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    <p>Collections.sort is implemented as mergesort. looking at the source, all compare conditions are <code>&gt;0</code> or <code>&lt;=0</code> which by chance treat the negative case the same as the equal case. a different sort implementation can fail.</p> <p>Per jbellis's comment, "<em>It's not quite "just luck," though -- Collections.sort is guaranteed to be "stable," meaning that equal elements must be in the same relative order post-sort. I'm not sure it's impossible to come up with a stable sort implementation that fails with this comparator, but I can't think of one off the top of my head.</em>"</p> <pre><code>private static void mergeSort(Object[] src, Object[] dest, int low, int high, int off, Comparator c) { int length = high - low; // Insertion sort on smallest arrays if (length &lt; INSERTIONSORT_THRESHOLD) { for (int i=low; i&lt;high; i++) for (int j=i; j&gt;low &amp;&amp; c.compare(dest[j-1], dest[j])&gt;0; j--) swap(dest, j, j-1); return; } // Recursively sort halves of dest into src int destLow = low; int destHigh = high; low += off; high += off; int mid = (low + high) &gt;&gt; 1; mergeSort(dest, src, low, mid, -off, c); mergeSort(dest, src, mid, high, -off, c); // If list is already sorted, just copy from src to dest. This is an // optimization that results in faster sorts for nearly ordered lists. if (c.compare(src[mid-1], src[mid]) &lt;= 0) { System.arraycopy(src, low, dest, destLow, length); return; } // Merge sorted halves (now in src) into dest for(int i = destLow, p = low, q = mid; i &lt; destHigh; i++) { if (q &gt;= high || p &lt; mid &amp;&amp; c.compare(src[p], src[q]) &lt;= 0) dest[i] = src[p++]; else dest[i] = src[q++]; } } </code></pre>
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    1. COIt's not quite "just luck," though -- Collections.sort is guaranteed to be "stable," meaning that equal elements must be in the same relative order post-sort. I'm not sure it's impossible to come up with a stable sort implementation that fails with this comparator, but I can't think of one off the top of my head.
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