Constants with final

Values that should never change

Some values must stay the same for the whole program, like the number of days in a week. Put final in front, and Java won't let anyone change it:

public class Main {
    public static void main(String[] args) {
        final int DAYS_IN_WEEK = 7;
        final double GST_RATE = 0.18;
        System.out.println("A week has " + DAYS_IN_WEEK + " days");
        System.out.println("Tax on 100 is " + (100 * GST_RATE));
    }
}

Output

A week has 7 days
Tax on 100 is 18.0

Constants are written in UPPER_CASE with underscores between words, so they stand out.

Trying to change a constant

public class Main {
    public static void main(String[] args) {
        final int DAYS_IN_WEEK = 7;
        DAYS_IN_WEEK = 8;
    }
}

Output (the program does not compile)

Main.java:4: error: cannot assign a value to final variable DAYS_IN_WEEK
        DAYS_IN_WEEK = 8;
        ^

Tip: Use a constant instead of typing the same number in many places. If it ever needs to change, you change it in one place only.