Booleans

A boolean is a value that can only be true or false. We’ve already seen them inside if conditions. Java has its own type for them, called boolean

public class Main {
    public static void main(String[] args) {
        boolean isOnline = true;
        boolean hasKey = false;
        System.out.println(isOnline);
        System.out.println(hasKey);
    }
}

Important: in Java, true and false are lowercase. Be careful not to capitalize them

Conditions like age < 18 or x == 5 also produce booleans

public class Main {
    public static void main(String[] args) {
        int age = 20;
        System.out.println(age < 18);    // false
        System.out.println(age >= 18);   // true
    }
}

We can combine booleans with logical operators

• && means and (both conditions must be true)
• || means or (at least one must be true)
• ! means not (flips the value)

Imagine: in order to drive a car, you need to be at least 18 years old AND have a license

public class Main {
    public static void main(String[] args) {
        int age = 20;
        boolean hasLicense = true;

        if (age >= 18 && hasLicense) {
            System.out.println("You can drive");
        } else {
            System.out.println("Sorry, no driving today");
        }
    }
}

Both conditions must be true for && to be true. If even one is false, the whole thing is false

|| (or) is more relaxed. Just one of the conditions being true is enough

public class Main {
    public static void main(String[] args) {
        boolean isVIP = false;
        boolean hasInvitation = true;

        if (isVIP || hasInvitation) {
            System.out.println("Welcome to the club");
        } else {
            System.out.println("Access denied");
        }
    }
}

isVIP is false, hasInvitation is true, so the or is true and the person enters

! flips a boolean. !true becomes false, !false becomes true

public class Main {
    public static void main(String[] args) {
        boolean isLoggedIn = false;
        if (!isLoggedIn) {
            System.out.println("Please log in first");
        }
    }
}

Reads almost like English: if not logged in, log in

Truth tables, just to have them in one place

true  && true  = true
true  && false = false
false && true  = false
false && false = false

true  || true  = true
true  || false = true
false || true  = true
false || false = false

!true  = false
!false = true

Mission: Station Access Gate

You are writing the access system for the station. A person can enter if they are an employee AND it is a working day, OR if they are a guest WITH an invitation.

Store in four boolean variables (employee, workingDay, guest, and invitation) whether the person is an employee, whether today is a working day, whether they are a guest, and whether they have an invitation. Then write an if / else using && and || that prints Access granted or Access denied.

Example

If the person is an employee and today is a working day (even if they are not a guest and have no invitation), your program should print

Access granted