Java Interview Preparation

# CHAPTER 28

Java Interview Preparation #

1. Introduction #

2. Core Java Interview Questions #

Q1: What is the difference between JDK, JRE, and JVM? JDK = Development Kit (compiler + tools + JRE). JRE = Runtime Environment (JVM + libraries). JVM = Virtual Machine that executes bytecode.

Q2: Why is Java platform-independent? Java compiles to bytecode (.class files) which runs on any JVM, regardless of the underlying OS.

Q3: What is the difference between == and .equals()? == compares memory addresses (reference equality). .equals() compares content (value equality). For Strings, always use .equals().

Q4: What is autoboxing and unboxing? Autoboxing: primitive → wrapper (int → Integer). Unboxing: wrapper → primitive (Integer → int).

Q5: What is the String Pool? A special memory area in the Heap where String literals are stored and reused to save memory.

Q6: Why are Strings immutable in Java? Security (passwords), thread safety, and performance (String Pool caching).

Q7: What is the difference between final, finally, and finalize()? final = constant/prevent override. finally = always-execute block. finalize() = garbage collection hook (deprecated).

Q8: Can we override a static method? No. Static methods are resolved at compile time (method hiding, not overriding).

Q9: What is the transient keyword? Marks a field to be excluded from serialization.

Q10: What is the difference between throw and throws? throw creates/throws an exception. throws declares that a method might throw an exception.

3. OOP Interview Questions #

Q11: What are the four pillars of OOP? Encapsulation, Inheritance, Polymorphism, Abstraction.

Q12: What is the difference between an abstract class and an interface? Abstract class can have constructors, instance variables, and concrete methods. Interface defines a contract with abstract methods (Java 8+ allows default/static methods). A class can implement multiple interfaces but extend only one class.

Q13: What is the diamond problem? When a class inherits from two classes that have the same method, creating ambiguity. Java avoids this by not allowing multiple class inheritance.

Q14: What is method overloading vs overriding? Overloading: same name, different parameters (compile-time). Overriding: child redefines parent method (runtime).

Q15: What is the super keyword used for? Access parent class members (methods, fields, constructors) from a child class.

Q16: Can a constructor be inherited? No. Constructors are not inherited, but a child can call super() to invoke the parent constructor.

Q17: What is a Singleton pattern? A design pattern ensuring a class has only one instance. Achieved with a private constructor and a static getInstance() method.

Q18: What is the difference between composition and inheritance? Inheritance = IS-A (Dog IS-A Animal). Composition = HAS-A (Car HAS-A Engine). Prefer composition over inheritance.

4. Collections Interview Questions #

Q19: ArrayList vs LinkedList? ArrayList: fast random access O(1), slow insert/delete O(n). LinkedList: slow access O(n), fast insert/delete O(1).

Q20: HashMap vs TreeMap vs LinkedHashMap? HashMap: O(1) lookup, no order. TreeMap: O(log n), sorted by key. LinkedHashMap: O(1), maintains insertion order.

Q21: How does HashMap work internally? Uses hashing. Key's hashCode() determines the bucket index. Collisions are resolved using linked lists (or trees in Java 8+).

Q22: What is the difference between HashSet and TreeSet? HashSet: O(1) operations, no order. TreeSet: O(log n), elements are sorted.

Q23: What is the fail-fast iterator? An iterator that throws ConcurrentModificationException if the collection is modified during iteration.

Q24: How to make a collection thread-safe? Use Collections.synchronizedList(), ConcurrentHashMap, or CopyOnWriteArrayList.

5. Exception Handling Questions #

Q25: Checked vs Unchecked exceptions? Checked: must handle at compile time (IOException). Unchecked: runtime errors (NullPointerException).

Q26: Can we have try without catch? Yes, with finally or try-with-resources.

Q27: What is the order of catch blocks? Most specific exception first, most general last.

