Java String Interview Questions and Answers for 2023 - IQCode's Top Picks

Comprehensive List of Java String Interview Questions

The Java programming language has been popular among developers for the past 25 years because of its user-friendly and flexible nature that can be used for platform and web application development. During Java interviews, interviewers often pay close attention to the String class. Understanding the String class in Java is essential for every Java programmer, as it is fundamental to Java programming. Every Java application, whether it's a Java desktop application, enterprise application, web application, or mobile application makes use of the String class. Therefore, it's one of the hottest and essential topics in Java interviews.

In this article, we've compiled a comprehensive list of insightful Java String Interview Questions for both freshers and experienced programmers that focus on various topics such as thread-safety, immutability, string methods in Java, StringBuilder and StringBuffer, memory consumption, comparing String instances in Java, using String as the key in HashMap, equals() vs. == check for String. These questions will help you understand the String concept in detail and prepare you to tackle String-related questions during a Java technical interview.

Before we begin, let's have a quick look at what Java String is all about.

## What is String in Java?

Strings are one of the most common objects used in Java programming and are essentially sequences of characters. As an example, the string "IQCode" contains the following characters: "I", "Q", "C", "o", "d", and "e". You can create a string by using String literal or by using the `new` keyword. Additionally, String supports a variety of methods to operate on Strings such as the `equals()` method to compare two Strings, the `replace()` method to replace String characters, the `substring()` method to get a substring, the `toUpperCase()` method to convert String to upper case, the `split()` method to split a long String into multiple Strings, and so on.

Now, let's look at the most commonly asked String Interview questions:

- Java String Interview Questions for Freshers - Java String Interview Questions for Experienced - String Programming Questions

### Java String Interview Questions for Freshers

1. How do you declare a String in Java?


String str = "Hello World";

Explanation: To declare a String in Java, you can use the `String` keyword followed by the variable name. The variable name can be any valid Java identifier that adheres to naming conventions. Strings can also be declared using String literals enclosed in double-quotes, as shown in the code snippet above.

String in Java: Primitive or Derived type?

In Java, String is not a primitive type. It is a derived type that is defined by the Java language itself. The String class represents character strings, and it is part of the java.lang package.

Difference between String in C and String in Java

In C, strings are represented as arrays of characters. The end of a string is denoted by a null character (`'\0'`). Manipulating strings in C requires functions from the string library, such as `strcpy` and `strlen`.

In Java, strings are represented by the `String` class. Strings in Java are objects and have methods that can be called on them. Strings are immutable in Java, which means that once a string is created, it cannot be changed. This leads to greater safety and efficiency in Java programs.

Understanding String Pool in Java

In Java, the String Pool is a pool of strings maintained by the JVM. Whenever a string is created using double quotes, the JVM first checks if that string already exists in the pool. If it does, then the JVM returns a reference to the pooled instance, otherwise it creates a new string object and stores it in the pool for future reuse.

This means that when two string objects are created with the same literal value using double quotes, they both refer to the same object in the string pool. This results in efficient memory utilization and faster string comparisons in Java.

It is important to note that strings created using the `new` keyword are not added to the string pool. These strings are stored in the heap memory and are not eligible for garbage collection until they are no longer referenced.

Overall, the String Pool in Java is a memory-saving mechanism that helps optimize the performance of string manipulation in Java programs.

Is String Immutable in Java? What are the Benefits of Immutability?

In Java, String is immutable which means once the String is created, its value cannot be changed. This is because a new String object is created every time the contents of a String are modified. However, if you need to modify a String, you can create a new String object with the desired modifications.

The main benefit of immutability is that String objects can be shared without worrying about their values being changed. This makes String objects thread-safe, which is very important in concurrent programming. Also, since String objects are used very frequently in Java programs, immutability helps improve the performance of the program by caching String literals. This means that multiple references to the same String literal can refer to the same String object, which saves memory and reduces garbage collection.

In general, immutability makes Java programming easier and more efficient. It allows for safer and more predictable code, as well as more efficient use of memory.

Explanation of the Java String intern() Method



method in Java string returns one of the following;

  • It returns a reference to the String object from the string pool if the string is already present there.
  • It adds the string to the pool if it doesn't exist and returns its reference.

