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    <p>Here is the code snippet for <strong>random sample problem</strong> (thanks Eyal for naming it). Not sure if that is what you are looking for.</p> <p>Be aware, that this solution would run into an infinete loop if not enough lowercase letters are in the string. So you would need to tackle that as well, but I guess it is a starting point. ;-)</p> <pre><code>String myString = "9aie3ra3nr23rr5r21t"; System.out.println(upperCaseRandom(myString, 10)); public static String upperCaseRandom(String input, int n) { StringBuilder output = new StringBuilder(input); Random r = new Random(); for (int i = 0; i &lt; n; i++) { // Pick a place int position = r.nextInt(input.length()); // Check if lowercase alpha if (Character.isLowerCase(output.charAt(position))) { output.setCharAt(position, Character.toUpperCase(output.charAt(position))); } else { i--; } } return output.toString(); } </code></pre> <p><strong>Edit:</strong> Here is an improved version. It does change exactly n lowercase letters into uppercase letters (if there are enough, otherwise it changes all of them). The programm does not run into infinite loops, but still running time is a problem though.</p> <pre><code>public static String upperCaseRandom(String input, int n) { final int length = input.length(); final StringBuilder output = new StringBuilder(input); final boolean[] alreadyChecked = new boolean[length]; final Random r = new Random(); for (int i = 0, checks = 0; i &lt; n &amp;&amp; checks &lt; length; i++) { // Pick a place int position = r.nextInt(length); // Check if lowercase alpha if (!alreadyChecked[position]) { if (Character.isLowerCase(output.charAt(position))) { output.setCharAt(position, Character.toUpperCase(output.charAt(position))); } else { i--; } checks++; alreadyChecked[position] = true; } else { i--; } } return output.toString(); } </code></pre>
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    1. POuppercase random number of characters from a random alphanumeric string
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    1. USNils Schmidt
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    1. POuppercase random number of characters from a random alphanumeric string
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    1. COThis doesn't work correctly in all cases (try it). You may "switch" numeric chars, resulting in no effect. If you add an isLetter() condition the problem will be fixed, but this solution will have a poor performance on large strings, specially with many digits, due to the unbounded nature of the trial and error approach.
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      1. USEyal Schneider
    2. COisLowerCase() will return false for a numeric char, thus it wont "switch" them. I cant recreate your wrong cases, but maybe I just did not understand so far. Could you recheck and/or maybe reexplain? The performance problems are indeed a problem, an improved version should not double check the same positions (just add an boolean[] to identify those. And still the infinite loop has to be avoided.
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      1. USNils Schmidt
    3. COSorry, you are 100% right regarding the correctness of the code, my mistake. It is also shorter than my last solution. The only disadvantage is performance, in the asymptotic meaning. For converting all N chars, the expected number of trials is N/N+N/(N-1)+...+N/2+N/1 which behaves like N * log N. In other words, the worst case complexity is O(N log N) instead of O(N). This is a general problem with this particular random sample algorithm.
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