Java StringBuilder huge overhead

Question

I read somewhere that the Java StringBuilder uses around 1 mb for 500 characters. Is this true and if so, isn't that a bit extreme? Is the StringBuilder doing some incredible things with this amount of memory? What is the reason and does this mean I should not make too much use of this class?

Answer 1

No, that's complete rubbish - unless you create a StringBuilder with a mammoth capacity, of course.

Java in general uses 2 bytes per char. There's a little bit of overhead in String and StringBuilder for the length and the array itself, but not a lot.

Now 1K for 500 characters is about right... I suspect that was the cause of confusion. (Either you misheard, or the person talking to you was repeating something they'd misheard.)

Answer 2

This information is erroneous, do you remember what the source of this information was? If so you should correct it. Java normally uses 2 bytes per character.

Answer 3

Because of the doubling reallocation 2K for 500 characters would also be right, but not more. Here is a similar question.

Answer 4

I have seen two cases where StringBuilder's tend to use large amounts of memory:

• When the StringBuilder is created with an insane initial-capacity.
• StringBuilder's who were "cached" to "save" object-allocation time.

So in the second case a StringBuilder might consume 1Mb of memory if some code, which used the SB earlier, stored a very big string in it. That's because it will only grow but not shrink its internal char-array.

Both cases can (and should) easy be avoided.

Answer 5

I find that hard to believe as internally Java converts all String objects to StringBuilders for performance reasons.