The boolean keyword in Java is one of the eight primitive data types. It represents a logical value that can be either true or false. It’s fundamental to decision-making and control flow in Java programs. Think of it as the on/off switch of logic.

Declaring boolean Variables:

You declare a boolean variable using the boolean keyword followed by the variable name:

Java boolean Keyword

boolean isLoggedIn;
boolean isRunning;
boolean hasCompleted;

Assigning Values:

You can assign either true or false to a boolean variable:

Java boolean Keyword

isLoggedIn = true;
isRunning = false;
hasCompleted = true;

Using boolean in Expressions:

boolean values are primarily used in logical expressions and conditional statements. They are the result of comparisons and logical operations.

  • Comparison Operators: Comparison operators (e.g., ==, !=, >, <, >=, <=) produce boolean results:
Java

int age = 25;
boolean isAdult = age >= 18; // isAdult will be true
  • Logical Operators: Logical operators (e.g., && (AND), || (OR), ! (NOT)) combine or modify boolean values:
Java

boolean hasPermission = isLoggedIn && isRunning; // true only if both are true
boolean isWeekend = day == "Saturday" || day == "Sunday"; // true if either is true
boolean isNotLoggedIn = !isLoggedIn; // true if isLoggedIn is false

Using boolean in Control Flow:

boolean values are essential for controlling the flow of execution in your programs. They are used in:

  • if statements:
Java

if (isLoggedIn) {
    // Code to execute if isLoggedIn is true
    System.out.println("Welcome!");
} else {
    // Code to execute if isLoggedIn is false
    System.out.println("Please log in.");
}
  • while loops:
Java

while (isRunning) {
    // Code to execute as long as isRunning is true
    // ... potentially change isRunning to false at some point
}
  • for loops (in the conditional part):
Java

for (int i = 0; i < 10 && !hasCompleted; i++) {
    // Code to execute
}
  • Ternary Operator:
Java

String message = isLoggedIn ? "Welcome!" : "Please log in.";

Example:

Java

public class LoginExample {
    public static void main(String[] args) {
        String username = "myuser";
        String password = "mypassword";

        boolean isLoggedIn = checkCredentials(username, password);

        if (isLoggedIn) {
            System.out.println("Login successful!");
        } else {
            System.out.println("Invalid username or password.");
        }
    }

    static boolean checkCredentials(String username, String password) {
        // In a real application, you would check against a database or other secure storage.
        // This is a simplified example.
        return username.equals("myuser") && password.equals("mypassword");
    }
}

Key Points about boolean:

  • boolean variables can only hold two values: true or false.
  • They are the result of comparisons and logical operations.
  • They are crucial for controlling program flow.
  • They are a primitive data type in Java.

Let’s explore more examples of the boolean keyword in Java, focusing on different usage scenarios and demonstrating its versatility.

1. Checking Range:

Java

public class RangeCheck {
    public static void main(String[] args) {
        int temperature = 25;

        boolean isComfortable = temperature >= 20 && temperature <= 30;

        if (isComfortable) {
            System.out.println("The temperature is comfortable.");
        } else {
            System.out.println("The temperature is not comfortable.");
        }
    }
}

This example checks if the temperature is within a specific range (20 to 30 degrees). The && (AND) operator ensures that both conditions must be true for isComfortable to be true.

2. Even or Odd:

Java

public class EvenOdd {
    public static void main(String[] args) {
        int number = 17;

        boolean isEven = number % 2 == 0; // Remainder 0 means even

        if (isEven) {
            System.out.println(number + " is even.");
        } else {
            System.out.println(number + " is odd.");
        }
    }
}

Here, we use the modulo operator (%) to check if a number is even or odd. If the remainder after dividing by 2 is 0, the number is even.

3. Checking for Null or Empty String:

Java

public class StringCheck {
    public static void main(String[] args) {
        String text = ""; // Or null

        boolean isEmpty = text == null || text.isEmpty();

        if (isEmpty) {
            System.out.println("The string is either null or empty.");
        } else {
            System.out.println("The string is: " + text);
        }
    }
}

This example demonstrates how to check if a string is either null or empty. The || (OR) operator checks if either condition is true. This is a common pattern when handling user input or data from external sources.

4. Combining Conditions:

Java

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

        boolean canDrive = age >= 18 && hasLicense && hasInsurance;

        if (canDrive) {
            System.out.println("This person can drive.");
        } else {
            System.out.println("This person cannot drive.");
        }
    }
}

This example combines multiple conditions using && (AND). The person can only drive if they are at least 18 years old, have a license, and have insurance.

5. Using boolean Flags:

Java

public class Game {
    boolean gameOver = false;
    int score = 0;

    public void play() {
        while (!gameOver) {  // Game continues as long as gameOver is false
            // ... game logic ...
            score++;
            if (score > 100) {
                gameOver = true; // Game ends when score reaches 100
            }
        }
        System.out.println("Game Over! Final score: " + score);
    }

    public static void main(String[] args) {
        Game myGame = new Game();
        myGame.play();
    }
}

Here, gameOver acts as a flag to control the game loop. The game continues as long as gameOver is false. This is a common way to manage the state of a program or loop.

6. Returning boolean from a Method:

Java

public class Validation {
    public static boolean isValidEmail(String email) {
        // ... complex email validation logic ...
        return email.contains("@") && email.contains("."); // Simplified check
    }

    public static void main(String[] args) {
        String email = "test@example.com";
        boolean isValid = isValidEmail(email);

        if (isValid) {
            System.out.println("The email is valid.");
        } else {
            System.out.println("The email is invalid.");
        }
    }
}

This example shows a method isValidEmail that returns a boolean value. This is a common practice for validation or checking conditions.

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