Why is String class immutable or final in Java?

The string is immutable means that we cannot change the object itself, but we can change the reference to the object. The string is made final to not allow others to extend it and destroy its immutability.

• Security Parameters are typically represented as String in network connections, database connection URLs, usernames/passwords, etc. If it was mutable, these parameters could be changed easily.
• Synchronization and Concurrency making String immutable automatically makes them thread safe thereby solving the synchronization issues.
• Caching when compiler optimizes our String objects, it seems that if two objects have the same value (a =" test", and b =" test") and thus we need only one string object (for both a and b, these two will point to the same object).
• Class loading String is used as arguments for class loading. If mutable, it could result in the wrong class being loaded (because mutable objects change their state).

Example:

public class StringImmutableDemo {
   public static void main(String[] args) {
      String st1 = "Tutorials";
      String st2 = "Point";
      System.out.println("The hascode of st1 = " + st1.hashCode());
      System.out.println("The hascode of st2 = " + st2.hashCode());
      st1 = st1 + st2;
      System.out.println("The Hashcode after st1 is changed : "+ st1.hashCode());
   }
}

输出：

The hascode of st1 = -594386763
The hascode of st2 = 77292912
The Hashcode after st1 is changed : 962735579

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