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Learn Data Structures and Algorithms in Java

String Immutability

Explore the concept of string immutability in Java, understanding why strings cannot be modified directly and how this affects string manipulation and algorithm design. Learn to differentiate mutable and immutable objects and grasp the practical implications of immutability for performance and code behavior. This lesson helps you develop a solid foundation to handle string operations effectively and avoid common pitfalls in Java programming.

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By this point, you know that individual characters in strings can be accessed and traversed, and that the length of a string can be determined using the same ideas that apply to arrays. That structural similarity makes strings feel familiar. However, there is one place where strings and arrays part ways in a significant and consequential way, and it shows up the moment you try to modify a string. Understanding this difference is not just a matter of knowing a language rule. It directly affects how string algorithms are designed, how their performance is analyzed, and why certain patterns appear in string code again and again.

Attempting to modify a string

Consider a simple array of integers. Changing the first element is a straightforward operation in which the value at that position is simply overwritten.

Java 25
public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        arr[0] = 99;
        System.out.println(java.util.Arrays.toString(arr));  // [99, 2, 3, 4, 5]
    }
}

Now consider trying to do the same thing with a string.

Java 25
public class Main {
    public static void main(String[] args) {
        String s = "hello";
        s.charAt(0) = 'H';  // This does not compile — charAt() returns a value, not a reference
        // Strings in Java are immutable; there is no way to change a character in place
    }
}

Java does not even allow the syntax s[0] = 'H' — this is a compile-time error because String objects do not support index assignment. This difference is not accidental. It reflects a fundamental distinction between arrays and strings. An array allows direct modification of its contents, whereas a String does not.

Mutable and immutable objects

The distinction observed above is expressed through the concepts of mutability and ...