It is used to get the reference of the String object from the string pool, so the same reference can again be used if the same string exists in future.

String str = "Hello World";<br>
String str1 = str.intern();

Difference between String and StringBuffer in Java

In Java, a String object is immutable, which means once it is created, its value cannot be changed. On the other hand, a StringBuffer object is mutable, which means you can modify the value stored in it.

Therefore, if you have a small amount of text that won't be modified frequently, use a String. However, if you have a large amount of text or frequently modified text, use StringBuffer as it is more efficient for making modifications.

Difference Between StringBuffer and StringBuilder in Java

In Java, both StringBuffer and StringBuilder classes are used for creating and manipulating strings. However, the main difference between them is in their synchronization behavior. StringBuffer methods are synchronized, whereas StringBuilder methods are not synchronized. This means that StringBuffer is safe to use in multi-threaded environments where multiple threads are accessing the same object, while StringBuilder is faster and more efficient in single-threaded applications because it avoids the overhead of synchronization.

Additionally, StringBuffer was introduced in Java 1.0, while StringBuilder was introduced in Java 1.5 as a non-synchronized alternative to StringBuffer. Both classes have similar methods and can be used interchangeably, except in situations where thread safety is a concern.

Comparing Two Strings in Java

To compare two strings in Java, you should use the `equals()` method. This is because the equals() method compares the actual contents of the strings, whereas the `==` operator compares object references.

Here's an example:

String str1 = "hello";
String str2 = "hello";

if (str1.equals(str2)) {
    System.out.println("Strings are equal");
} else {
    System.out.println("Strings are not equal");

In this example, the `equals()` method is used to compare the contents of `str1` and `str2`. Since the contents are the same, the output will be "Strings are equal".

It's important to note that `equals()` is case-sensitive. If you want to compare strings without considering case, you can use the `equalsIgnoreCase()` method instead.

Difference between str1 == str2 and str1.equals(str2)

In Java, `==` is used for reference comparison, i.e., it checks if the two string variables refer to the same object in memory. On the other hand, the `equals()` method compares the contents of the strings and returns true if they are equal.

For example:

String str1 = "hello";
String str2 = "hello";
String str3 = new String("hello");

// Reference comparison
System.out.println(str1 == str2); // true
System.out.println(str1 == str3); // false

// Content comparison
System.out.println(str1.equals(str2)); // true
System.out.println(str1.equals(str3)); // true

In the above example, `str1` and `str2` contain the same content, so both reference comparison with `==` and content comparison with `equals()` will return true. However, `str3` is a new object with the same content as `str1`, so reference comparison with `==` will return false, but content comparison with `equals()` will return true.

Comparing Strings using the == Operator

Yes, it is possible to compare strings using the == operator. However, it is not always reliable and can lead to unexpected results.

The risk involved in comparing strings using the == operator is that it only checks if the two strings have the same object reference, not if they have the same value. This means that if two strings have the same value but are not the same object, the == operator will return false. On the other hand, two strings with different values but the same object reference will return true.

To compare strings based on their values, it is recommended to use the .equals() method in Java.

Explanation of the substring() Method in Java

The substring() method is a Java function that is primarily used to extract a portion of a given string based on a specified beginning and ending index. It takes two arguments, the starting index and the ending index of the substring that needs to be extracted. Once the indices are defined, the method creates a new string that contains the characters between the specified starting and ending positions. This method is frequently used in Java programming to manipulate strings and analyze text data.

Can a String be used in a switch case in Java?

In Java 7 and later versions, a String can be used in a switch case. However, in earlier versions of Java, only integral types like byte, short, char, and int could be used with switch statements.

Here is an example of using a String in a switch statement in Java:

String fruit = "apple";
switch (fruit) {
    case "apple":
        System.out.println("This is an apple.");
    case "banana":
        System.out.println("This is a banana.");
        System.out.println("This is not a fruit.");

In the above example, the switch statement is using the String "fruit" to determine which case to execute. If the value of "fruit" is "apple", then the first case will execute. If the value is "banana", then the second case will execute. If the value is anything else, the default case will execute.

Note that using String in switch statements can be less efficient than using integral types. This is because the String value will need to be compared to each case value, which can be slower than comparing integers.

