Both classes declare variable 'x'--this is variable hiding, not overriding.
Step 3: super.x accesses the parent class's x (10).
Step 4: x (or this.x) accesses B's x (20).
Step 5: Therefore, output is '10 20'.

Example 3

What is the output?

public class Test {
public static void main(String[] args) {
try {
int $a = 5$ , $b = 0$ ;
int $c = a / b$ ;
System.out.println(c);
} catch (ArithmeticException e) {
System.out.println("Division by zero");
} finally {
System.out.println("Finally block");
}
}
}

Solution

Output:
Division by zero
Finally block

Explanation:
Step 1: Division a/b where $b = 0$ throws ArithmeticException.
Step 2: try block executes until exception occurs.
Step 3: Control jumps to catch (ArithmeticException e) block.
Step 4: 'Division by zero' is printed.
Step 5: finally block always executes, printing 'Finally block'.

Example 4

What is the output?

import java.util.*;
public class Test {
public static void main(String[] args) {
HashSet<Integer> $se t = n e w$ HashSet<>();
set.add(5);
set.add(3);
set.add(5);
set.add(1);
System.out.println(set);
}
}

Solution

Output:
[1, 3, 5] (order may vary)

Explanation:
Step 1: HashSet stores unique elements only.
Step 2: set.add(5) adds 5.
Step 3: set.add(3) adds 3.
Step 4: set.add(5) is ignored—duplicate not added.
Step 5: set.add(1) adds 1.
Step 6: HashSet does not maintain insertion order, so output order depends on hash codes.
Step 7: Contains {1, 3, 5} regardless of display order.

Representative solved questions

See the kind of question in this topic before opening the full practice set.

Question 1

What is the output of:
System.out.println("5" + 3 + 2);

10
532
53 + 2
Compilation error

Answer: B. 532

Explanation

Step 1: "5" is a String
Step 2: "5" + 3 = "53" (string concatenation)
Step 3: "53" + 2 = "532"
Step 4: Left to right evaluation
Output: 532

Sources and review notes

This is an AISEA-authored practice question.

Review status: accepted · Reviewed 2026-08-13 · structure and answer-key checks, editorial quality checks, duplicate screening

Question 2

Which inheritance concept is NOT supported in Java?

Single inheritance
Multilevel inheritance
Hierarchical inheritance
Multiple inheritance of classes

Answer: D. Multiple inheritance of classes

Explanation

Step 1: Java supports single inheritance (A extends B)
Step 2: Multilevel inheritance (A extends B extends C) works
Step 3: Multiple class inheritance (A extends B, C) is not allowed
Step 4: Interfaces can achieve similar effect
Answer: Multiple inheritance of classes

Sources and review notes

This is an AISEA-authored practice question.

Review status: accepted · Reviewed 2026-08-13 · structure and answer-key checks, editorial quality checks, duplicate screening

Question 3

What is the result of:
Integer.parseInt("123");

"123"
123
Compilation error
NumberFormatException

Answer: B. 123

Explanation

Step 1: parseInt converts String to int
Step 2: "123" becomes integer 123
Step 3: Returns primitive int value
Step 4: ValueOf() would return Integer object
Answer: 123

Sources and review notes

This is an AISEA-authored practice question.

Review status: accepted · Reviewed 2026-08-13 · structure and answer-key checks, editorial quality checks, duplicate screening

Common mistakes and useful habits

Use .equals() for String content comparison, never == (except String literals).
Prefer ArrayList over Vector unless thread safety is required—Vector methods are synchronized and slower.
String is immutable; use StringBuilder for string concatenation in loops to avoid creating many objects.
Always handle checked exceptions or declare with throws; unchecked exceptions don't require handling.
For thread safety, consider ConcurrentHashMap over synchronized HashMap for better performance.
The finally block executes even if return statement is in try/catch—use for cleanup code.

Ready to test your understanding?

Work through 100 questions with explanations after each answer.

Start practice

Next topic: Python

Java MCQs with answers for placement preparation

100 practice questionsPage updated 24 Sept 2026Review metadata pendingPractice questions

Practise this concept

Key formulas and rules

7 rules

String Character Count: str.length() returns total UTF-16 code units in the String instance

Array Dimension: arr.length is an immutable public field denoting array capacity

Collection Size: list.size() and set.size() return the current element count in the data structure

Standard Loop Pattern: for (int i = 0; i < n; i++) iterates indices from $0$ to $n - 1$

Enhanced For-Each: for (T item : collection) iterates over elements implementing Iterable<T>

Math Utilities: Math.max(a, b), Math.min(a, b), and Math.sqrt(x) evaluate basic scalar functions

Type Conversions: Integer.parseInt(s) parses a string to a primitive int; String.valueOf(x) converts primitives to string

Key concepts

Object-Oriented Programming (OOP)

Java follows four OOP pillars:

1. Encapsulation: Bundling data and methods, controlling access via private fields with public getters/setters.

2. Inheritance: Creating new classes from existing ones using 'extends', promoting code reuse.

3. Polymorphism: Same method name, different behaviors—achieved through method overloading (compile-time) and overriding (runtime).

