Java junit test case for the class?

Your void output() method is declared to return void . That means it returns nothing. But in the below code you are expecting it to return "c,b,a" .

Either you have to change the output() to return String like String output() or you check for side-effects i.e. Any changes that output is performing in the method.

You should break up your program into smaller parts. I think what you are really trying to test is sortStringBubble. So create a test method that calls sortStringBubble with a certain set of characters and then checks that the method did the right thing by checking the result.

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