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    <p>Here's (1) a really efficient, generic way to do it and (2) a simple and effective way.</p> <p>(1) In the first sort, I compute and cache the number of occurrences of the digit 5 once per value in the array. Then I use a generic sort method to sort based on the cached values. This is better than a custom comparator because the number of occurrences is only computed once per element in the list.</p> <p>(2) In the second sort, I simply use a custom comparator, which is more simple to implement.</p> <p>Sorts (3) and (4) do the same thing as (1) and (2) respectively using Strings rather than Integers.</p> <pre><code>import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; import java.util.Comparator; import java.util.List; import java.util.ListIterator; public class DigitSort { public static void main(String[] args) { List&lt;Integer&gt; initInt = Arrays.asList(555, 55, 5, 101, 202); final int sortDigit = 5; // sort #1 List&lt;Integer&gt; ints = new ArrayList&lt;&gt;(initInt); sort(ints, new ToComparable&lt;Integer, DigitComparable&gt;() { @Override public DigitComparable toComparable(Integer number) { return number == null ? null : new DigitComparable(number, digitCount(sortDigit, number)); } }); System.out.println("Results of int generic sort:"); for (Integer number : ints) System.out.println(number); // sort #2 ints = new ArrayList&lt;&gt;(initInt); Collections.sort(ints, new Comparator&lt;Integer&gt;() { @Override public int compare(Integer number1, Integer number2) { if (number1 == null &amp;&amp; number2 == null) return 0; if (number1 == null) return -1; if (number2 == null) return 1; int digitCount1 = digitCount(sortDigit, number1); int digitCount2 = digitCount(sortDigit, number2); return digitCount1 &gt; 0 &amp;&amp; digitCount2 &gt; 0 ? digitCount1 - digitCount2 : digitCount2 &gt; 0 ? -1 : digitCount1 &gt; 0 ? 1 : number1 - number2; } }); System.out.println("\nResults of int comparator sort:"); for (Integer number : ints) System.out.println(number); List&lt;String&gt; initString = new ArrayList&lt;&gt;(initInt.size()); for (Integer number : initInt) initString.add(number == null ? null : number.toString()); // sort #3 List&lt;String&gt; strings = new ArrayList&lt;&gt;(initString); sort(strings, new ToComparable&lt;String, DigitComparable&gt;() { @Override public DigitComparable toComparable(String number) { if (number == null) return null; int asInt = Integer.parseInt(number); return new DigitComparable(asInt, digitCount(sortDigit, asInt)); } }); System.out.println("\nResults of string generic sort:"); for (String number : strings) System.out.println(number); // sort #4 strings = new ArrayList&lt;&gt;(initString); Collections.sort(strings, new Comparator&lt;String&gt;() { @Override public int compare(String number1, String number2) { if (number1 == null &amp;&amp; number2 == null) return 0; if (number1 == null) return -1; if (number2 == null) return 1; int digitCount1 = digitCount(sortDigit, number1); int digitCount2 = digitCount(sortDigit, number2); return digitCount1 &gt; 0 &amp;&amp; digitCount2 &gt; 0 ? digitCount1 - digitCount2 : digitCount2 &gt; 0 ? -1 : digitCount1 &gt; 0 ? 1 : Integer.parseInt(number1) - Integer.parseInt(number2); } }); System.out.println("\nResults of string comparator sort:"); for (String number : strings) System.out.println(number); } private static class DigitComparable implements Comparable&lt;DigitComparable&gt; { private final int number, digitCount; DigitComparable(int number, int digitCount) { this.number = number; this.digitCount = digitCount; } @Override public int compareTo(DigitComparable other) { return digitCount &gt; 0 &amp;&amp; other.digitCount &gt; 0 ? digitCount - other.digitCount : other.digitCount &gt; 0 ? -1 : digitCount &gt; 0 ? 1 : number - other.number; } } protected static int digitCount(int digit, String number) { char asChar = Character.forDigit(digit, 10); int count = 0; for (char c : number.toCharArray()) if (c == asChar) ++count; return count; } protected static int digitCount(int digit, int number) { number = Math.abs(number); int count = 0; while (number != 0) { if (number % 10 == digit) ++count; number /= 10; } return count; } public interface ToComparable&lt;T, C extends Comparable&lt;? super C&gt;&gt; { C toComparable(T t); } public static &lt;T, C extends Comparable&lt;? super C&gt;&gt; void sort(List&lt;T&gt; list, ToComparable&lt;T, C&gt; function) { class Pair implements Comparable&lt;Pair&gt; { final T original; final C comparable; Pair(T original, C comparable) { this.original = original; this.comparable = comparable; } @Override public int compareTo(Pair other) { return comparable == null &amp;&amp; other.comparable == null ? 0 : comparable == null ? -1 : other.comparable == null ? 1 : comparable.compareTo(other.comparable); } } List&lt;Pair&gt; pairs = new ArrayList&lt;&gt;(list.size()); for (T original : list) pairs.add(new Pair(original, function.toComparable(original))); Collections.sort(pairs); ListIterator&lt;T&gt; iter = list.listIterator(); for (Pair pair : pairs) { iter.next(); iter.set(pair.original); } } } </code></pre>
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