BeginnerPractice in the editor

Java Booleans

Many questions in a program have only two possible answers: is the user logged in? Is the number even? Is the shop open? Java has a special data type for such answers, boolean, with exactly two values: true and false.

Boolean variables

ExampleTry it in the editor

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

Output

true
false

Name boolean variables like a yes-or-no question: isReady, hasTicket, canVote. Then the code reads almost like English.

Comparisons give booleans

Every comparison from the Operators topic produces a boolean. You can print it or store it in a variable:

ExampleTry it in the editor

public class Main {
    public static void main(String[] args) {
        int x = 10, y = 9;
        System.out.println(x > y);
        System.out.println(x == 10);
        System.out.println(x != y);
        boolean isEven = x % 2 == 0;
        System.out.println("10 is even: " + isEven);
    }
}

Output

true
true
true
10 is even: true

Combining booleans

With && (and), || (or) and ! (not) you build bigger conditions from small ones. A small table of all cases is called a truth table:

a	b	`a && b`	`a \|\| b`	`!a`
true	true	true	true	false
true	false	false	true	false
false	true	false	true	true
false	false	false	false	true

ExampleTry it in the editor

public class Main {
    public static void main(String[] args) {
        int age = 19;
        boolean hasId = true;
        boolean member = false;
        boolean canEnter = age >= 18 && hasId;
        boolean getsDiscount = member || age < 21;
        System.out.println("Can enter: " + canEnter);
        System.out.println("Discount: " + getsDiscount);
        System.out.println("Not a member: " + !member);
    }
}

Output

Can enter: true
Discount: true
Not a member: true

Storing a long condition in a well-named variable, like canEnter, makes the program much easier to read than repeating the whole expression.

Booleans and decisions

The real power of booleans comes with decisions. An if statement runs a block only when its condition is true. You will learn it properly in the next topics, User Input and If ... Else, but here is a first taste:

ExampleTry it in the editor

public class Main {
    public static void main(String[] args) {
        int temperature = 31;
        boolean isHot = temperature > 30;
        if (isHot) {
            System.out.println("Drink plenty of water!");
        }
        System.out.println("Have a nice day.");
    }
}

Output

Drink plenty of water!
Have a nice day.

Note that you write if (isHot), not if (isHot == true): the variable already is true or false.

Practice exercises

Store whether 2025 is a leap year in a boolean (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) and print it.
Make two booleans hasUmbrella and isRaining and print whether you will get wet.
Print the truth table of && for all four combinations of two booleans.