String is immutable in java. And trim()
returns a new string so you have to get it back by assigning it.
String a = " example ";
System.out.println(a);
a.trim(); // String trimmed.
System.out.println(a);// still old string as it is declared.
a = " example ";
a = a.trim(); //got the returned string, now a is new String returned ny trim()
System.out.println(a);// new string
Edit:
she said that it's because I'm using a newer version of java (jdk1.7) and a.trim() works in the previous versions of java.
Please find a new java teacher. That's completely a false statement with no evidence.