Java Interview Questions for Freshers: Your Complete Preparation Guide
- Sujeet Prajapati
- Aug 20
- 7 min read
Landing your first Java developer role can feel overwhelming, but with the right preparation, you'll walk into that interview room with confidence. This comprehensive guide covers the most frequently asked Java interview questions for freshers, complete with detailed explanations and examples.
Table of Contents
Basic Java Concepts
Q1: What is Java and what are its key features?
Answer: Java is a high-level, object-oriented programming language developed by Sun Microsystems (now Oracle). Key features include:
Platform Independence: "Write Once, Run Anywhere" (WORA)
Object-Oriented: Everything is based on objects and classes
Automatic Memory Management: Garbage collection handles memory cleanup
Multi-threading: Built-in support for concurrent programming
Security: Strong security features and sandboxing
Robustness: Strong error handling and type checking
Q2: Explain the difference between JDK, JRE, and JVM.
Answer:
JVM (Java Virtual Machine): Runtime environment that executes Java bytecode
JRE (Java Runtime Environment): JVM + libraries and components needed to run Java applications
JDK (Java Development Kit): JRE + development tools (compiler, debugger, etc.)
Think of it as: JDK ⊃ JRE ⊃ JVM
Q3: What is bytecode in Java?
Answer: Bytecode is intermediate code generated by the Java compiler (javac) from Java source code. It's platform-independent and gets executed by the JVM. This enables Java's "write once, run anywhere" capability.
Object-Oriented Programming
Q4: What are the four pillars of OOP?
Answer:
Encapsulation: Bundling data and methods together, hiding internal implementation
Inheritance: Creating new classes based on existing classes
Polymorphism: Same interface, different implementations
Abstraction: Hiding complex implementation details, showing only essential features
Q5: Difference between abstract class and interface?
Answer:
Abstract Class | Interface |
Can have concrete methods | Only abstract methods (Java 8+ allows default/static) |
Can have instance variables | Only public, static, final variables |
Single inheritance | Multiple inheritance supported |
Can have constructors | Cannot have constructors |
Access modifiers allowed | Methods are public by default |
Q6: What is method overloading vs method overriding?
Answer:
Method Overloading (Compile-time polymorphism):
Same method name, different parameters
Occurs within the same class
Resolved at compile time
public void print(int x) { }
public void print(String s) { }
public void print(int x, int y) { }Method Overriding (Runtime polymorphism):
Same method signature in parent and child class
Child class provides specific implementation
Resolved at runtime
// Parent class
public void display() {
System.out.println("Parent display");
}
// Child class
@Override
public void display() {
System.out.println("Child display");
}Data Types and Variables
Q7: What are the primitive data types in Java?
Answer:
Type | Size | Range | Default Value |
byte | 8 bits | -128 to 127 | 0 |
short | 16 bits | -32,768 to 32,767 | 0 |
int | 32 bits | -2³¹ to 2³¹-1 | 0 |
long | 64 bits | -2⁶³ to 2⁶³-1 | 0L |
float | 32 bits | IEEE 754 | 0.0f |
double | 64 bits | IEEE 754 | 0.0d |
boolean | 1 bit | true/false | false |
char | 16 bits | 0 to 65,535 | '\u0000' |
Q8: Difference between == and equals()?
Answer:
== compares references (memory addresses) for objects, values for primitives
equals() compares actual content/values of objects
String s1 = new String("Hello");
String s2 = new String("Hello");
String s3 = "Hello";
String s4 = "Hello";
System.out.println(s1 == s2); // false (different objects)
System.out.println(s1.equals(s2)); // true (same content)
System.out.println(s3 == s4); // true (string pool)Control Structures
Q9: Explain different types of loops in Java.
Answer:
for loop: When you know the number of iterations
for(int i = 0; i < 10; i++) {
System.out.println(i);
}while loop: When condition is checked before execution
int i = 0;
while(i < 10) {
System.out.println(i);
i++;
}do-while loop: When you want to execute at least once
int i = 0;
do {
System.out.println(i);
i++;
} while(i < 10);Enhanced for loop: For iterating collections/arrays
int[] numbers = {1, 2, 3, 4, 5};
for(int num : numbers) {
System.out.println(num);
}Arrays and Collections
Q10: Difference between Array and ArrayList?
Answer:
Array | ArrayList |
Fixed size | Dynamic size |
Can store primitives and objects | Only objects (autoboxing for primitives) |
Better performance | Slightly slower due to dynamic resizing |
No built-in methods | Rich set of methods |
// Array
int[] arr = new int[5];
arr[0] = 10;
// ArrayList
ArrayList<Integer> list = new ArrayList<>();
list.add(10);
list.add(20);Q11: What are the main Collection interfaces?
Answer:
List: Ordered collection, allows duplicates (ArrayList, LinkedList, Vector)
Set: No duplicates allowed (HashSet, LinkedHashSet, TreeSet)
Queue: FIFO operations (LinkedList, PriorityQueue)
Map: Key-value pairs (HashMap, LinkedHashMap, TreeMap)
Exception Handling
Q12: What is exception handling and its keywords?