6. Top 20 Coding Challenges #

C1: Reverse a String

public static String reverse(String s) {
    return new StringBuilder(s).reverse().toString();
}

C2: Check Palindrome

public static boolean isPalindrome(String s) {
    String reversed = new StringBuilder(s).reverse().toString();
    return s.equals(reversed);
}

C3: Fibonacci Series

public static void fibonacci(int n) {
    int a = 0, b = 1;
    for (int i = 0; i < n; i++) {
        System.out.print(a + " ");
        int temp = a + b; a = b; b = temp;
    }
}

C4: Check Prime Number

public static boolean isPrime(int n) {
    if (n < 2) return false;
    for (int i = 2; i <= Math.sqrt(n); i++) {
        if (n % i == 0) return false;
    }
    return true;
}

C5: Find Largest in Array

public static int findMax(int[] arr) {
    int max = arr[0];
    for (int n : arr) if (n > max) max = n;
    return max;
}

C6: Factorial

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

C7: Count Vowels

public static int countVowels(String s) {
    return (int) s.toLowerCase().chars()
        .filter(c -> "aeiou".indexOf(c) != -1).count();
}

C8: Remove Duplicates from Array

public static int[] removeDuplicates(int[] arr) {
    return Arrays.stream(arr).distinct().toArray();
}

C9: Check Anagram

public static boolean isAnagram(String a, String b) {
    char[] c1 = a.toLowerCase().toCharArray();
    char[] c2 = b.toLowerCase().toCharArray();
    Arrays.sort(c1); Arrays.sort(c2);
    return Arrays.equals(c1, c2);
}

C10: FizzBuzz

for (int i = 1; i <= 100; i++) {
    if (i % 15 == 0) System.out.println("FizzBuzz");
    else if (i % 3 == 0) System.out.println("Fizz");
    else if (i % 5 == 0) System.out.println("Buzz");
    else System.out.println(i);
}

C11: Swap Two Numbers Without Temp Variable

a = a ^ b; b = a ^ b; a = a ^ b;

C12: Check Armstrong Number

public static boolean isArmstrong(int n) {
    int original = n, sum = 0, digits = String.valueOf(n).length();
    while (n > 0) { sum += Math.pow(n % 10, digits); n /= 10; }
    return sum == original;
}

C13: Binary Search

public static int binarySearch(int[] arr, int target) {
    int low = 0, high = arr.length - 1;
    while (low <= high) {
        int mid = (low + high) / 2;
        if (arr[mid] == target) return mid;
        if (arr[mid] < target) low = mid + 1;
        else high = mid - 1;
    }
    return -1;
}

C14: Bubble Sort

public static void bubbleSort(int[] arr) {
    for (int i = 0; i < arr.length - 1; i++)
        for (int j = 0; j < arr.length - i - 1; j++)
            if (arr[j] > arr[j + 1]) { int t = arr[j]; arr[j] = arr[j+1]; arr[j+1] = t; }
}

C15: Count Words in a String

String[] words = text.trim().split("\\s+");
System.out.println("Word count: " + words.length);

C16: Find Second Largest

int max = Integer.MIN_VALUE, second = Integer.MIN_VALUE;
for (int n : arr) {
    if (n > max) { second = max; max = n; }
    else if (n > second && n != max) second = n;
}

C17: String to Integer (Without parseInt)

public static int stringToInt(String s) {
    int result = 0;
    for (char c : s.toCharArray()) result = result * 10 + (c - '0');
    return result;
}

C18: Matrix Transpose

for (int i = 0; i < rows; i++)
    for (int j = 0; j < cols; j++)
        transposed[j][i] = matrix[i][j];

C19: Power Without Math.pow

public static long power(int base, int exp) {
    long result = 1;
    for (int i = 0; i < exp; i++) result *= base;
    return result;
}

C20: Find Missing Number in Array (1 to n)

int n = arr.length + 1;
int expectedSum = n * (n + 1) / 2;
int actualSum = Arrays.stream(arr).sum();
int missing = expectedSum - actualSum;

7. Summary #

8. Next Chapter Recommendation #