问题
As we know, there is a attribute in StringBuilder called capacity, it is always larger than the length of StringBuilder object. However, what is capacity used for? It will be expanded if the length is larger than the capacity. If it does something, can someone give an example?
回答1:
You can use the initial capacity to save the need to re-size the StringBuilder while appending to it, which costs time.
If you know if advance how many characters would be appended to the StringBuilder and you specify that size when you create the StringBuilder, it will never have to be re-sized while it is being used.
If, on the other hand, you don't give an initial capacity, or give a too small intial capacity, each time that capacity is reached, the storage of the StringBuilder has to be increased, which involves copying the data stored in the original storage to the larger storage.
回答2:
The string builder has to store the string that is being built somewhere. It does so in an array of characters. The capacity is the length of this array. Once the array would overflow, a new (longer) array is allocated and contents are transferred to it. This makes the capacity rise.
If you are not concerned about performance, simply ignore the capacity. The capacity might get interesting once you are about to construct huge strings and know their size upfront. Then you can request a string builder with a capacity being equal to the expected size (or slightly larger if you are not sure about the size).
Example when building a string with a content size of 1 million:
StringBuilder sb = new StringBuilder(1000000);
for(int i = 0; i < 1000000; i++){
sb.append("x");
}
Initializing the string builder with one million will make it faster in comparison to a default string builder which has to copy its array repeatedly.
回答3:
StringBuilder is backed by an array of characters. The default capacity is 16 + the length of the String argument. If you append to the StringBuilder and the number of characters cannot be fit in the array, then the capacity will have to be changed which will take time. So, if you have some idea about the the number of characters that you might have, initialize the capacity.
回答4:
The answer is: performance. As the other answers already say, StringBuilder uses an internal array of some original size (capacity). Every time the building up string gets to large for the array to hold it, StringBuilder has to allocate a new, larger array, copy the data from the previous array to the new one and delete the previous array.
If you know beforehand what size the resulting string might be and pass that information to the constructor, StringBuilder can create a large enough array right away and thus can avoid the allocating and copying.
While for small strings, the performance gain is negelectible, it make quite a difference if you build really large strings.
来源：https://stackoverflow.com/questions/27562016/why-do-we-need-capacity-in-stringbuilder