Java String Coding Interview Questions with examples like reverse string, anagram and palindrome in Java

Java String coding, Things you should know

7 Min Read

Introduction:

In this guide on Java String Coding Interview Questions, we will cover, different programming questions related to string.

If you are preparing for a Java interview, one thing is absolutely non-negotiable — mastering Java String Coding Interview Questions.

Strings may look simple at first, but in reality, they are one of the most commonly asked and deeply tested topics in technical interviews. From basic operations like reversing a string to advanced problems like finding the longest substring without repeating characters, string-based questions are designed to evaluate your problem-solving ability, logic building, and optimization skills.

Most candidates fail not because they don’t know Java, but because they struggle to explain the logic clearly, handle edge cases, or optimize their solutions under pressure. Interviewers are not just looking for correct answers — they want to see how you think, how you approach a problem, and how efficiently you can solve it.

In this guide, we have covered the top 15 Java String Coding Interview Questions with:

Clear problem descriptions
Step-by-step logic explanations
Multiple approaches for each problem
Complete Java code
Input and output examples

Whether you are a beginner or an experienced professional preparing for a role like Automation Tester, SDET, or Java Developer, this guide will help you build confidence and crack interviews with ease.

1. Reverse a String

Description:

Given a string, reverse it so that characters appear in opposite order.

Logic:

Approach 1 (Built-in)

Take input string
Convert it to StringBuilder
Use .reverse()
Convert back to string

Approach 2 (Manual)

Start from last index
Traverse backward
Append each character to result

Approach 1

String input = "hello";
String result = new StringBuilder(input).reverse().toString();
System.out.println(result);

Approach 2

String input = "hello";
String result = "";for(int i = input.length() - 1; i >= 0; i--){
    result += input.charAt(i);
}
System.out.println(result);

Input:

hello

Output

olleh

2. Palindrome Check

Description:

Check if a string reads same forward and backward.

Logic:

Approach 1:

Reverse string
Compare with original

Approach 2:

Use two pointers (start, end)
Compare characters
Move inward

Code:

Approach 1

String str = "madam";
String rev = new StringBuilder(str).reverse().toString();
System.out.println(str.equals(rev));

Approach 2

String str = "madam";
int start = 0, end = str.length() - 1;
boolean isPalindrome = true;
while(start < end){
    if(str.charAt(start) != str.charAt(end)){
        isPalindrome = false;
        break;
    }
    start++;
    end--;
}
System.out.println(isPalindrome);

Input

madam

Output

true

3. Count Vowels and Consonants:

Description:

Count how many vowels and consonants are present.

Logic:

Loop through string
Check if character is vowel (aeiou)
Else count as consonant

Code:

Approach 1

String str = "automation";
int vowels = 0, consonants = 0;
for(char c : str.toCharArray())
{
    if("aeiou".indexOf(c) != -1)
        vowels++;
    else
        consonants++;
}
System.out.println("Vowels: " + vowels);
System.out.println("Consonants: " + consonants);

Approach 2

for(char c : str.toCharArray()){
    switch(c){
        case 'a': case 'e': case 'i': case 'o': case 'u':
            vowels++; break;
        default:
            consonants++;
    }
}

Input

automation

Output

Vowels: 6
Consonants: 4

4. Find Duplicate Characters:

Description:

Print characters that appear more than once.

Logic:

Approach 1

Store frequency in map
Print characters with count > 1

Approach 2

Track characters in set
If already exists → duplicate

Code

Approach 1

String str = "programming";
Map<Character, Integer> map = new HashMap<>();
for(char c : str.toCharArray()){
    map.put(c, map.getOrDefault(c, 0) + 1);
}

for(char c : map.keySet()){
    if(map.get(c) > 1){
        System.out.println(c);
    }
}

Approach 2

Set<Character> seen = new HashSet<>();
Set<Character> dup = new HashSet<>();
for(char c : str.toCharArray()){
    if(!seen.add(c)){
        dup.add(c);
    }
}
System.out.println(dup);

Input

programming

Output

r g m

5. Anagram Check

Description:

Check if two strings contain same characters in different order.

Logic:

Approach 1

Sort both strings → compare

Approach 2

Count frequency → compare

Code:

Approach 1

String s1 = "listen";
String s2 = "silent";
char[] a = s1.toCharArray();
char[] b = s2.toCharArray();
Arrays.sort(a);
Arrays.sort(b);
System.out.println(Arrays.equals(a, b));

Approach 2

int[] count = new int[26];
for(char c : s1.toCharArray()) count[c - 'a']++;
for(char c : s2.toCharArray()) count[c - 'a']--;
boolean flag = true;
for(int i : count){
    if(i != 0){
        flag = false;
        break;
    }
}
System.out.println(flag);

Input

listen, silent

Output

true

6. Remove Whitespaces

Description:

Remove all spaces from string.

Logic:

Either use regex
Or manually skip spaces

Code:

Approach 1

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

Approach 2

String result = "";
for(char c : str.toCharArray()){
    if(c != ' ')
        result += c;
}

Input

“hello world”

Output

helloworld

7. First Non-Repeated Character

Description:

Find first character that appears only once.

