How to Check If a Boolean Expression Is Valid in Java

Introduction

In this lab, you will learn how to check if a boolean expression is valid in Java. We will start by evaluating simple boolean expressions using comparison operators.

Next, you will explore how to handle compound boolean expressions by combining simple expressions with logical operators. Finally, you will learn how to identify and catch errors that can occur when working with invalid boolean expressions in your Java code.

Skills Graph

Evaluate Simple Boolean Expression

In this step, we will learn about boolean expressions in Java and how to evaluate simple ones. Boolean expressions are fundamental in programming as they allow us to make decisions based on whether a condition is true or false.

A boolean expression is a statement that evaluates to either true or false. In Java, we use comparison operators to create these expressions. Here are some common comparison operators:

• ==: Equal to
• !=: Not equal to
• >: Greater than
• <: Less than
• >=: Greater than or equal to
• <=: Less than or equal to

Let's create a simple Java program to evaluate some boolean expressions.

1. Open the HelloJava.java file in the WebIDE editor if it's not already open.

2. Replace the entire contents of the file with the following code:

   public class HelloJava {
       public static void main(String[] args) {
           int x = 10;
           int y = 20;
   
           boolean isEqual = (x == y);
           boolean isNotEqual = (x != y);
           boolean isGreater = (x > y);
           boolean isLess = (x < y);
           boolean isGreaterOrEqual = (x >= y);
           boolean isLessOrEqual = (x <= y);
   
           System.out.println("Is x equal to y? " + isEqual);
           System.out.println("Is x not equal to y? " + isNotEqual);
           System.out.println("Is x greater than y? " + isGreater);
           System.out.println("Is x less than y? " + isLess);
           System.out.println("Is x greater than or equal to y? " + isGreaterOrEqual);
           System.out.println("Is x less than or equal to y? " + isLessOrEqual);
       }
   }

   In this code:

   • We declare two integer variables, x and y.
   • We create several boolean variables (isEqual, isNotEqual, etc.) and assign the result of a boolean expression to each.
   • We use System.out.println to print the results of these boolean expressions.

3. Save the file (Ctrl+S or Cmd+S).

4. Compile the program by running the following command in the Terminal:

   javac HelloJava.java

   If there are no errors, a HelloJava.class file will be created.

5. Run the compiled program:

   java HelloJava

   You should see the output showing the results of each boolean expression.

   Is x equal to y? false
   Is x not equal to y? true
   Is x greater than y? false
   Is x less than y? true
   Is x greater than or equal to y? false
   Is x less than or equal to y? true

You have successfully evaluated simple boolean expressions in Java! Understanding how to compare values and get a true or false result is crucial for controlling the flow of your programs.

Handle Compound Expressions

In this step, we will learn how to combine simple boolean expressions to create more complex ones using logical operators. These are called compound expressions, and they allow us to check multiple conditions at once.

Java provides the following logical operators:

• &&: Logical AND. The expression is true only if both conditions are true.
• ||: Logical OR. The expression is true if at least one of the conditions is true.
• !: Logical NOT. This operator reverses the boolean value of an expression. If an expression is true, ! makes it false, and vice versa.

Let's modify our HelloJava.java program to use these logical operators.

1. Open the HelloJava.java file in the WebIDE editor.

2. Replace the current code with the following:

   public class HelloJava {
       public static void main(String[] args) {
           int age = 25;
           int score = 85;
           boolean isStudent = true;
   
           // Using Logical AND (&&)
           boolean isEligibleForDiscount = (age > 18 && isStudent);
           System.out.println("Is eligible for student discount? " + isEligibleForDiscount);
   
           // Using Logical OR (||)
           boolean passedExam = (score >= 70 || age < 18);
           System.out.println("Passed the exam? " + passedExam);
   
           // Using Logical NOT (!)
           boolean isNotStudent = !isStudent;
           System.out.println("Is not a student? " + isNotStudent);
   
           // Combining multiple operators
           boolean complexCondition = (age > 20 && score > 80 || !isStudent);
           System.out.println("Complex condition result: " + complexCondition);
       }
   }

   In this updated code:

   • We introduce new variables age, score, and isStudent.
   • We use && to check if someone is eligible for a student discount (older than 18 AND is a student).
   • We use || to check if someone passed an exam (score is 70 or higher OR they are under 18).
   • We use ! to negate the isStudent boolean.
   • We show an example of combining &&, ||, and ! in a single expression.

