What += Really Does, Once per Pass, Inside a Loop

A method that builds a CSV line out of a thousand values takes a noticeable moment. The same method over ten thousand values takes far more than ten times as long.

java

String row = "";
for (String value : values) {
    row += value + ",";
}

row += value cannot add to row, because nothing can add to a string. What it does instead, on every pass, is build a whole new string containing everything row already held plus the new piece, and then point row at that.

What each pass copies when building a string of n pieces
Pass	Characters copied	Strings left for the collector
1	The first piece	1
2	Everything so far, again	2
n	Everything so far, again	n

The work per pass grows with what has already been built, so the total grows with the square of the number of pieces. Ten times the input is a hundred times the copying, which is exactly the shape the timings showed.

A single + between two values is fine and always has been — the cost only appears when the result of one concatenation is the input to the next. That is what a loop does, and it is the one place to reach for a builder instead.

Java
JVM
Performance