Indexes

Each character has an index starting at 0. The last index is length() - 1. Using an index outside that range throws a StringIndexOutOfBoundsException.

The Quick Reference methods

substring(from, to) includes from and stops before to, so its length is to - from. substring(from) goes to the end. indexOf(str) returns the first match's index, or −1. One character is s.substring(i, i + 1).

String s = "computer";
System.out.println(s.length());
System.out.println(s.substring(3, 6));
System.out.println(s.substring(5));
System.out.println(s.substring(0, 1));
System.out.println(s.indexOf("ute"));
System.out.println(s.indexOf("z"));
System.out.println(s.substring(2, 2).length());

8
put
ter
c
4
-1
0

Comparing strings

equals compares contents. == only checks whether two references point to the same object, so never use it for content. compareTo returns a negative number if this string comes first alphabetically, 0 if equal, and positive if it comes after. Uppercase letters come before lowercase.

String a = "apple";
String b = new String("apple");
System.out.println(a.equals(b));
System.out.println(a == b);
System.out.println("apple".compareTo("banana") < 0);
System.out.println("Zoo".compareTo("ant") < 0);

true
false
true
true

Immutability and split

Strings are immutable: methods return new strings. To keep a result, assign it. split(delim) breaks a string into an array of pieces.

String s = "data";
s.substring(1);
System.out.println(s);
s = s.substring(1);
System.out.println(s);
String[] parts = "red,green,blue".split(",");
System.out.println(parts.length + " " + parts[1]);

data
ata
3 green

Pattern: walk a string one character at a time

String word = "banana";
int count = 0;
for (int i = 0; i < word.length(); i++) {
  if (word.substring(i, i + 1).equals("a")) {
    count++;
  }
}
System.out.println(count);

3