Strings

A string is an object of the String class, and it comes with plenty of methods: length() to count its characters (with parentheses, unlike an array's length), charAt(i) to get the character at position i, trim() to remove spaces from both ends, toLowerCase() to convert letters to lower case, replace(" ", "") to swap one piece of text for another, and split(" +") to break the string into an array at every run of one or more spaces. A string never changes, so methods like trim and replace return a new string instead of changing the original. That's why name.trim(); on its own does nothing.

The most famous Java trap is that == between two strings asks "are these the same object?", not "do they have the same characters?". It may give you the right answer with strings written in your code, then get it wrong with a string the user typed or one built while the program runs. So always compare content with a.equals(b), or with equalsIgnoreCase when you want to ignore the difference between upper and lower case.

Since a string never changes, joining with += inside a loop builds a brand-new string on every iteration. A StringBuilder, on the other hand, is text you can modify: you add to it with append, reverse it in place with reverse(), and get the final string with toString().