In Java, the String class is immutable, meaning once a String object is created, its value cannot be changed. This design choice is intentional and offers several benefits related to security, performance, and thread-safety. Here’s a breakdown of the reasons:
1. Security
- Strings are widely used for sensitive information like usernames, passwords, URLs, and database connection strings.
- If Strings were mutable, a change in one part of the code could unintentionally or maliciously alter the value, leading to security vulnerabilities.
- For example,
Class.forName("com.mysql.jdbc.Driver")uses a String. If this String were mutable, it could be altered after being used, causing a potential exploit.
2. String Pool (Interning)
- Java maintains a string pool (a cache of string literals) to save memory.
- Immutable strings ensure that the same literal reference can be shared across the application without concern that it will change.
- For example:
String s1 = "hello";String s2 = "hello";System.out.println(s1 == s2); // true- This wouldn’t be safe if strings were mutable.
3. Thread Safety
- Because Strings are immutable, they can be safely shared between threads without synchronization.
- No thread can change the value of a String once it is created, so there’s no risk of race conditions.
4. Caching HashCode
- The
Stringclass caches its hash code after the first computation. - Since the contents never change, the cached hash code remains valid for future uses (like in hash-based collections such as
HashMap).
5. Ease of Use
- Immutable objects are simpler to reason about, debug, and use.
- They reduce the likelihood of bugs caused by unintended changes.
