Random alphabet character with no duplicate

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public static String RandomAlpha(int len){ Random r = new Random(); String key = ""; for(int x=0;x<len;x++) key = key + (char) (r.nextInt(26) + 'A'); return key; } public static String RandomAlphaNoDuplicate(int len){ Random r = new Random(); String key = ""; for (int i = 0; i < len;) { char c = (char) (r.nextInt(26) + 'A'); if(!key.toString().contains(""+c)){ key = key + c; i++; } } return key; }