Slow string concatenation over large input

Question

I\'ve written an n-ary tree ADT which works fine. However, I need to store its serialization in a variable a calling class. eg.

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Answer 1

Let me say the reason that string concatenation is slow is because strings are immutable. This means every time you write "+=", a new String is created. This means the way you build up your string is in the worst case, O(n²). That's because if you +='ed 1 char at a time, the cost of building a new string would be 2 + 3 + 4 + ... + n, which is O(n²).

Use StringBuilder as other's suggest (over the slower, but threadsafe StringBuffer).

I suppose I should add, StringBuilder will give you O(n) amortized time, because it works like a vector behind the scenes, since it is mutable. So build up your string there, and then call toString().

StringBuilder builder = new StringBuilder();
builder.append("blah"); // append more as needed.
String text = builder.toString();

I would also like to add that this problem is similar in Python. The idiom in python is to append all your strings to concatenate into a list, and then join the list. "".join(the_list).

UPDATE: As Bill points out, concatenation is not the root of all evil. One off string concatenations are fine, and may even be optimized! (They are also worst case linear). But, when you are concatenating in a loop, as you are above, the performance will drastically change as the number of iterations goes up. In that case, my above analysis is flawless, as I specifically stated it is "worst case", which means you assume no optimizations. (Which the JVM can't even optimize the concatenation in loops as well as it can outside).