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String Immutability

What is String Immutability?​

In Java, String objects are immutable, which means once a String object is created, its value cannot be changed.

If you try to modify a string, Java creates a new object instead of changing the existing one.

String Immutability = The value of a String object cannot be changed after creation.

Example​

String s = "Hello";

s.concat(" World");

System.out.println(s); // Hello

Explanation:

The method concat() creates a new string object, but since it is not assigned to a variable, the original string remains unchanged.

Correct Way​

String s = "Hello";

s = s.concat(" World");

System.out.println(s); // Hello World

Here:

  • "Hello" remains unchanged.
  • "Hello World" is created as a new object.

Memory Representation​

String s1 = "Java";
s1 = s1.concat(" Programming");

Memory concept:

    String Constant Pool

    +--------+
    | "Java" |
    +--------+
         |
         v
    +---------------------+
    | "Java Programming"  |
    +---------------------+

The original "Java" string is not modified.
A new object "Java Programming" is created.

Why Strings Are Immutable​

Java designers made strings immutable for several reasons:

1. Security​

Strings are widely used in:

  • file paths
  • network connections
  • database URLs

Immutability prevents accidental modification.

2. String Pool Optimization​

Since strings cannot change, Java safely reuses objects in the String Pool, saving memory.

Example:

String s1 = "Java";
String s2 = "Java";

Both variables point to the same object.

3. Thread Safety​

Immutable objects are automatically thread-safe because their values cannot change.

Multiple threads can use the same string safely.

4. Hashcode Caching​

Strings are frequently used as keys in:

  • HashMap
  • HashSet

Immutability allows Java to cache hashcodes, improving performance.

Demonstration Program​

public class Test {

    public static void main(String[] args) {

        String s1 = "Java";

        String s2 = s1.concat(" Programming");

        System.out.println(s1); // Java
        System.out.println(s2); // Java Programming

    }

}

Summary​

  • String objects in Java are immutable.
  • Any modification creates a new object.
  • Original strings remain unchanged.
  • Benefits include: Security , Memory optimization (String Pool) , Thread safety & Performance improvements.