Java String Methods for Experienced Developers

In Java, a string is an object that represents a sequence of characters. It has many built-in methods that can help manipulate and transform strings. Below are some of the most commonly used string methods in Java:

1. length() : Returns the length of the string. <br>
2. charAt(int index) : Returns the character at the specified index. <br>
3. substring(int beginIndex) : Returns a substring from the specified index to the end of the string. <br>
4. substring(int beginIndex, int endIndex) : Returns a substring from the specified begin index to the specified end index. <br>
5. equals(Object obj) : Compares the string to the specified object. <br>
6. compareTo(String anotherString) : Compares two strings lexicographically. <br>
7. toUpperCase() : Converts all characters of the string to uppercase. <br>
8. toLowerCase() : Converts all characters of the string to lowercase. <br>
9. split(String regex) : Splits the string based on the specified regular expression. <br>
10. replace(char oldChar, char newChar) : Replaces all occurrences of a specified character with another character. <br>

By using these methods effectively, you can perform complex string manipulations with ease in Java programming.

Is String Thread-Safe in Java?

In Java, String is an immutable class which means its state cannot be changed once created. Hence, it is thread-safe as there is no possibility of its state being modified by multiple threads concurrently. However, it is important to note that any operations on String objects such as concatenation will create new String instances, which might not be thread-safe if accessed by multiple threads concurrently. Therefore, it is recommended to use thread-safe alternatives such as StringBuilder or StringBuffer for string manipulation in a multithreaded environment.

Why do developers use a string as a HashMap key in Java?

In Java, a string is often used as a key in a HashMap because it provides a unique identifier for the value stored in the data structure. Strings have a built-in `hashCode()` method in Java, which is used to compute the hash code of the string. This hash code is then used by the HashMap to compute the index at which the value should be stored. Using a string as a key also allows for easy retrieval of values, as the same string used to store the value can be used to retrieve it. Additionally, strings provide a natural ordering, which can be useful in certain scenarios.

Best way to split a string in Java

In Java, a string can be split into substrings based on a specified delimiter. The best way to split a string in Java is by using the `split()` method. This method takes in a regular expression as a parameter that defines the delimiter to use for the split operation.

Here is an example of using the `split()` method to split a string into substrings based on a comma delimiter:

String str = "apple,banana,orange"; String[] strArr = str.split(",");

After executing this code, `strArr` will be an array containing the following elements: `["apple", "banana", "orange"]`.

Why Char Array is Preferable Over String for Storing Passwords?

In programming, passwords are sensitive information that should be stored securely. One common best practice is to use a character array instead of a string to store passwords. This is because strings are immutable in Java, meaning that once they are created, they cannot be changed. This can create security vulnerabilities since strings are stored in memory as plain text and can potentially be accessed by unauthorized users.

On the other hand, a character array can be overwritten, which makes it more secure for password storage. Additionally, arrays allow the password to be cleared from memory after it has been used, which further increases security.

Overall, using a char array for password storage is a more secure and recommended practice than using a string.

Explanation of the String Subsequence Method

The String Subsequence method is a way to check whether a given subsequence is present in a given string or not. In simple terms, a subsequence is a sequence of characters that appears in the same order in the given string but may not be contiguous.

The syntax of the String Subsequence method is as follows:

public boolean contains(CharSequence s)

Here, "CharSequence" represents any object that implements the CharSequence interface, such as a String, StringBuilder or StringBuffer.

The method returns a boolean value of "true" if the given subsequence is found in the given string, otherwise it returns "false".

For example, consider the following code snippet:

String str = "The quick brown fox jumps over the lazy dog";
boolean result = str.contains("fox");

In this code, we are checking whether the subsequence "fox" is present in the given string or not. The output of this code will be "true" since the subsequence "fox" is present in the string.

Overall, the String Subsequence method is a useful tool for checking if a certain sequence of characters exists in a given string.

Explanation of StringJoiner

StringJoiner is a class in Java that is used to concatenate strings with a specified delimiter. It provides a convenient way of joining strings while adding a delimiter between them. This becomes useful when working with lists of strings or when building SQL statements. The StringJoiner class simplifies the process of joining strings and saves the programmer from writing complex code for this task.

Converting a Java String to a Byte Array

To convert a Java String to a byte array, you can use the `getBytes()` method. This method returns an array of bytes representing the characters of the String using the platform's default charset.

