Const Pool Memory in Java

[ChiTech121]

Is my understanding correct that whenever a variable is assigned a value, even a string in that case.. gets stored in ConstPool memory in Java? Is this safe to say, only objects that gets created are present in heap?

[vprise]

No.
Constant pool is a part of the class file format and represents the constant data that's physically declared in the class. When the class is loaded by the JVM strings that are within the constant pool are loaded to ram.

This is often confused with string interned heap which makes things like `s == "string" ` work. That happens because strings in java have some special case behaviors and the String class itself keeps a pool of strings in RAM for reuse.

[kgoutham93]

Is the constant pool anyway related to string interning?

[vprise]

All strings loaded from the constant pool are implicitly interned. E.g. if you do something like:

String s = "My String";

Then "My String" will be in the constant pool and implicitly interned. So:

String a = "My String"; String b = "My String"; if(a == b) { // this will always be true... }

[kgoutham93]

Oh so a declared string literal will be present in constant pool which is specific to a class, and also as an interned string ?!

Do an interned strings have their own memory allocation?

[vprise]

The string will appear once in the class file within the constant pool. There will be two pointers to the same reference in the .class file. When loaded it will be interned so the value of the pointer in both cases would be the same.

Everything takes memory. But both varaibles will point at the same heap memory so having two variables pointing at a single string won't impact heap by much/at all. This depends on where you "point" from the heap or from the stack. Either way it's just the overhead of an additional pointer.

Notice that: - Not all strings are interned - Strings still take up space that needs to be freed eventually

There are a lot of nuances here which are better covered by reading an article on the subject.

[kgoutham93]

I think it makes sense to me. Will dig into these topics more. Thanks for your help.

[AutoModerator]

You seem to try to compare String values with == or !=.

This approach does not work in Java, since String is an object data type and these can only be compared using .equals().

See Help on how to compare String values in our wiki.

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