What is the output of the following program(string concatenation)? : Questions « Utility Classes « SCJP

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public class Question {
       public static void main(String args[]) {
        String s1 = "abc";
        String s2 = "def";
        String s3 = s1.concat(s2.toUpperCase());
        System.out.println(s1+s2+s3);
       }
      }