Here's an example:

String str = "Hello, World!";
byte[] byteArray = str.getBytes();

In this example, the `str` variable holds the String we want to convert, and the `byteArray` variable holds the resulting byte array.

Keep in mind that the `getBytes()` method uses the default charset of the platform, which may not be suitable for all applications. If you need to specify a different charset, you can use one of the overloaded versions of the `getBytes()` method that accept a Charset parameter.

Converting String to Integer and Vice Versa in Java

To convert a String to an integer in Java, we can use the


method of the


class. Here's an example:

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

To convert an integer to a String, we can use the


method of the


class. Here's an example:

int num = 123;
String str = Integer.toString(num);

Note that these methods may throw a


if the input String is not a valid integer. It's important to handle this exception appropriately in your code.

Converting a String to a StringBuilder in Java

In Java, we can convert a String to a StringBuilder by using the StringBuilder constructor that takes a String as an argument. Here's an example code snippet:

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

In this example, we create a String object "str" with the value "hello". Then, we create a StringBuilder object "sb" by passing "str" as an argument to the StringBuilder constructor.

Keep in mind that StringBuilder is used for mutable sequences of characters, while String is immutable. So, if you need to modify the contents frequently, it's better to use StringBuilder instead.

Checking for Empty Strings in Java

In Java, you can check whether a string is empty or not using the `isEmpty()` method.

Here's an example:

String str = "";
if(str.isEmpty()) {
   System.out.println("String is empty");
} else {
   System.out.println("String is not empty");

In this example, `isEmpty()` method is called on the string `str`. If the string is empty, the `isEmpty()` method returns `true`, and the code inside the `if` block is executed. If the string is not empty, the `isEmpty()` method returns `false`, and the code inside the `else` block is executed.

You can also use the `length()` method to check whether a string is empty or not. If the length of the string is `0`, then the string is empty.


String str = "";
if(str.length() == 0) {
   System.out.println("String is empty");
} else {
   System.out.println("String is not empty");


String str1 = "Hello";
String str2 = new String("Hello");
String str3 = "Hello";

Two objects will be created for this code:

The first object will be for `str1` which will be stored in the string constant pool.

The second object will be for `str2` which will be stored in the heap memory.

`str3` will reference the same object as `str1` because it is also stored in the string constant pool.

Printing All Permutations of a String in Java

Here is an example program in Java that prints all permutations of a given string:

public class Permutations {
    public static void main(String[] args) {
        String str = "abc";
        permute(str, 0, str.length()-1);
     * Recursive method to print all permutations of a string
     * @param str The input string
     * @param l The left index of the substring
     * @param r The right index of the substring
    private static void permute(String str, int l, int r) {
        if (l == r) {
        } else {
            for (int i = l; i <= r; i++) {
                str = swap(str,l,i);
                permute(str, l+1, r);
                str = swap(str,l,i);
     * Utility method to swap two characters in a string
     * @param str The input string
     * @param i The index of the first character
     * @param j The index of the second character
     * @return A new string with the characters swapped
    private static String swap(String str, int i, int j) {
        char temp;
        char[] charArray = str.toCharArray();
        temp = charArray[i];
        charArray[i] = charArray[j];
        charArray[j] = temp;
        return String.valueOf(charArray);

This program uses a recursive approach to find all permutations of a given string. The


method takes the input string, a left index, and a right index as parameters. If the left index and right index are the same, the program prints the string. Otherwise, it swaps the character at the left index with each character in the substring to the right of it and then recursively calls itself with the left index incremented by 1.

Python Program to Calculate the Total Number of Characters in a String

string = input("Enter a string: ") 
count = 0   # initialize counter to 0
for char in string:
    if char != " ":  # ignore spaces
        count += 1   # increment counter for every non-space character

print("Total number of characters in the string:", count)

Explanation of the code:

  • The user is prompted to enter a string.
  • The count variable is initialized to 0 to keep track of the total number of non-space characters in the string.
  • A for loop is used to iterate through each character in the string.
  • If the character is not a space, the count is incremented by 1.
  • Finally, the total number of characters is displayed to the user.

How to reverse a string in Java?

To reverse a string in Java, you can use either a loop or a StringBuilder.

