Is “new String()” immutable as well?

Question

I\'ve been studying Java String for a while. The following questions are based on the below posts

Java String is special
Immutability of String in java

Answer 1

It's the other way around, actually.

[...] the String class has been designed so that the values in the common pool can be reused in other places/variables.

No, the String class is immutable so that you can safely reference an instance of it, without worrying about it being modified from another part of your program. This is why pooling is possible in the first place.

So, consider this:

// this string literal is interned and referenced by 'a'
String a = "Hello World!";

// creates a new instance by copying characters from 'a'
String b = new String(a);

Now, what happens if you simply create a reference to your newly created b variable?

// 'c' now points to the same instance as 'b'
String c = b;

Imagine that you pass c (or, more specifically, the object it is referencing) to a method on a different thread, and continue working with the same instance on your main thread. And now imagine what would happen if strings were mutable.

Why is this the case anyway?

If nothing else, it's because immutable objects make multithreading much simpler, and usually even faster. If you share a mutable object (that would be any stateful object, with mutable private/public fields or properties) between different threads, you need to take special care to ensure synchronized access (mutexes, semaphores). Even with this, you need special care to ensure atomicity in all your operations. Multithreading is hard.

Regarding the performance implications, note that quite often copying the entire string into a new instance, in order to change even a single character, is actually faster than inducing an expensive context switch due to synchronization constructs needed to ensure thread-safe access. And as you've mentioned, immutability also offers interning possibilities, which means it can actually help reduce memory usage.

It's generally a pretty good idea to make as much stuff immutable as your can.

Answer 2

1) The immutable part isn't because of the pool; it just makes the pool possible in the first place. Strings are often passed as arguments to other functions or even shared with other threads; Making Strings immutable was a design decision to make reasoning in such situations easier. So yes - Strings in Java are always immutable, no matter how you create them (note that it's possible to have mutable strings in java - just not with the String class).

2) Yes. Probably. I'm not actually 100% sure, but that should be the case.

Answer 3

String is immutable irrespective of how it is instantiated

1) Short answer is yes, new String() is immutable too.

Because every possible mutable operation (like replace,toLowerCase etcetra) that you perform on String does not affect the original String instance and returns you a new instance.

You may check this in Javadoc for String. Each public method of String that is exposed returns a new String instance and does not alter the present instance on which you called the method.

This is very helpful in Multi-threaded environment as you don't have to think about mutability (someone will change the value) every time you pass or share the String around. String can easily be the most used data type, so the designers have blessed us all to not think about mutability everytime and saved us a lot of pain.

Immutability allowed String pool or caching

It is because of immutability property that the internal pool of string was possible, as when same String value is required at some other place then that immutable reference is returned. If String would have been mutable then it would not have been possible to share Strings like this to save memory.

String immutablity was not because of pooling, but immutability has more benefits attached to it.

String interning or pooling is an example of Flyweight Design pattern

2) Yes it will be interned like any other String as a blank String is also as much a String as other String instances.

References:

• Immutability benefits of String

Answer 4

new String() is an expression that produces a String ... and a String is immutable, no matter how it is produced.

^{(Asking if new String() is mutable or not is nonsensical. It is program code, not a value. But I take it that that is not what you really meant.)}

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If I create a string object as String c = ""; is an empty entry created in the pool?

Yes; that is, an entry is created for the empty string. There is nothing special about an empty String.

^{(To be pedantic, the pool entry for "" gets created long before your code is executed. In fact, it is created when your code is loaded ... or possibly even earlier than that.)}

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So, I was wanted to know whether the new heap object is immutable as well, ...

Yes it is. But the immutability is a fundamental property of String objects. All String objects.

You see, the String API simply does not provide any methods for changing a String. So (apart from some dangerous and foolish¹ tricks using reflection), you can't mutate a String.

and if so what was the purpose?.

The primary reason that Java String is designed as an immutable class is simplicity. It makes it easier to write correct programs, and read / reason about other people's code if the core string class provides an immutable interface.

An important second reason is that the immutability of String has fundamental implications for the Java security model. But I don't think this was a driver in the original language design ... in Java 1.0 and earlier.

Going by the answer, I gather that other references to the same variable is one of the reasons. Please let me know if I am right in understanding this.

No. It is more fundamental than that. Simply, all String objects are immutable. There is no complicated special case reasoning required to understand this. It just >>is<<.

For the record, if you want a mutable "string-like" object in Java, you can use StringBuilder or StringBuffer. But these are different types to String.

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^{1 - The reason these tricks are (IMO) dangerous and foolish is that they affect the values of strings that are potentially shared by other parts of your application via the string pool. This can cause chaos ... in ways that the next guy maintaining your code has little chance of tracking down.}

Answer 5

This isn't strictly an answer to your question, but if behind your question is a wish to have mutable strings that you can manipulate, you should check out the StringBuilder class, which implements many of the exact same methods that String has but also adds methods to change the current contents.

Once you've built your string in such a way that you're content with it, you simply call toString() on it in order to convert it to an ordinary String that you can pass to library routines and other functions that only take Strings.

Also, both StringBuilder and String implements the CharSequence interface, so if you want to write functions in your own code that can use both mutable and immutable strings, you can declare them to take any CharSequence object.

Answer 6

The Java libraries are heavily optimized around the constraint that any String object is immutable, regardless of how that object is constructed. Even if you create your b using new, other code that you pass that instance to will treat the value as immutable. This is an example of the Value Object pattern, and all of the advantages (thread-safety, no need to make private copies) apply.

The empty string "" is a legitimate String object just like anything else, it just happens to have no internal contents, and since all compile-time constant strings are interned, I'll virtually guarantee that some runtime library has already caused it to be added to the pool.