Java — Interview Questions

Commonly asked interview questions based on real placement patterns — the kind of thing interviewers actually ask, not textbook trivia.

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OOP & Class Design

What are the 4 pillars of OOP?

Difference between method overloading and overriding?

Overloading: same method name, different parameters — resolved at compile time (static polymorphism). Overriding: child class redefines parent method with same signature — resolved at runtime (dynamic polymorphism). Override rules: same name, same return type, same parameters, cannot reduce access modifier.

What is the difference between abstract class and interface?

Abstract class: can have constructors, instance variables, concrete methods + abstract methods, single inheritance only. Interface: no constructors, all fields are public static final, methods are public abstract by default (Java 8+ allows default and static methods), supports multiple implementation. Use abstract class when classes share common code; use interface when unrelated classes share a contract.

What is super keyword?

super refers to the parent class. Uses: super() calls parent constructor (must be first statement), super.method() calls parent method, super.variable accesses parent variable. Cannot use super in static context.

What is this keyword?

this refers to current object instance. Uses: distinguish instance variables from parameters (this.name = name), call another constructor in same class (this()), pass current object as argument.

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Exception Handling

What is the exception hierarchy in Java?

Throwable is the root. Two children: Error (JVM-level, don't catch) and Exception. Under Exception: Checked (FileNotFoundException, IOException, ClassNotFoundException, InterruptedException) and Unchecked/RuntimeException (NullPointerException, ArrayIndexOutOfBoundsException, ArithmeticException, ClassCastException, NumberFormatException).

What is the difference between throw and throws?

throw is used inside a method to explicitly throw an exception object: throw new ArithmeticException("message"). throws is used in method signature to declare that the method may throw a checked exception: void readFile() throws IOException. You throw ONE exception; you declare MULTIPLE with throws.

Can we have try without catch?

Yes — try with finally (no catch) is valid. The finally block always runs.

What happens if exception is thrown in finally block?

The original exception is lost and the finally block exception propagates. Avoid throwing exceptions in finally.

When does finally NOT execute?

When System.exit() is called, or when JVM crashes (OutOfMemoryError, kill -9).

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Multithreading

What is the difference between Process and Thread?

Process: independent program with its own memory space. Thread: lightweight unit within a process, shares memory with other threads of same process. Threads are faster to create/destroy, communicate easier but need synchronization.

What is race condition and how do you prevent it?

Race condition: two threads read/modify shared data simultaneously, giving inconsistent results. Prevention: use synchronized keyword on methods or blocks. Synchronized ensures only one thread executes that block at a time.

Difference between sleep() and wait()?

sleep(ms) pauses the thread for given milliseconds — does NOT release lock. wait() pauses thread AND releases the lock — thread stays in waiting state until notify() or notifyAll() is called. sleep() is in Thread class; wait() is in Object class.

What is deadlock?

Two or more threads blocked forever, each waiting for a resource held by the other. Example: Thread 1 holds Lock A, waiting for Lock B. Thread 2 holds Lock B, waiting for Lock A. Prevention: always acquire locks in the same order.

What does volatile keyword do?

Variables declared volatile are read directly from main memory (not thread's local cache). Ensures visibility of changes across threads. Does NOT ensure atomicity — use synchronized or AtomicInteger for compound operations.

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Collections

Difference between ArrayList and LinkedList?

ArrayList: backed by dynamic array, O(1) random access, O(n) insert/delete in middle. LinkedList: doubly linked list, O(n) random access, O(1) insert/delete if you have the node. Use ArrayList for frequent reads, LinkedList for frequent inserts/deletes at ends.

Difference between HashMap and Hashtable?

HashMap: not synchronized, allows one null key and multiple null values, faster. Hashtable: synchronized, no null keys or values, legacy class. For thread-safe needs, use ConcurrentHashMap over Hashtable.

What is the difference between Set, List, and Map?

List: ordered, allows duplicates (ArrayList, LinkedList). Set: unordered, no duplicates (HashSet, TreeSet). Map: key-value pairs, no duplicate keys (HashMap, TreeMap, LinkedHashMap).

How does HashMap work internally?

Uses an array of buckets. Key's hashCode() determines the bucket index. Within a bucket, entries are stored as a linked list (Java 8+: converts to balanced tree when list > 8). For retrieval: compute hash → find bucket → search by equals(). Load factor (default 0.75) triggers resize when 75% full.

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Java 8+ Features

What is a functional interface?

An interface with exactly ONE abstract method. Can be used as lambda expression target. Example: Runnable, Comparator, Predicate. @FunctionalInterface annotation enforces this.

What is Stream API?

Allows functional-style processing of collections. Operations are lazy (nothing runs until terminal operation). Pipeline: stream() → zero or more intermediate ops (filter, map, sorted) → terminal op (collect, count, reduce, forEach).

