Boolean. Booleans are often used in Java programs. We can use the literals "true" and "false." We often use booleans inside if-statements, or while-loops.

Boolean features. It is possible to invert the value of a boolean with a simple expression. Booleans are often useful as local variables.

Example. In this program we use the literal constants true and false. Often we assign a boolean to true or false as we declare it. Here we test for truth in two ifs.

public class Program {
    public static void main(String[] args) {

        // Test true and false booleans.
        boolean value = true;
        if (value) {
            System.out.println("A");
        }
        value = false;
        if (!value) {
            System.out.println("B");
        }
    }
}
A
B

Flip. We can negate (or invert, or flip) a boolean value. This changes true to false, and false to true. We use the exclamation mark to perform this operation.

public class Program {
    public static void main(String[] args) {

        boolean value = true;
        System.out.println(value);
        // Flip from true to false.
        value = !value;
        System.out.println(value);
        // Flip back to true from false.
        value = !value;
        System.out.println(value);
    }
}
true
false
true

Expression variable. A boolean variable can store the result of an expression evaluation. It can then be reused, without reevaluating the expression.

public class Program {
    public static void main(String[] args) {

        int size = 100;
        String color = "blue";

        // Use expression to compute a boolean value.
        boolean largeAndBlue = size >= 50 && color == "blue";
        // Test our boolean.
        if (largeAndBlue) {
            System.out.println("Large and blue = " + largeAndBlue);
        }
    }
}
Large and blue = true

Boolean arguments. Methods often receive boolean arguments. We can pass them anything that evaluates to a boolean—this includes the constants true and false.

public class Program {

    static void display(boolean console) {
        // Display a message if boolean argument is true.
        if (console) {
            System.out.println("The cat is cute.");
        }
    }

    public static void main(String[] args) {

        // A message is displayed.
        display(true);
        // These calls do not print anything to the console.
        display(false);
        display(false);
    }
}
The cat is cute.

Return boolean. Here we see a method that returns a boolean based on its arguments. We prefix the method with the word "is." This is a convention.

public class Program {

    static boolean isValid(String color, int size) {
        // Return boolean based on three expressions.
        return color.length() >= 1 && size >= 10 && size <= 100;
    }

    public static void main(String[] args) {

        // This call returns true.
        if (isValid("blue", 50)) {
            System.out.println("A");
        }
        // This call is not valid because the size is too large.
        if (!isValid("orange", 200)) {
            System.out.println("B");
        }
    }
}
A
B

While-loop. We can use a boolean in a while-loop. Here we continue looping while the boolean is true, and we end the loop by setting the boolean to false.

public class Program {
    public static void main(String[] args) {

        boolean value = true;

        // Continue looping while true.
        while (value) {
            System.out.println("OK");
            value = false;
        }

        System.out.println("DONE");
    }
}
OK
DONE

Memory usage. In this program we measure the memory usage of an array of booleans. We allocate one million boolean elements. With freeMemory(), we determine how much memory is used.

public class Program {
    public static void main(String[] args) {

        // Get free memory.
        long memory1 = Runtime.getRuntime().freeMemory();

        // Allocate and use an array of one million booleans.
        boolean[] array = new boolean[1000000];
        array[0] = true;

        // Get free memory again.
        long memory2 = Runtime.getRuntime().freeMemory();
        array[1] = true;

        // See how much memory was used.
        System.out.println(memory1 - memory2);
    }
}
1000016

Convert to int. We cannot directly convert a boolean to an int (like 0 or 1). Often a ternary expression is helpful for this requirement.

An optimization. Sometimes a program checks a set of conditions repeatedly in if-statements. We can place the result of this check in a boolean.

Summary. With booleans, we store true and false values. We store the results of complex expressions. And we can return, and receive, booleans from methods.