In Java, when we print a string literal on to the terminal, does this string literal also be stored in the string pool?

Question

I am aware that when we initialize a string literal to a variable this literal will be stored in the string pool by the JVM. Consider the piece of code below.

System.out.println("This is a string literal");

Does the string literal within the quotes also be stored in the string pool even if I don't initialize it to a variable?

Answer 1

I will preface this answer by saying that there is little practical use in gaining a deep understanding of the Java string pool. From a practical perspective, you just need to remember two things:

• Don't use == to compare strings. Use equals, compareTo, or equivalent methods.

• Don't use explicit String.intern calls in your code. If you want to avoid potential problems with duplicate strings, enable the string de-duplication feature that is available in modern Java GCs.

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I am aware that when we initialize a string literal either using the 'new' keyword or not, this literal will be stored in the string pool by the JVM.

This is garbled.

Firstly, you don't "initialize" a string literal. You initialize a variable.

String hi = "hello";  // This initializes the variable `hi`.

Secondly you typically don't / shouldn't use a string literal with new.

String hi = new String("hello");  // This is bad.  You should write this as above.

The normal use-case for creating a string using new is something like this:

String hi = new String(arrayOfCharacters, offset, count);

In fact, creation and interning of the String object that corresponds to a string literal, happens either at the first time that the literal is used in an expression or at an earlier time. The precise details (i.e. when it happens) are unspecified and (I understand) version dependent.

The first usage might be in a variable initialization, or it might be in something else; e.g. a method call.

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So to your question:

Consider the piece of code below:

System.out.println("This is a string literal");

Does the string literal within the quotes also be stored in the string pool even if I do not initialize it?

Yes, it does. If that was the first time the literal was used, the code above may be the trigger for this to happen. But it could have happened previously; e.g. if the above code was run earlier.

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As a followup, you asked:

Why does the String Pool collect string literals which are not stored in a variable and just displayed in the console?

Because the JLS 3.10.5 requires that the String objects which correspond to string literals are interned:

"Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.28) - are "interned" so as to share unique instances, using the method String.intern (§12.5)."

And you asked:

The Presence of the String Pool help optimize the program. By storing literals as such (which is actually not required because it is just to be displayed in the console), isn't it the case that it goes against its whole purpose (which is optimization)?

The original idea for interning and the string pool was to save memory. That made sense 25 years ago when the Java language was designed and originally specified. These days even a low-end Android phone has 1GB of RAM, and interning of string literals to save a few thousand bytes is kind of pointless. Except that the JLS says that this must happen.

But the answer is No, it doesn't go against the (original) purpose. This statement:

System.out.println("This is a string literal");

could be executed many times. You don't want / need to create a new String object for the literal each time that you execute it. The thing is that the JVM doesn't know what is going to happen.

Anyway, the interning must happen because that is what the spec says.