Top 25+ Interview Questions On String in Java with Answers

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Java is one of the most widely used programming languages. Did you know? According to Statista, it is used by over 63.6% of developers worldwide. However, when it comes to Java interviews, they can cover a wide range of topics, from collections framework to exception handling. And, if the job requires proficiency in string, you’ll need to be prepared for string-related interview questions. But these questions alone can be vast, leaving little time to focus on other concepts. That’s why we have prepared this guide for you. We’ve compiled a list of common interview questions on string in Java to help you prepare. 

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Basic Interview Questions on String in Java

Here are some important string based interview questions in Java and their answers. 

1. What is a string in Java?

A string in Java is a series of characters enclosed within double quotes used to represent textual data. 

Examples include “hello” and “Java is fun!” Strings are essential for storing and manipulating text-based information in Java programs, allowing for tasks such as displaying messages, reading input, and processing data.

2. How do you create a string object in Java?

You can create a string object in Java using either the String class constructor or by directly assigning a string literal. 

For example:

String str1 = new String(“hello”);

String str2 = “Java is fun!”;

3. Is String a primitive or derived type in Java?

The string is a derived type in Java, not a primitive type. Unlike primitive types, which represent basic data types like integers and booleans, strings are objects that encapsulate a sequence of characters. 

They belong to the Java.lang package and offer various methods for string manipulation and comparison.

4. Explain the String pool in Java.

The String pool in Java is a special area of memory where string literals are stored. When a string is created using double quotes, Java checks if it already exists in the pool. If so, it reuses it; otherwise, it adds it to the pool.

5. Difference between string in C and string in Java.

In C, strings are represented as arrays of characters terminated by a null character (‘\0’). They are handled using pointers and array operations. 

In contrast, Java strings are objects belonging to the String class, offering various methods for manipulation, comparison, and string-specific operations, providing more functionality and safety.

6. What is the length of a string in Java? 

The length of a string in Java refers to the number of characters it contains. You can get the length of a string using the length() method. 

For example:

String str = “Java”;

int length = str.length(); // length will be 4

String Programming Interview Questions in Java for Experienced

Here are some important interview questions on string in Java and their answers for experience. 

7. What is the difference between StringBuilder and StringBuffer? 

StringBuilder is not synchronized, making it faster but not thread-safe. StringBuffer, on the other hand, is synchronized, ensuring thread safety, but may be slower due to synchronization overhead.

8. Explain the concept of string immutability in Java.

In Java, strings are immutable, meaning their value cannot be changed after they are created. Any operation that appears to modify a string actually creates a new string object.

9. Can you explain the concept of string interning in Java?

String interning is the process of storing only one copy of each distinct string value, regardless of how many references point to it. This optimization is performed automatically by the Java runtime to conserve memory.

10. What is StringTokenizer in Java?

StringTokenizer in Java is a class used to break a string into tokens or smaller parts based on a specified delimiter. It provides methods to iterate over these tokens and extract them one by one.

Java String Interview Questions for 5 Years Experience

If you have 5 years or less experience, learn these key string based interview questions in Java. 

11. Explain the difference between == and equals() when comparing strings. 

The == operator checks if two string references point to the same memory location, while the equals() method checks if two strings have the same content.

12. How do you find the index of a specific character or substring in a string? 

You can use the indexOf() method to find the index of a specific character or substring within a string.

13. Why is a string used as a HashMap key in Java?

Strings are used as HashMap keys in Java due to their immutability, which ensures that the key’s value remains constant. Additionally, strings provide efficient hashing and easy comparison, making them ideal for key-value pair associations in HashMaps.

14. Why char array is preferred over a String in storing passwords?

Char arrays are preferred over strings for storing passwords because they are mutable, allowing for secure erasure after use. Unlike strings, which are immutable and remain in memory, char arrays offer better protection against password exposure.

Also Read - Top 20+ HashMap Interview Questions With Answers

Java String Programming Questions for 10+ Years’ Experience

These Java string questions are for candidates with 10 or more years of experience. 

15. How do you handle large text files in Java efficiently? 

Reading large text files efficiently involves using BufferedReader and FileReader classes for buffered reading and processing data in chunks to minimize memory usage.

16. What are the differences between regular expressions and string manipulation methods in Java?

Regular expressions provide powerful pattern-matching capabilities, while string manipulation methods like substring() and indexOf() are simpler but less flexible for complex pattern-matching and manipulation tasks.

