Java String Interview Questions & Programs (With Examples)

String questions are one of the most important topics in Java interviews.

If you're preparing for Java interviews, String-based questions are very common. They help interviewers test your logic, problem-solving, and understanding of core Java concepts.

Written by Shivkumar Udas – Engineering student sharing practical Java guides for beginners.

💡 Why String Questions Matter

Strings are used in almost every Java application, so interviewers often ask String-related questions.

These are some of the most frequently asked Java String interview questions.

Optimization Tip: In many cases, use StringBuilder instead of String for better performance.

1. Reverse a String in Java

String str = "Java";
StringBuilder sb = new StringBuilder(str);
System.out.println(sb.reverse());

Time Complexity: O(n)

Interview Variation: Reverse without using built-in methods.

2. Check Palindrome String

String str = "madam";
StringBuilder sb = new StringBuilder(str);

if(str.equals(sb.reverse().toString())) {
    System.out.println("Palindrome");
}

Time Complexity: O(n)

3. Count Vowels and Consonants

String str = "Java Programming";
int vowels = 0, consonants = 0;

for(char c : str.toLowerCase().toCharArray()) {
    if(c >= 'a' && c <= 'z') {
        if("aeiou".indexOf(c) != -1)
            vowels++;
        else
            consonants++;
    }
}

System.out.println("Vowels: " + vowels);
System.out.println("Consonants: " + consonants);

Time Complexity: O(n)

4. Check Anagram Strings

import java.util.Arrays;

String str1 = "listen";
String str2 = "silent";

char[] a = str1.toCharArray();
char[] b = str2.toCharArray();

Arrays.sort(a);
Arrays.sort(b);

if(Arrays.equals(a, b)) {
    System.out.println("Anagram");
}

Time Complexity: O(n log n)

5. Remove White Spaces

String str = "Java Programming";
str = str.replaceAll("\\s", "");

System.out.println(str);

Time Complexity: O(n)

6. Find Duplicate Characters

String str = "programming";

for(int i = 0; i < str.length(); i++) {
    for(int j = i + 1; j < str.length(); j++) {
        if(str.charAt(i) == str.charAt(j)) {
            System.out.println(str.charAt(j));
            break;
        }
    }
}

Time Complexity: O(n²)

7. Check if String Contains Only Digits

String str = "12345";

boolean isDigit = str.matches("\\d+");

System.out.println(isDigit);

Time Complexity: O(n)

8. Convert String to Integer

String str = "123";
int num = Integer.parseInt(str);

System.out.println(num);

Time Complexity: O(n)

9. Count Frequency of Characters

String str = "java";

for(char c : str.toCharArray()) {
    int count = 0;
    for(char ch : str.toCharArray()) {
        if(c == ch) count++;
    }
    System.out.println(c + " : " + count);
}

Time Complexity: O(n²)

10. Remove Duplicate Characters

String str = "programming";
String result = "";

for(char c : str.toCharArray()) {
    if(result.indexOf(c) == -1) {
        result += c;
    }
}

System.out.println(result);

Time Complexity: O(n²)

📌 Most Important String Questions

• Reverse String
• Palindrome
• Anagram
• Duplicate Characters
• Frequency Count

🎯 Interview Tip

Focus on writing logic clearly. Many interviewers ask you to optimize String programs.

Understanding String methods like length(), charAt(), and equals() is very important.

🧪 Practice Task

Try solving these programs without looking at the code and test with different inputs.

🔗 Related Guides

• Java Roadmap
• Java Coding Programs
• Java Interview Questions

🎯 Conclusion

String questions are very important for Java interviews and help improve your logical thinking.

Practice regularly and try to optimize your solutions.