3. Save the file (Ctrl+S or Cmd+S).

4. Compile the program in the Terminal:

   javac HelloJava.java

   Ensure there are no compilation errors.

5. Run the compiled program:

   java HelloJava

   Observe the output, which will show the results of the compound boolean expressions.

   Is eligible for student discount? true
   Passed the exam? true
   Is not a student? false
   Complex condition result: true

You have now learned how to create and evaluate compound boolean expressions using logical AND (&&), OR (||), and NOT (!) operators. This allows you to build more sophisticated conditions in your Java programs.

Catch Invalid Expression Errors

In this step, we will explore what happens when we try to evaluate expressions that are not valid in Java and how the compiler helps us catch these errors. Understanding common errors is an important part of learning any programming language.

Java is a strongly-typed language, which means that the type of data matters. Boolean expressions specifically require operands that can be compared or evaluated to a boolean value. Trying to use incompatible types in a boolean expression will result in a compilation error.

Let's intentionally introduce an error into our HelloJava.java program to see how the compiler reacts.

1. Open the HelloJava.java file in the WebIDE editor.

2. Modify the code to include an invalid comparison. For example, let's try to compare a boolean with an integer:

   public class HelloJava {
       public static void main(String[] args) {
           int age = 25;
           boolean isStudent = true;
   
           // This line will cause a compilation error
           // boolean invalidComparison = (isStudent == age);
   
           System.out.println("Age: " + age);
           System.out.println("Is student: " + isStudent);
       }
   }

   We've commented out the line that will cause the error for now, but we'll uncomment it in the next step. The commented-out line boolean invalidComparison = (isStudent == age); attempts to compare a boolean variable (isStudent) with an int variable (age) using the equality operator (==). Java does not allow this direct comparison because boolean and int are different data types that cannot be meaningfully compared in this way.

3. Save the file (Ctrl+S or Cmd+S).

4. Now, let's uncomment the line that will cause the error. Change the code to:

   public class HelloJava {
       public static void main(String[] args) {
           int age = 25;
           boolean isStudent = true;
   
           // This line will cause a compilation error
           boolean invalidComparison = (isStudent == age);
   
           System.out.println("Age: " + age);
           System.out.println("Is student: " + isStudent);
           System.out.println("Invalid comparison result: " + invalidComparison);
       }
   }

5. Save the file again.

6. Try to compile the program in the Terminal:

   javac HelloJava.java

   You should see an error message from the Java compiler. The exact message might vary slightly depending on the Java version, but it will indicate a type mismatch or incompatible types.

   ~/project/HelloJava.java:7: error: incomparable types: boolean and int
           boolean invalidComparison = (isStudent == age);
                                        ^
   1 error

   This error message tells you the file name (HelloJava.java), the line number (7), and a description of the error ("incomparable types: boolean and int"). This is the compiler helping you catch mistakes before you even try to run the program.

7. To fix the error and make the program compilable again, you need to remove or comment out the invalid line. Change the code back to the version where the line is commented out:

   public class HelloJava {
       public static void main(String[] args) {
           int age = 25;
           boolean isStudent = true;
   
           // This line will cause a compilation error
           // boolean invalidComparison = (isStudent == age);
   
           System.out.println("Age: " + age);
           System.out.println("Is student: " + isStudent);
       }
   }

8. Save the file.

9. Compile the program again:

   javac HelloJava.java

   This time, the compilation should be successful, and you won't see any error messages.

Understanding compilation errors and how to read them is a crucial skill for any programmer. The Java compiler is a powerful tool that helps you find many common mistakes early in the development process.

Summary

In this lab, we learned the fundamentals of boolean expressions in Java. We started by understanding what a boolean expression is and how it evaluates to either true or false. We then practiced evaluating simple boolean expressions using common comparison operators such as ==, !=, >, <, >=, and <=. Through a practical example, we saw how to declare integer variables, apply these comparison operators to create boolean expressions, and print the resulting boolean values.

Building upon simple expressions, we explored how to handle compound boolean expressions by combining simple ones using logical operators like && (AND), || (OR), and ! (NOT). We learned how these operators work and how to use parentheses to control the order of evaluation in complex expressions. Finally, we addressed the importance of handling potential errors that can arise from invalid boolean expressions, understanding how to identify and address issues that might prevent an expression from being evaluated correctly.