Search⌘ K
AI Features
Home
Courses
Learn Data Structures and Algorithms in Java

String Immutability

Explore the concept of string immutability in Java, understanding why strings cannot be changed after creation. Learn how this property influences string algorithm design, performance analysis, and safe handling of string data in your Java programs.

We'll cover the following...

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' ...