Java String Methods Reference

›Short versionOpen if you only have a minute.

A String is a fixed sequence of UTF-16 code units. Nothing on this page edits that sequence in place.
trim, substring, replace, and toUpperCase return a new String. Keep the result or it is gone.
equals compares characters. == compares whether two variables point at the same object.
indexOf returns the first index, or -1 when the text is missing. -1 is not a valid index.
substring takes a start that is included and an end that is excluded. replace is literal; replaceAll and split use a regex.

String represents a sequence of UTF-16 code units. Most methods inspect text or return a new String; they do not edit the original object.

Methods

API	Parameters	Returns	Behavior	Watch for
length()	None.	int count of UTF-16 code units.	Reports storage length, which matches simple character count for most beginner examples.	Emoji and some non-BMP characters can count as two code units.
charAt(int index)	Zero-based index from 0 to length() - 1.	char at that code-unit index.	Reads a single UTF-16 code unit without modifying the string.	Throws StringIndexOutOfBoundsException for invalid indexes.
substring(int begin, int end)	begin inclusive, end exclusive.	New String containing the requested range.	Copies the selected section into a separate String value.	begin > end or indexes outside range throw StringIndexOutOfBoundsException.
indexOf(String target)	Target substring to locate.	First index or -1 if not found.	Scans from left to right using literal matching.	Check for -1 before using the result as an index.
split(String regex)	Regular expression delimiter.	String[] tokens.	Breaks text around regex matches.	Dots, pipes, and backslashes are regex metacharacters and often need escaping.
equals(Object other)	Any object reference.	boolean content equality.	Returns true only when other is a String with the same characters.	Use "literal".equals(value) when value might be null.

Which call fits

Option	Best for	Tradeoff
equals()	Checking whether two strings have the same content.	Case-sensitive and null-sensitive when called on a nullable variable.
equalsIgnoreCase()	Simple case-insensitive comparisons such as yes/no input.	Not a substitute for full locale-aware text rules.
==	Reference identity checks.	Usually wrong for string content comparisons.

Examples

Extract and validate parts

java

String email = "ada@example.com";
int at = email.indexOf("@");

if (at != -1 && email.endsWith(".com")) {
    String user = email.substring(0, at);
    System.out.println(user); // ada
}

Remember immutability

java

String name = " ada ";
name.trim();
System.out.println(name); // still has spaces

name = name.trim();
System.out.println(name); // ada

String name = " ada "; then name.trim(); then you print name. What is printed, and what would keep the trimmed text?

The print shows ada , spaces included. trim() returned a new string, and that return value was discarded. name = name.trim(); stores it, and the next print shows ada.