String Methods

We already saw how to declare a String and combine strings with +. Time to look at the most useful String methods in Java

How long is a string? .length()

public class Main {
    public static void main(String[] args) {
        String name = "Tommy Vercetti";
        System.out.println(name.length());
    }
}

Output 14. Spaces count too. Notice the parentheses () at the end: for a String, length is a method, so we call it with parentheses

Uppercase and lowercase

public class Main {
    public static void main(String[] args) {
        String name = "tommy vercetti";
        System.out.println(name.toUpperCase());   // TOMMY VERCETTI
        System.out.println(name.toLowerCase());   // tommy vercetti
    }
}

Important: these methods do not change the original variable. They give back a new string

public class Main {
    public static void main(String[] args) {
        String name = "tommy";
        name.toUpperCase();
        System.out.println(name);   // still tommy
    }
}

To actually keep the uppercase version, we must reassign

public class Main {
    public static void main(String[] args) {
        String name = "tommy";
        name = name.toUpperCase();
        System.out.println(name);   // TOMMY
    }
}

This trap catches everyone in the beginning. Strings in Java are immutable: they cannot be changed. Methods always return a new string

Get a substring (a piece of the string)

public class Main {
    public static void main(String[] args) {
        String name = "Tommy Vercetti";
        System.out.println(name.substring(0, 5));   // Tommy
        System.out.println(name.substring(6));      // Vercetti
    }
}

substring(start, end) gives the characters from position start up to (but not including) position end, the end is exclusive

substring(start) with one argument gives everything from start to the end of the string

Counting starts from 0. name.substring(0, 5) means positions 0, 1, 2, 3, 4 which spell Tommy

Single character at a position

public class Main {
    public static void main(String[] args) {
        String name = "Tommy Vercetti";
        System.out.println(name.charAt(0));   // T
        System.out.println(name.charAt(6));   // V
    }
}

charAt(i) gives back a char (a single character). Combined with length(), you can grab the last character too: name.charAt(name.length() - 1)

Comparing strings, and the most common Java trap of all time

public class Main {
    public static void main(String[] args) {
        String a = "hello";
        String b = "hello";

        if (a == b) {
            System.out.println("equal");
        } else {
            System.out.println("not equal");
        }
    }
}

This might print equal, but it's wrong. With objects (and String is an object in Java), == compares whether they are the same object in memory, not whether they have the same content. The correct way is

public class Main {
    public static void main(String[] args) {
        String a = "hello";
        String b = "hello";

        if (a.equals(b)) {
            System.out.println("equal");
        }
    }
}

Always use .equals() to compare strings in Java. Memorize this right now

Mission: Signal Decoder

A garbled name arrived over Tommy's comms. The decryption module needs you to display it in several formats so the analysts can check it.

Store the name in a String named name. Then print these six values on separate lines, in this order:

• the name in uppercase
• the name in lowercase
• the length of the name
• the first character
• the first 5 characters
• the last 5 characters

For "the last 5 characters" use length() to compute the start position, so it works for any name (of at least 5 letters).

Example for the name lance vance:

LANCE VANCE
lance vance
11
l
lance
vance

Example for the name tommy vercetti:

TOMMY VERCETTI
tommy vercetti
14
t
tommy
cetti

Example for the name lance (exactly 5 letters: the first 5 and the last 5 are the whole name):

LANCE
lance
5
l
lance
lance