Java Interview Questions - Part 1: All About Strings

Welcome to our new series: Java Interview Questions - Part 1. If you're preparing for your next IT or Software Developer interview, especially for Java roles, this post is just for you. Today, we’re going to talk about one of the favourite topics of interviewers — Strings in Java. 😄
We’ll keep the tone light and explanations simple. Just like how you would explain it to your friend during a late-night study session.
Whether you are a fresher or someone with a couple of years of experience, these string-based questions are often asked in companies like TCS, Infosys, Accenture and so on. So let’s get into it. 💻☕

1. Reverse a String / How to reverse a string in Java?

String input = "hello";

StringBuilder sb = new StringBuilder(input);

System.out.println(sb.reverse());
Output: olleh

Why StringBuilder instead of StringBuffer?

You might wonder why we used StringBuilder here and not StringBuffer. Both help in creating mutable strings (strings that can change), but there’s a key difference:

• StringBuffer is thread-safe (synchronized), meaning it can safely be used by multiple threads at once.
• StringBuilder is not thread-safe, but faster because it doesn’t have the overhead of synchronization.

2. Check if a String is a Palindrome

String input = "madam";
StringBuffer sb = new StringBuffer(input);

System.out.println(s.length()>=3 && s.equalsIgnoreCase(sb.reverse().toString()));
Output: true

3. Find Duplicate Characters in a String

String input = "programming";
Map<Character,Integer> map = new HashMap<>();
for(int i=0; i< input.length(); i++) {

char c = input.charAt(i);

map.put(c, map.getOrDefault(c, 0)+1);
}
for(Entry<Character,Integer> entry: map.entrySet()) {
if(entry.getValue()>1) {System.out.println(entry.getKey());}}
Output: r,g,m

4. Remove All Whitespaces

String input = "  Java Interview   ";

String result = input.replaceAll("\\s", "");

System.out.println(result);
Output: JavaInterview

5. First Non-Repeated Character

String input = "swiss";

LinkedHashMap<Character, Integer> map = new LinkedHashMap<>();

for (char c : input.toCharArray()) {

    map.put(c, map.getOrDefault(c, 0) + 1);

}

for (Map.Entry<Character, Integer> entry : map.entrySet()) {

    if (entry.getValue() == 1) {

        System.out.println("First non-repeating character: " + entry.getKey());

        break;

    }

}

Output: w

6. Check if Two Strings are Anagrams

String a = "listen", b = "silent";

char[] arr1 = a.toCharArray();

char[] arr2 = b.toCharArray();

Arrays.sort(arr1);

Arrays.sort(arr2);

System.out.println(Arrays.equals(arr1, arr2));

Output: true

7. Count Occurrences of a Character

String s = "banana";
char c = 'a';
HashMap<Character, Integer> map = new HashMap<>();

for( char c1 : s.toCharArray()) {

map.put(c1, map.getOrDefault(c1, 0)+1);}

for(Entry<Character, Integer> entry : map.entrySet()) {

if(entry.getKey().equals(c)) {

System.out.println(entry.getValue());}}

Output: 3

8. Convert String to Integer(without using built-in methods)

String str = "123";

int result = 0;

for (char c : str.toCharArray()) {

    result = result * 10 + Character.getNumericValue(c);

}

System.out.println(result);
Output: 123

9. Check if One String Contains Another

String big = "Hello India";

String small = "India";

System.out.println(big.contains(small));

Output: true

10. Replace All Occurrences of a Character

String input = "banana";

String output = input.replace('a', '@');

System.out.println(output);
Output: b@n@n@

11. Split a String by Delimiter

String input = "apple,banana,grapes";

String[] fruits = input.split(",");

for(String fruit: fruits) {

System.out.println(fruit);}
Output:
apple
banana
grapes

12. Sort Characters in a String Alphabetically

String input = "java";

char[] arr = input.toCharArray();

Arrays.sort(arr);

System.out.println(new String(arr)); 

Output: aajv

13. Find the Longest Word in a Sentence

String sentence = "Java is a beautiful language";

String[] words = sentence.split(" ");

 if (words.length == 0 || (words.length == 1 && words[0].isEmpty())) {

        System.out.println(""); }

String longestWord = ""; 

for (String word : words) {

if (word.length() > longestWord.length()) {

longestWord = word; }}

System.out.println(longestWord);

Output: beautiful

14. Find Repeated Words in a String

String input = "Java is great and Java is powerful";

String[] words = input.toLowerCase().split(" ");

Map<String, Integer> wordCount = new HashMap<>();

for (String word : words) {

    wordCount.put(word, wordCount.getOrDefault(word, 0) + 1);

}

for(Entry<String, Integer> entry: wordCount.entrySet()) {

if(entry.getValue()>1) {

System.out.println(entry.getKey() + " - " + entry.getValue() + " times ");

}

}

Output: 

java - 2 times 

is - 2 times 

Final Thoughts

Practicing string problems is one of the easiest ways to become confident in Java. These types of questions are favourites in both coding rounds and technical interviews.

Pro Tip: Try rewriting these solutions using Java 8 Streams for better practice!

If you found this helpful, please share this article with your friends, classmates or teammates who are also preparing for interviews. 💬📤

Till then, happy coding! 💻🌟