String methods

The String class has dozens of methods. Here are the most useful ones: finding a substring, cutting out a part, changing case, replacing, splitting. Remember the main rule from the strings lesson: a string never changes, and every method returns a new one.

Searching in a string

contains() checks whether a string contains a substring, startsWith() and endsWith() whether it starts or ends with it. indexOf() returns the index of the first occurrence, or -1 if there is none:

void main() {
    String email = "anna@example.com";
    IO.println(email.contains("@"));
    IO.println(email.endsWith(".com"));
    IO.println(email.indexOf("@"));
    IO.println(email.indexOf("#"));
}

Output:

true
true
4
-1

lastIndexOf() works the same way but finds the last occurrence.

substring

substring(start, end) cuts out part of a string: the character at index start is included, the one at index end is not. If no end is given, everything up to the end of the string is taken:

void main() {
    String email = "anna@example.com";
    int at = email.indexOf("@");
    IO.println(email.substring(0, at));
    IO.println(email.substring(at + 1));
}

Output:

anna
example.com

Case and whitespace

toUpperCase() and toLowerCase() change the case. strip() removes whitespace at both ends — often needed for user input:

void main() {
    String input = "   Oleg  ";
    IO.println("[" + input.strip() + "]");
    IO.println(input.strip().toUpperCase());
    IO.println("JAVA".toLowerCase());
}

Output:

[Oleg]
OLEG
java

For a case-insensitive comparison there is equalsIgnoreCase(): "Java".equalsIgnoreCase("JAVA") returns true.

replace

replace() replaces every occurrence of one substring with another:

void main() {
    String phone = "555-123-4567";
    IO.println(phone.replace("-", ""));
    IO.println("cat and cat".replace("cat", "dog"));
}

Output:

5551234567
dog and dog

split and join

split() cuts a string at a separator and returns an array of the parts. String.join() does the opposite — it glues the parts together with a separator:

void main() {
    String csv = "Anna,Oleg,Maria";
    String[] names = csv.split(",");
    IO.println(names.length);
    IO.println(names[1]);
    IO.println(String.join(" | ", names));
}

Output:

3
Oleg
Anna | Oleg | Maria

split() drops empty parts at the end: "a,b,,".split(",") gives an array of two elements.

Checks and repetition

isEmpty() is true for a zero-length string, isBlank() also for a string of only spaces. repeat() repeats a string a given number of times:

void main() {
    IO.println("".isEmpty());
    IO.println("   ".isEmpty());
    IO.println("   ".isBlank());
    IO.println("=".repeat(12));
}

Output:

true
false
true
============

Common mistake

String methods do not change the original string. If you do not store the result, it is simply lost:

void main() {
    String name = "anna";
    name.toUpperCase();
    IO.println(name);
}

Output:

anna

Assign the result to a variable: name = name.toUpperCase();.