In short: strip() is “Unicode-aware” evolution of trim(). Meaning trim() removes only characters <= U+0020 (space); strip() removes all Unicode whitespace characters (but not all control characters, such as \0)
Problem
String::trim has existed from early days of Java when Unicode
had not fully evolved to the standard we widely use today.
The definition of space used by String::trim is any code point less
than or equal to the space code point (\u0020), commonly referred to
as ASCII or ISO control characters.Unicode-aware trimming routines should use
Character::isWhitespace(int).Additionally, developers have not been able to specifically remove
indentation white space or to specifically remove trailing white
space.
Solution
Introduce trimming methods that are Unicode white space aware
and provide additional control of leading only or trailing only.
A common characteristic of these new methods is that they use a different (newer) definition of “whitespace” than did old methods such as String.trim(). Bug JDK-8200373.
The current JavaDoc for String::trim does not make it clear which
definition of “space” is being used in the code. With additional
trimming methods coming in the near future that use a different
definition of space, clarification is imperative. String::trim uses
the definition of space as any codepoint that is less than or equal to
the space character codepoint (\u0020.) Newer trimming methods will
use the definition of (white) space as any codepoint that returns true
when passed to the Character::isWhitespace predicate.
The method isWhitespace(char) was added to Character with JDK 1.1, but the method isWhitespace(int) was not introduced to the Character class until JDK 1.5. The latter method (the one accepting a parameter of type int) was added to support supplementary characters. The Javadoc comments for the Character class define supplementary characters (typically modeled with int-based “code point”) versus BMP characters (typically modeled with single character):
The set of characters from U+0000 to U+FFFF is sometimes referred to
as the Basic Multilingual Plane (BMP). Characters whose code points
are greater than U+FFFF are called supplementary characters. The Java
platform uses the UTF-16 representation in char arrays and in the
String and StringBuffer classes. In this representation, supplementary
characters are represented as a pair of char values … A char value,
therefore, represents Basic Multilingual Plane (BMP) code points,
including the surrogate code points, or code units of the UTF-16
encoding. An int value represents all Unicode code points, including
supplementary code points. … The methods that only accept a char
value cannot support supplementary characters. … The methods that
accept an int value support all Unicode characters, including
supplementary characters.
OpenJDK Changeset.
Benchmark comparison between trim() and strip() – Why is String.strip() 5 times faster than String.trim() for blank string In Java 11