Correct Way To Trim A String In Java

Answer :

You are doing it right. From the documentation:

Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared.

Also from the documentation:

trim

public String trim()

Returns a copy of the string, with leading and trailing whitespace
omitted. If this String object represents an empty character sequence,
or the first and last characters of character sequence represented by
this String object both have codes greater than '\u0020' (the space
character), then a reference to this String object is returned.

Otherwise, if there is no character with a code greater than '\u0020'
in the string, then a new String object representing an empty string
is created and returned.

Otherwise, let k be the index of the first character in the string
whose code is greater than '\u0020', and let m be the index of the
last character in the string whose code is greater than '\u0020'. A
new String object is created, representing the substring of this
string that begins with the character at index k and ends with the
character at index m-that is, the result of this.substring(k, m+1).

This method may be used to trim whitespace (as defined above) from the
beginning and end of a string.

Returns:

A copy of this string with leading and trailing white space removed, or this string if it has no leading or trailing white
space.

As strings in Java are immutable objects, there is no way to execute trimming in-place. The only thing you can do to trim the string is create new trimmed version of your string and return it (and this is what the trim() method does).

In theory you are not assigning a variable to itself. You are assigning the returned value of method trim() to your variable input.

In practice trim() method implementation is optimized so that it is creating (and returning) another variable only when necessary. In other cases (when there is actually no need to trim) it is returning a reference to original string (in this case you are actually assigning a variable to itself).

See http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/String.java#String.trim%28%29

Anyway trim() does not modify original string, so this is the right way to use it.