How to randomly select 7 letters(a to z ) in java

I want to select 7 random letters from a to z

so far i have found only ways to select a single letter http://ift.tt/1Jb5Wt2

Random r = new Random();
char c = (char) (r.nextInt(26) + 'a');

Why seven letters because , i need to apply some conditions to those 7 letters - to check the words formed by those seven letters , the word length should be minimum of 4 letters ......