Top 10 Difficult Interview Questions on Java Strings Asked in FAANG/MAANG Companies

Java Strings are a fundamental aspect of the Java programming language, and a strong understanding of string manipulation is essential when interviewing for top-tier companies like FAANG (Facebook, Amazon, Apple, Netflix, Google) or MAANG (Microsoft, Amazon, Adobe, Netflix, Google). In this blog post, we will explore the top 10 difficult interview questions related to Java Strings that are frequently asked in these companies. Each question will be accompanied by a detailed explanation and example solutions. Let’s get started!

  1. How do you reverse a string without using any built-in functions or libraries?

This question tests your understanding of string manipulation and problem-solving skills.

Example Solution:

public static String reverseString(String str) {
    char[] charArray = str.toCharArray();
    int left = 0;
    int right = charArray.length - 1;
    while (left < right) {
        char temp = charArray[left];
        charArray[left] = charArray[right];
        charArray[right] = temp;
        left++;
        right--;
    }
    return new String(charArray);
}
  1. Explain the difference between String, StringBuilder, and StringBuffer in terms of thread safety and performance.

This question assesses your knowledge of different string classes and their characteristics.

Example Explanation:

  • String: Immutable class, not thread-safe, and offers good performance for read-only operations.
  • StringBuilder: Mutable class, not thread-safe, provides better performance for single-threaded string manipulation.
  • StringBuffer: Mutable class, thread-safe due to synchronized methods, suitable for multi-threaded environments but with slightly reduced performance compared to StringBuilder.

3. How would you find the first non-repeated character in a given string?

This question evaluates your problem-solving skills and understanding of string manipulation.

Example Solution:

public static char findFirstNonRepeatedChar(String str) {
    Map<Character, Integer> charCount = new LinkedHashMap<>();
    for (char c : str.toCharArray()) {
        charCount.put(c, charCount.getOrDefault(c, 0) + 1);
    }
    for (char c : str.toCharArray()) {
        if (charCount.get(c) == 1) {
            return c;
        }
    }
    throw new IllegalArgumentException("No non-repeated character found.");
}
  1. How can you check if two strings are anagrams?

This question tests your ability to compare and manipulate strings effectively.

Example Solution:

public static boolean areAnagrams(String str1, String str2) {
    if (str1.length() != str2.length()) {
        return false;
    }
    int[] charCount = new int[26];
    for (int i = 0; i < str1.length(); i++) {
        charCount[str1.charAt(i) - 'a']++;
        charCount[str2.charAt(i) - 'a']--;
    }
    for (int count : charCount) {
        if (count != 0) {
            return false;
        }
    }
    return true;
}
  1. Explain the concept of string interning and how it relates to the String Pool.

This question assesses your knowledge of string interning and its connection to the String Pool.

Example Explanation:
String interning is a process where multiple string objects with the same value are represented by a single shared copy. The String Pool, a memory area in the Java heap, is responsible for storing unique string literals. When a new string literal is created, Java checks the String Pool to determine if an identical string already exists. If it does, the existing string reference is returned, optimizing memory usage.

  1. How can you remove duplicate characters from a string?

This question evaluates your ability to manipulate strings and efficiently solve string-related problems.

Example Solution:

public static String removeDuplicates(String str) {
    StringBuilder result = new StringBuilder();
    boolean[] visited = new boolean[256];
    for (char c : str.toCharArray()) {
        if (!visited[c]) {
            result.append(c);
            visited[c] = true;
        }
    }
    return result.toString();
}
  1. How would you check if a string is a palindrome?

This question tests your understanding of string manipulation and problem-solving skills.

Example Solution:

public static boolean isPalindrome(String str) {
    int left = 0;
    int right = str.length() - 1;
    while (left < right) {
        if (str.charAt(left) != str.charAt(right)) {
            return false;
        }
        left++;
        right--;
    }
    return true;
}
  1. Explain the concept of string immutability and why it is important.

This question evaluates your understanding of string immutability and its significance.

Example Explanation:
In Java, strings are immutable, meaning their values cannot be changed once they are created. Any operation that appears to modify a string actually creates a new string object. Immutability ensures string integrity and enhances security, thread safety, and memory efficiency. Additionally, it allows for efficient string sharing and optimization.

  1. How can you efficiently check if a string contains only unique characters?

This question tests your problem-solving skills and understanding of string manipulation.

Example Solution:

public static boolean hasUniqueCharacters(String str) {
    boolean[] visited = new boolean[256];
    for (char c : str.toCharArray()) {
        if (visited[c]) {
            return false;
        }
        visited[c] = true;
    }
    return true;
}
  1. Explain the concept of a substring and how you can extract it from a larger string.

This question assesses your knowledge of substring extraction and manipulation.

Example Explanation:
A substring is a contiguous sequence of characters within a larger string. In Java, you can extract a substring using the substring() method. It takes the starting index as the first parameter and an optional ending index as the second parameter, allowing you to extract a specific portion of the original string.

Free Tip:
Preparing for interviews with top-tier companies like FAANG/MAANG requires a solid understanding of Java Strings and the ability to solve complex string-related problems. By studying and practicing the answers to these difficult interview questions, you can boost your confidence and increase your chances of success in your interviews. Keep learning, practicing, and exploring Java Strings to excel in your career aspirations. Good luck!

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