Using loop:

Code snippet:

String input = "hello";
String output = "";
for(int i = input.length() - 1; i >= 0; i--){
   output += input.charAt(i);

This iterates through the string in reverse order, and then adds each character to the output string.

Using StringBuilder:

Code snippet:

String input = "hello";
String output = new StringBuilder(input).reverse().toString();

This creates a new StringBuilder object initialized with the input string, calls its


method to reverse the string, and then converts it back to a string using



Using StringBuilder is more efficient if you're working with large strings.

Converting an Array to a String in Java

In Java, you can convert an array to a string using the `Arrays.toString()` method. This method takes the array as an argument and returns a string representation of the array elements.

Here's an example code snippet:

int[] numbers = { 1, 2, 3, 4, 5 };
String numbersAsString = Arrays.toString(numbers);

This code will output the following:

[1, 2, 3, 4, 5]

Note that the `Arrays.toString()` method will return a string with square brackets around the array elements and comma-separated values.

If you want to remove the brackets and have a space-separated string, you can use the `String.join()` method instead. Here's an example:

int[] numbers = { 1, 2, 3, 4, 5 };
String numbersAsString = String.join(" ",

This code will output the following:

1 2 3 4 5

Here, we converted the `int` array to a stream of `String` objects, mapped each element to its string representation, and joined them using a space separator.

Counting the Occurrences of a Character in a String

Yes, it is possible to count the number of times a specific character appears in a string in Python. Here is an example using a function:

def count_char_occurrences(string, char):
    count = 0
    for c in string:
        if c == char:
            count += 1
    return count

This function takes in two parameters: the string to be analyzed and the character to count occurrences of. The function then initializes a variable called "count" to 0, which will be incremented every time the character is found in the string.

Using a for loop, the function iterates through each character in the string. If the character being looked at is equal to the target character (i.e. "char"), the function increments the count. Finally, the function returns the total count of occurrences.

Here is an example of how to use this function:

string = "Hello world!"
char = "o"
count = count_char_occurrences(string, char)
print("The character \'{}\' appears {} times in the string.".format(char, count))

In this example, we are looking for the character 'o' in the string "Hello world!". The function returns that 'o' appears twice in the string.

How to Compare Two Strings to Determine if They are Anagrams?

Anagrams are words or phrases made by rearranging the letters of another word or phrase. To compare two strings and determine if they are anagrams, we can follow the following steps:

1. Remove all spaces and convert the strings to lowercase. 2. Sort the characters of both strings alphabetically. 3. Compare the sorted strings. If they are equal, then the original strings are anagrams.

Removing a Specific Character from a String in Python

In Python, you can remove a specific character from a string using the `replace()` method. Here's an example:

string = "Hello World!"
char_to_remove = "l"
new_string = string.replace(char_to_remove, "")

This code will output "Heo Word!" because it removes all occurrences of the character 'l' from the original string.

Alternatively, if you only want to remove the first occurrence of a character, you can use the `replace()` method with an optional third argument that specifies the maximum number of replacements to make. Here's an example:

string = "Hello World!"
char_to_remove = "l"
new_string = string.replace(char_to_remove, "", 1)

This code will output "Helo World!" because it only removes the first occurrence of the character 'l' from the original string.

Program to check if a string is a palindrome

def is_palindrome(s):
    Function to check if a string is a palindrome
    s = s.lower() # convert string to lowercase
    reversed_s = s[::-1] # reverse the string
    if s == reversed_s:
        return True
        return False

# Test the function
string = "racecar"
if is_palindrome(string):
    print(string + " is a palindrome")
    print(string + " is not a palindrome")

This program checks whether the given input string is a palindrome or not. A palindrome is a word, phrase, number, or other sequences of characters that reads the same forward and backward. To check for palindrome, we first convert the string to lowercase using the built-in lower() method. Then, we reverse the string using slicing and store it in a new variable called reversed_s. Finally, we compare the original string and the reversed string. If they are equal, the input string is a palindrome, otherwise, it is not a palindrome. We test the function using a sample string to demonstrate the working of the function.

I'm sorry, I cannot answer that question without seeing the program. Can you please provide the code or describe it?Unfortunately, there is no program provided for me to give the output of. Can you please provide the program in question?

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