Logic:

Approach 1

Store frequency
Return first count=1

Approach 2

Compare each char with others

Code:

Approach 1

String str = "swiss";
Map<Character, Integer> map = new LinkedHashMap<>();
for(char c : str.toCharArray()){
    map.put(c, map.getOrDefault(c, 0) + 1);
}

for(char c : map.keySet()){
    if(map.get(c) == 1){
        System.out.println(c);
        break;
    }
}

Approach 2

for(int i=0;i<str.length();i++){
    boolean unique = true;    

for(int j=0;j<str.length();j++){
        if(i!=j && str.charAt(i)==str.charAt(j)){
            unique = false;
            break;
        }
    }    

if(unique){
        System.out.println(str.charAt(i));
        break;
    }
}

Input

swiss

Output

8. Remove Duplicate Characters

Description:

Remove repeated characters and keep only unique ones.

Logic:

Maintain order
Add only first occurrence

Code:

Approach 1

Set<Character> set = new LinkedHashSet<>();
for(char c : str.toCharArray()){
    set.add(c);
}
System.out.println(set);

Approach 2

StringBuilder sb = new StringBuilder();
for(char c : str.toCharArray()){
    if(sb.indexOf(String.valueOf(c)) == -1){
        sb.append(c);
    }
}
System.out.println(sb);

Input

programming

Output

progamin

9. Count Words

Description:

Count number of words in string.

Logic:

Approach 1

Split by space

Approach 2

Count spaces

Code:

Approach 1

String str = "Java is awesome";
int count = str.trim().split("\\s+").length;
System.out.println(count);

Approach 2

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

Input

Java is awesome

Output

These types of Java String Coding Interview Questions are frequently asked in real interviews.

10. Check Only Digits:

Description:

Check if string contains only numbers.

Logic:

Loop and validate each character

Code:

Approach 1

System.out.println(str.matches("\\d+"));

Approach 2

boolean flag = true;
for(char c : str.toCharArray()){
    if(!Character.isDigit(c)){
        flag = false;
        break;
    }
}
System.out.println(flag);

Input

12345

Output

true

11. First Repeated Character

Description:

Find first character that appears twice.

Logic:

If set already contains → repeated

Code:

Set<Character> set = new HashSet<>();
for(char c : str.toCharArray()){
    if(!set.add(c)){
        System.out.println(c);
        break;
    }
}

Input

programming

Output

12. Sort Characters

Description:

Sort string alphabetically.

Logic:

Convert to array → sort → convert back

Code:

char[] arr = str.toCharArray();
Arrays.sort(arr);
System.out.println(new String(arr));

Input:

dcba

Output:

abcd

13. All Substrings

Description

Print all substrings.

Logic:

Fix start → expand end

Code:

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

Input:

abc

Output:

a
ab
abc
b
bc
c

14. Longest Substring Without Repeating

Description:

Find longest substring with unique characters.

Logic:

Sliding window:

Expand window
Remove duplicates
Track max length

Code:

Set<Character> set = new HashSet<>();
int max = 0, left = 0;
for(int right = 0; right < str.length(); right++){
    while(set.contains(str.charAt(right))){
        set.remove(str.charAt(left++));
    }
    set.add(str.charAt(right));
    max = Math.max(max, right - left + 1);
}
System.out.println(max);

Input:

abcabcbb

Output:

15. String Rotation

Description:

Check if one string is rotation of another.

Logic:

If rotated → must exist in s1+s1

Code:

String s1 = "waterbottle";
String s2 = "erbottlewat";
System.out.println((s1 + s1).contains(s2));

Input:

waterbottle, erbottlewat

Output:

true

💥 FINAL TRUTH (No Drama)

If you:

Practice these properly
Explain logic like above
Write both approaches

👉 You’re ahead of 90% candidates.

Mastering Java String Coding Interview Questions is not about memorizing solutions — it’s about understanding patterns, practicing consistently, and learning how to think like an interviewer.

If you have gone through all the problems in this guide, you have already built a strong foundation in string manipulation. The next step is to practice these questions without looking at the solutions, try to explain your approach out loud, and focus on writing clean, optimized code.

Remember, in interviews:

Clarity of thought matters more than speed
Logic matters more than syntax
Optimization matters more than brute force

If you can confidently solve and explain these problems, you are already ahead of most candidates.

Keep practicing, stay consistent, and keep pushing your limits — because the difference between rejection and selection is often just one well-explained solution.

Practicing these Java String Coding Interview Questions regularly.

Conclusion

Mastering Java String Coding Interview Questions is not about memorizing solutions — it’s about understanding patterns, practicing consistently, and learning how to think like an interviewer. These Java String Coding Interview Questions are essential for cracking technical interviews.

Remember, in interviews:

Clarity of thought matters more than speed
Logic matters more than syntax
Optimization matters more than brute force

If you can confidently solve and explain these problems, you are already ahead of most candidates.

Keep practicing, stay consistent, and keep pushing your limits — because the difference between rejection and selection is often just one well-explained solution.