4. Abstraction: Hiding implementation details via abstract classes and interfaces.

Inheritance and Access Modifiers

Java supports single inheritance (one parent class). Access modifiers control visibility:

public: Accessible everywhere
private: Only within the class
protected: Within package and subclasses
default (no modifier): Within package only

The 'super' keyword calls parent class methods/constructors. 'this' refers to current instance.

Exception Handling

Java uses try-catch-finally blocks for error handling:

Checked exceptions: Must be handled (IOException, SQLException)
Unchecked exceptions: Runtime errors (NullPointerException, ArrayIndexOutOfBoundsException)

Syntax: try { riskyCode } catch (ExceptionType e) { handle } finally { always executes }

Custom exceptions extend Exception or RuntimeException.

Collections Framework

Core interfaces and implementations:

ArrayList offers O(1) get but O(n) insert/delete middle. HashMap provides O(1) average for put/get.

Generics

Generics enable type-safe collections and methods:

Class<T>: Type parameter for generic class
<?>: Unbounded wildcard
<? extends T>: Upper bounded (T or subclasses)
<? super T>: Lower bounded (T or superclasses)

Example: ArrayList<String> list = new ArrayList<>(); prevents adding non-String objects at compile time.

Threads and Concurrency

Java provides multithreading via:

Extending Thread class
Implementing Runnable interface (preferred)

Key methods: start(), run(), sleep(ms), join(), yield()

Synchronization prevents race conditions:
synchronized methods
synchronized blocks
ReentrantLock for finer control

The volatile keyword ensures visibility of changes across threads.

Check the idea while it is fresh.Open a focused exercise without losing this article.Practise this concept

Worked examples

Example 1

What is the output?

class Test {
public static void main(String[] args) {
String $s 1 = n e w$ String("hello");
String $s 2 = n e w$ String("hello");
System.out.println(s1 == s2);
System.out.println(s1.equals(s2));
}
}

Solution

Output:
false
true

Example 2

What is the output?

Solution

Output:
10 20

Example 3

What is the output?

Solution

Output:
Division by zero
Finally block

Example 4

What is the output?

Solution

Output:
[1, 3, 5] (order may vary)

Representative solved questions

See the kind of question in this topic before opening the full practice set.

Question 1

What is the output of:
System.out.println("5" + 3 + 2);

10
532
53 + 2
Compilation error

Answer: B. 532

Explanation

Step 1: "5" is a String
Step 2: "5" + 3 = "53" (string concatenation)
Step 3: "53" + 2 = "532"
Step 4: Left to right evaluation
Output: 532

Sources and review notes

This is an AISEA-authored practice question.

Review status: accepted · Reviewed 2026-08-13 · structure and answer-key checks, editorial quality checks, duplicate screening

Question 2

Which inheritance concept is NOT supported in Java?

Single inheritance
Multilevel inheritance
Hierarchical inheritance
Multiple inheritance of classes

Answer: D. Multiple inheritance of classes

Explanation

Sources and review notes

This is an AISEA-authored practice question.

Review status: accepted · Reviewed 2026-08-13 · structure and answer-key checks, editorial quality checks, duplicate screening

Question 3

What is the result of:
Integer.parseInt("123");

"123"
123
Compilation error
NumberFormatException

Answer: B. 123

Explanation

Step 1: parseInt converts String to int
Step 2: "123" becomes integer 123
Step 3: Returns primitive int value
Step 4: ValueOf() would return Integer object
Answer: 123

Sources and review notes

This is an AISEA-authored practice question.

Review status: accepted · Reviewed 2026-08-13 · structure and answer-key checks, editorial quality checks, duplicate screening

Common mistakes and useful habits

Use .equals() for String content comparison, never == (except String literals).
Prefer ArrayList over Vector unless thread safety is required—Vector methods are synchronized and slower.
String is immutable; use StringBuilder for string concatenation in loops to avoid creating many objects.
Always handle checked exceptions or declare with throws; unchecked exceptions don't require handling.
For thread safety, consider ConcurrentHashMap over synchronized HashMap for better performance.
The finally block executes even if return statement is in try/catch—use for cleanup code.

Ready to test your understanding?

Work through 100 questions with explanations after each answer.

Start practice

Next topic: Python

Key formulas and rules

Key concepts

Object-Oriented Programming (OOP)

Inheritance and Access Modifiers

Exception Handling

Collections Framework

Generics

Threads and Concurrency

Worked examples

Example 1

Solution

Example 2

Solution

Example 3

Solution

Example 4

Solution

Representative solved questions

Question 1

Question 2

Question 3

Common mistakes and useful habits

Ready to test your understanding?

Keep building the same skill family.

Key formulas and rules

Key concepts

Object-Oriented Programming (OOP)

Inheritance and Access Modifiers

Exception Handling

Collections Framework

Generics

Threads and Concurrency

Worked examples

Example 1

Solution

Example 2

Solution

Example 3

Solution

Example 4

Solution

Representative solved questions

Question 1

Question 2

Question 3

Common mistakes and useful habits

Ready to test your understanding?

Keep building the same skill family.