Answer: Exception handling manages runtime errors gracefully without crashing the program.
Keywords:
try: Block that might throw an exception
catch: Handles specific exceptions
finally: Always executes (cleanup code)
throw: Manually throws an exception
throws: Declares exceptions a method might throw
try {
int result = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Cannot divide by zero!");
} finally {
System.out.println("This always executes");
}Q13: Difference between checked and unchecked exceptions?
Answer:
Checked Exceptions: Compile-time exceptions that must be handled (IOException, SQLException)
Unchecked Exceptions: Runtime exceptions, handling is optional (NullPointerException, ArrayIndexOutOfBoundsException)
String Manipulation
Q14: Why are Strings immutable in Java?
Answer: Strings are immutable for:
Security: Prevents modification of sensitive data
Thread Safety: No synchronization needed
String Pool Optimization: Multiple references can point to same object
Hashcode Consistency: Hash value doesn't change
Q15: Difference between String, StringBuilder, and StringBuffer?
Answer:
String | StringBuilder | StringBuffer |
Immutable | Mutable | Mutable |
Thread-safe (immutable) | Not thread-safe | Thread-safe |
Slower for concatenation | Faster | Slower than StringBuilder |
String pool | Heap memory | Heap memory |
Memory Management
Q16: Explain Java memory structure.
Answer:
Heap Memory: Objects and instance variables (Young Generation, Old Generation)
Stack Memory: Method calls, local variables, partial results
Method Area: Class-level data, static variables, constant pool
PC Registers: Current executing instruction
Native Method Stacks: Native method calls
Q17: What is Garbage Collection?
Answer: Automatic memory management that removes unreferenced objects from heap memory. Main algorithms include:
Serial GC: Single-threaded
Parallel GC: Multi-threaded
G1 GC: Low-latency collector
ZGC/Shenandoah: Ultra-low latency collectors
Multithreading Basics
Q18: How do you create a thread in Java?
Answer: Two main ways:
Extending Thread class:
class MyThread extends Thread {
public void run() {
System.out.println("Thread running");
}
}
MyThread t = new MyThread();
t.start();Implementing Runnable interface:
class MyRunnable implements Runnable {
public void run() {
System.out.println("Thread running");
}
}
Thread t = new Thread(new MyRunnable());
t.start();Q19: What is synchronization?
Answer: Synchronization ensures that only one thread accesses a shared resource at a time, preventing data corruption.
synchronized void method() {
// Only one thread can execute this at a time
}
synchronized(this) {
// Synchronized block
}Common Coding Problems
Q20: Write a program to reverse a string.
Answer:
public class StringReverse {
public static String reverse(String str) {
StringBuilder sb = new StringBuilder(str);
return sb.reverse().toString();
}
// Alternative approach
public static String reverseManual(String str) {
char[] chars = str.toCharArray();
int left = 0, right = chars.length - 1;
while (left < right) {
char temp = chars[left];
chars[left] = chars[right];
chars[right] = temp;
left++;
right--;
}
return new String(chars);
}
}Q21: Find the factorial of a number.
Answer:
public class Factorial {
// Recursive approach
public static long factorial(int n) {
if (n <= 1) return 1;
return n * factorial(n - 1);
}
// Iterative approach
public static long factorialIterative(int n) {
long result = 1;
for (int i = 2; i <= n; i++) {
result *= i;
}
return result;
}
}Q22: Check if a number is prime.
Answer:
public class PrimeCheck {
public static boolean isPrime(int n) {
if (n <= 1) return false;
if (n <= 3) return true;
if (n % 2 == 0 || n % 3 == 0) return false;
for (int i = 5; i * i <= n; i += 6) {
if (n % i == 0 || n % (i + 2) == 0) {
return false;
}
}
return true;
}
}Interview Tips for Success
Before the Interview:
Practice coding on paper or whiteboard
Review basic algorithms and data structures
Prepare questions about the company and role
Get familiar with common IDEs (Eclipse, IntelliJ IDEA)
During the Interview:
Think out loud while solving problems
Ask clarifying questions
Start with a simple solution, then optimize
Test your code with examples
Be honest about what you don't know
Key Points to Remember:
Focus on understanding concepts, not just memorizing
Practice explaining technical concepts in simple terms
Be prepared to write code for basic programs
Show enthusiasm for learning and growing
Demonstrate problem-solving approach
Conclusion
Success in Java interviews comes from consistent practice and deep understanding of core concepts. Focus on building a strong foundation in OOP principles, basic programming constructs, and problem-solving skills. Remember, interviewers often value your thought process and approach as much as the final answer.
Keep practicing, stay curious, and approach each interview as a learning opportunity. With dedication and the right preparation, you'll land that dream Java developer role!
Good luck with your interviews!
This guide covers the most essential Java interview questions for freshers. For more advanced topics and company-specific questions, continue building your knowledge through practice and real-world project
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