You have probably written something like:
String result = m + " plus " + n + " is " + m+n;
The problem with this is that Java does the + operations from
left to right, so after forming "23 plus 57 is " it then attaches
the value of m to get "23 plus 57 is 23" and then
attaches the value of n to get "23 plus 57 is
2357".
It is true that the last + operator has no spaces round it, and
so looks to human eyes as if it should be done first. Java, though, takes no
notice, and always ignores any unnecessary spacing (outside of double
quotes). That means you have to put in brackets to make it clear to Java which
operation to do first.