What is Optional?

A container object that may or may not contain a value — avoids NullPointerException. Methods: isPresent(), get(), orElse(default), orElseGet(supplier), orElseThrow(), ifPresent(consumer).

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Common Coding Questions

Reverse a String:

String str = "Hello";
String reversed = new StringBuilder(str).reverse().toString();
// or manually:
char[] chars = str.toCharArray();
for (int i = 0, j = chars.length-1; i < j; i++, j--) {
    char temp = chars[i]; chars[i] = chars[j]; chars[j] = temp;
}
System.out.println(new String(chars));  // olleH

Check if String is palindrome:

public boolean isPalindrome(String s) {
    s = s.toLowerCase().replaceAll("[^a-z0-9]", "");
    int i = 0, j = s.length() - 1;
    while (i < j) {
        if (s.charAt(i++) != s.charAt(j--)) return false;
    }
    return true;
}

Find factorial recursively:

public int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

Convert Roman numeral to number (common interview Q from notes):

public int romanToInt(String s) {
    Map<Character, Integer> map = new HashMap<>();
    map.put('I',1); map.put('V',5); map.put('X',10);
    map.put('L',50); map.put('C',100); map.put('D',500); map.put('M',1000);

    int result = 0;
    for (int i = 0; i < s.length(); i++) {
        int curr = map.get(s.charAt(i));
        int next = (i+1 < s.length()) ? map.get(s.charAt(i+1)) : 0;
        if (curr < next) result -= curr;
        else result += curr;
    }
    return result;
}

Thread-based app: print numbers with delay (from original notes):

class NumberPrinter extends Thread {
    public void run() {
        for (int i = 1; i <= 10; i++) {
            try {
                System.out.println("Val of i = " + i);
                Thread.sleep(1000);  // wait 1 second
            } catch (InterruptedException ie) {
                System.out.println("Problem in thread execution");
            }
        }
    }
}
// Create and start:
NumberPrinter np = new NumberPrinter();
np.start();

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From the Handwritten Notes (Q&A Notebook)

These are direct questions from the 5-year notes collection — numbered as found in the original notebook.

Q3: What do you understand by JVM?

JVM (Java Virtual Machine) is a virtual machine that enables the computer to run a Java program. JVM acts like a runtime engine which calls the main method present in Java code. Java code is compiled by JVM to bytecode which is close to native code. JVM is platform dependent (different JVMs for Windows, Linux, Mac) but Java code is platform independent — write once, run anywhere.

Q4: How many types of memory are allocated by JVM?

JVM allocates 5 types of memory:

Q5: What is Platform?

Platform is a set of hardware and software environment in which a piece of software is executed. Java is a software-based platform — JVM is the platform on which Java programs run, not hardware.

Q16: How many types of constructors are used in Java?

Two types: Default constructor (no parameters — provided by compiler if you don't write one), and Parameterized constructor (takes parameters for initialization). Note: if you write a parameterized constructor, the compiler does NOT add a default constructor automatically.

Q17: What is the purpose of a default constructor?

A default constructor initializes instance variables to their default values — integers to 0, objects to null, booleans to false. Example:

class Student {
    int id;
    String name;
    void Student_Info() { System.out.println(id + " " + name); }
}
// With new Student(), id=0, name=null
// Output: 0 null

Q30: Can we make abstract methods static?

No. If we declare abstract methods as static, they become part of the class and can be called directly. But calling an undefined abstract method is completely useless, therefore it is not allowed.

Q31: Can we declare static methods and variables inside abstract class?

Yes. Static members belong to the class, not the instance. Abstract classes can have static members.

Q32: What is the this keyword in Java?

this is a reference variable that refers to the current object. It can be used to:

Note: this can be used to refer to static members but it is NOT good practice.

Q46: Can we declare constructor as final?

No. The constructor can never be declared as final, because it is never inherited. If you try to do so, it will throw a compile time error.

Q47: Can we declare interface as final?

No. Interface must be implemented by some class to provide its definition. Therefore, there is no sense to make it final. If you try to do so, it will throw compile time error.

Q48: Difference between final and abstract method?

Abstract method cannot be final as we need to override them in subclass to give its definition. Final method cannot be overridden — directly contradicts the purpose of abstract.

Q49: What is Java instanceof operator?

The instanceof operator in Java is also known as type comparison operator because it compares the instance with type.

class Simple {
    public static void main(String args[]) {
        Simple s = new Simple();
        System.out.println(s instanceof Simple);  // true
    }
}
// Returns true if the object is an instance of the specified class
// Used before casting to avoid ClassCastException
if (obj instanceof Dog) {
    Dog d = (Dog) obj;
    d.bark();
}

Properties of Java (from notes):

JVM Memory allocation — practical consequences: