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    <p>In some cases this is obsolete due to optimisations performed by the compiler, but the general issue is that code like:</p> <pre><code>string myString=""; for(int i=0;i&lt;x;i++) { myString += "x"; } </code></pre> <p>will act as below (each step being the next loop iteration):</p> <ol> <li>construct a string object of length 1, and value "x"</li> <li>Create a new string object of size 2, copy the old string "x" into it, add "x" in position 2.</li> <li>Create a new string object of size 3, copy the old string "xx" into it, add "x" in position 3.</li> <li>... and so on</li> </ol> <p>As you can see, each iteration is having to copy one more character, resulting in us performing 1+2+3+4+5+...+N operations each loop. This is an O(n^2) operation. If however we knew in advance that we only needed N characters, we could do it in a single allocation, with copy of just N characters from the strings we were using - a mere O(n) operation.</p> <p>StringBuffer/StringBuilder avoid this because they are mutable, and so do not need to keep copying the same data over and over (so long as there is space to copy into in their internal buffer). They avoid performing an allocation and copy proportional to the number of appends done by over-allocing their buffer by a proportion of its current size, giving amortized O(1) appending.</p> <p>However its worth noting that often the compiler will be able to optimise code into StringBuilder style (or better - since it can perform constant folding etc.) automatically.</p>
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