17. Can you explain the role of character encoding in string processing?

Character encoding defines how characters are represented as bytes in memory and affects string processing, especially when dealing with internationalization and encoding conversion tasks. Common encodings include UTF-8 and ISO-8859-1.

18. How can you split the string in Java?

In Java, you can split a string into substrings using the split() method from the String class. This method takes a delimiter as an argument and returns an array of strings, where each element is a substring separated by the delimiter. 

For example:

String str = “Hello,World”;

String[] parts = str.split(“,”);

String Coding Questions in Java

Here are some important string based interview questions in Java and their answers.

19. How do you compare two strings in Java?

You can compare strings in Java using the equals() method for content comparison or the == operator for reference comparison.

For example:

String str1 = “hello”;

String str2 = “hello”;

boolean isEqual = str1.equals(str2); // Content comparison

boolean isSameReference = (str1 == str2); // Reference comparison

20. How do you check whether a String is empty in Java?

To check if a string is empty in Java, you can use the isEmpty() method or check if its length is zero using length(). 

For example:

String str = “hello”;

boolean isEmpty = str.isEmpty(); // false

String emptyStr = “”;

boolean isEmptyStr = emptyStr.isEmpty(); // true

// Or check using length()

boolean isEmptyLength = str.length() == 0; // false

21. Implement a function to check if two strings are rotations of each other.

public boolean areRotations(String str1, String str2) {

   if (str1.length() != str2.length()) {

       return false;

   }

   String concatenated = str1 + str1;

   return concatenated.contains(str2);

}

Advanced Interview Programs on String in Java

Here are some advanced string interview program questions in Java and their answers.

22. Explain how you would check if two strings are anagrams of each other. 

To check if two strings are anagrams, compare their sorted character arrays. If both arrays are equal, the strings are anagrams.

For Example:

public boolean areAnagrams(String str1, String str2) {

   char[] charArray1 = str1.toCharArray();

   char[] charArray2 = str2.toCharArray();

   Arrays.sort(charArray1);

   Arrays.sort(charArray2);

   return Arrays.equals(charArray1, charArray2);

}

23. Write a function to count the occurrences of a specific character in a string.

public int countOccurrences(String str, char target) {

   int count = 0;

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

       if (str.charAt(i) == target) {

           count++;

       }

   }

   return count;

}

24. Write a function to find all permutations of a string.

import java.util.ArrayList;

import java.util.List;

public class StringPermutations {

   public List<String> generatePermutations(String str) {

       List<String> permutations = new ArrayList<>();

generatePermutationsHelper(str.toCharArray(), 0, permutations);

       return permutations;

   }

   private void generatePermutationsHelper(char[] chars, int index, List<String> permutations) {

       if (index == chars.length – 1) {

           permutations.add(new String(chars));

           return;

       }

       for (int i = index; i < chars.length; i++) {

           swap(chars, index, i);

           generatePermutationsHelper(chars, index + 1, permutations);

           swap(chars, index, i);

       }

   }

   private void swap(char[] chars, int i, int j) {

       char temp = chars[i];

       chars[i] = chars[j];

       chars[j] = temp;

   }

}

String Programs in Java for Practice

Practice these Java string interview programs to increase your success rate.

25. Write a function to find the first non-repeating character in a string.

public char firstNonRepeatingChar(String str) {

   int[] charCount = new int[256];

   for (char c : str.toCharArray()) {

       charCount[c]++;

   }

   for (char c : str.toCharArray()) {

       if (charCount[c] == 1) {

           return c;

       }

   }

   return ‘\0’; // No non-repeating character found

}

26. Write a function to compress a string by replacing repeated characters with their count.

public String compressString(String str) {

   StringBuilder compressed = new StringBuilder();

   int count = 1;

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

       if (i + 1 < str.length() && str.charAt(i) == str.charAt(i + 1)) {

           count++;

       } else {

compressed.append(str.charAt(i)).append(count);

           count = 1;

       }

   }

   return compressed.length() < str.length() ? compressed.toString() : str;

}

27. Write a function to count the occurrences of each character in a string.

public Map<Character, Integer> countCharacters(String str) {

   Map<Character, Integer> charCount = new HashMap<>();

   for (char c : str.toCharArray()) {

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

   }

   return charCount;

}

Wrapping Up

These 25+ interview questions on string in Java cover a wide range of topics, from basic concepts to advanced techniques. Learning these questions can greatly improve your readiness for Java interviews. For more tech job opportunities in India, visit Hirist, your go-to platform for the latest and best tech jobs. Start your job search today!