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Why String is immutable in Java ?

String is an immutable class in Java. An immutable class is simply a class whose instances cannot be modified. All information in an instance is initialized when the instance is created and the information can not be modified. There are many advantages of immutable classes. This article summarizes why String is designed to be immutable. A good answer depends on deep understanding of memory, synchronization, data structures, etc.

1. Requirement of String Pool

String pool (String intern pool) is a special storage area in Method Area. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

The following code will create only one string object in the heap.

String string1 = "abcd";
String string2 = "abcd";

Here is how it looks:
java-string-pool

If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

2. Caching Hashcode

The hashcode of string is frequently used in Java. For example, in a HashMap. Being immutable guarantees that hashcode will always the same, so that it can be cashed without worrying the changes.That means, there is no need to calculate hashcode every time it is used. This is more efficient.

In String class, it has the following code:

private int hash;//this is used to cache hash code.

3. Facilitating the Use of Other Objects

To make this concrete, consider the following program:

HashSet<String> set = new HashSet<String>();
set.add(new String("a"));
set.add(new String("b"));
set.add(new String("c"));
 
for(String a: set)
	a.value = "a";

In this example, if String is mutable, it's value can be changed which would violate the design of set (set contains unduplicated elements). This example is designed for simplicity sake, in the real String class there is no value field.

4. Security

String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Were String not immutable, a connection or file would be changed and lead to serious security threat. The method thought it was connecting to one machine, but was not. Mutable strings could cause security problem in Reflection too, as the parameters are strings.

Here is a code example:

boolean connect(string s){
    if (!isSecure(s)) { 
throw new SecurityException(); 
}
    //here will cause problem, if s is changed before this by using other references.    
    causeProblem(s);
}

5. Immutable objects are naturally thread-safe

Because immutable objects can not be changed, they can be shared among multiple threads freely. This eliminate the requirements of doing synchronization.

In summary, String is designed to be immutable for the sake of efficiency and security. This is also the reason why immutable classes are preferred in general.

Category >> Basics >> Java  
  • Ashish Thakran

    Please check out the below link. I think it may be some useful:

    http://newtechnobuzzz.blogspot.in/2014/07/why-string-is-immutable-or-final-in-java.html

  • jasondevj

    I don’t agree with the following statement “This is also the reason why immutable classes are preferred in general”

    I do agree immutable has its advantages and makes it easier to develop but most of the developers forget the memory overhead immutable objects bring in to the GC, when we have a immutable hashmap and when its changed, all the references has to recreated and redone for the new variable when we do this multiple times it puts an overhead on the GC and leads to stop the world GC which slows down the programme significantly.

    Where immutable objects makes things easier and is really useful for simple objects like String when it comes to more and more complex objects we need to check simplicity vs performance

  • David Johnston

    a.value = “a”;

    The above will typically work but because strings are objects it performs an object equality. The immutable nature of strings, and the fact that they are cached, makes this incorrect usage of object equality normally work and extremely difficult to figure out when it doesn’t. “a”.equals(a.value) is the error-proof way to compare two strings.

  • http://javarevisited.blogspot.com/ Javin Paul

    From Java 7 String pool is on heap space rather than permgen space. By the way, I have also shared my thoughts about String class in Java, you may find it useful.

  • niu

    what a amazing.

  • iMaplezhou

    String Pool is in Method Area, not heap

  • 王正一

    I think String pool in Method Area, not in Heap

  • Bernie Zhang

    Why String pool in Java Heap? Why not Method?

  • lordstark

    amazingly simple..