Scramble a String

I'm trying to scramble a String and I have it working, I was just wondering if there is a more efficient way of doing it than what I have. Does anyone have a suggestion on how to make this more efficient?

Code:

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String abc = "abcdefghijklmnopqrstuvwxyz";
String scrambled = "";
Random randGen = new Random();

while(scrambled.length() != abc.length())
{
  char rand = abc.charAt(randGen.nextInt(abc.length()));
  if(!scrambled.contains("" + rand))
      scrambled += rand;
}

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What is more efficient:
Fewer machine instructions executed
or fewer source statements.

Should the output String contain the same number of letters as the input String?
Your algorithm appears to only allow one copy of any letter. The longest that your output string could be would be 26 letters for any long string input to it.

I have no idea which is more efficient... The output String should contain the exact characters of the input String just mixed up.

Quote:

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The output String should contain the exact characters of the input String just mixed up.

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Have you tested it with some different input to see what happens?

Quote:

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I have no idea which is more efficient.

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I was asking you what you meant by being efficient. How do you determine if one piece of code is more efficient than another?

Which is worth more a one dollar bill or 30 quarters. The one dollar bill is more efficient to carry.

One thing I can say immediately is to use a stringbuilder instead of string concatenation. Then use append. Norm made some good points as well, this should probably take a string input and scramble it.