How to sort alphabetically in desceding order in java

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Java

I am writing a method to sort alphabets in descending order. it holds that B<a, and b<A: if two letters only differ in upper and lower case, the upper case is put directly in front of corresponding lower case character A<a.
i have this code this far and dont know how to proceed further to achieve my purpose.

What I have tried:

private boolean isPredecessor(char value)
 {
     return this.value>value;
 }

  }