java - string trim function is not working

You need to re-assign the result of trim back to s:

s = s.trim();

Remember, Strings in Java are immutable, so almost all the String class methods will create and return a new string, rather than modifying the string in place.

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Although this is off-topic, but (as I said there almost), it's worth knowing that, exception to this rule is when creating a substring of same length, or any time a method returns the string with the same value, which will be optimized and will not create a new string, but simply return this.

String s = "Rohit";
String s2 = s.substring(0, s.length());

System.out.println(s == s2); // will print true