Q.1

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

Ans. abcdefabcDEF - I got this answer and it contradicts with the concept that String Objects are immutable.
How does it work?

Thanks,
Deepa