Finding parts of a string

substring cuts a piece out of a string. Like every String method, it gives back a new string and leaves the original whole.

• substring(start) gives everything from the index start to the end.
• substring(start, end) gives the characters from start up to, but not including, end.

String word = "carpet";
System.out.println(word.substring(3));
System.out.println(word.substring(0, 3));
System.out.println(word.substring(1, 4));

pet
car
arp

Because the end isn't included, substring(start, end) is always end - start characters long: substring(1, 4) has 3. An index past the end crashes the program with the same StringIndexOutOfBoundsException as charAt.

Searching in a string

indexOf looks for a piece of text and returns the index where it first starts, or -1 when it isn't there. Together with substring, it cuts a string at a certain character, wherever that character happens to be:

String email = "ada@example.com";
int at = email.indexOf("@");
System.out.println(at);
System.out.println(email.substring(0, at));
System.out.println(email.substring(at + 1));
System.out.println(email.indexOf("#"));

3
ada
example.com
-1

Three more methods answer yes-or-no questions with a boolean:

String file = "holiday-photo.jpg";
System.out.println(file.contains("photo"));
System.out.println(file.startsWith("holiday"));
System.out.println(file.endsWith(".png"));

true
true
false

All of these count capitals: "Photo.JPG".endsWith(".jpg") is false. When capitals shouldn't matter, make the text small first: file.toLowerCase().endsWith(".jpg").