Useful String Methods

Strings come with tools

A String is not just text – it comes with many built-in methods (tools). You use a method by writing a dot after the string: name.length().

public class Main {
    public static void main(String[] args) {
        String word = "Java Rocks";
        System.out.println(word.length());
        System.out.println(word.toUpperCase());
        System.out.println(word.toLowerCase());
        System.out.println(word.contains("Rock"));
        System.out.println(word.replace("Rocks", "Rules"));
        System.out.println(word.startsWith("Ja"));
    }
}

Output

10
JAVA ROCKS
java rocks
true
Java Rules
true

Positions start at 0

Every character in a string has a position number called an index. The first character is at index 0, not 1:

Think of it like this: Think of floors in some buildings: the bottom floor is the “ground floor” (0), then floor 1, floor 2...

public class Main {
    public static void main(String[] args) {
        String word = "Java";
        System.out.println(word.charAt(0));
        System.out.println(word.charAt(3));
        System.out.println(word.indexOf("v"));
        System.out.println(word.indexOf("z"));
    }
}

Output

J
a
2
-1

indexOf gives the position of the first match, or -1 if it isn't there.

Cutting out a piece: substring

substring(start, end) gives the characters from start up to, but not including, end. With only one number, it goes to the end:

public class Main {
    public static void main(String[] args) {
        String email = "asha@vulntech.com";
        int at = email.indexOf("@");
        System.out.println(email.substring(0, at));
        System.out.println(email.substring(at + 1));
    }
}

Output

asha
vulntech.com

Cleaning and splitting

public class Main {
    public static void main(String[] args) {
        String messy = "   hello   ";
        System.out.println("[" + messy.trim() + "]");

        String fruits = "apple,banana,mango";
        String[] parts = fruits.split(",");
        System.out.println(parts[0]);
        System.out.println(parts[2]);
        System.out.println(parts.length);
    }
}

Output

[hello]
apple
mango
3

trim() removes spaces at both ends. split cuts a string into pieces – you'll learn more about the String[] (array) it gives back in the Arrays module.

Common mistake: Asking for an index that doesn't exist crashes the program:

public class Main {
    public static void main(String[] args) {
        String word = "Java";
        System.out.println(word.charAt(4));
    }
}

Output (Java stops with an error)

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 4