Logical operators in Java - Mini Project: Build & Apply

Logical operators

📖 Scenario: You are creating a simple program to check if a person is eligible for a special discount based on their age and membership status.

🎯 Goal: Build a Java program that uses logical operators to decide if a person qualifies for a discount.

📋 What You'll Learn

Create variables for age and membership status

Create a boolean variable for discount eligibility using logical operators

Use if-else to print the eligibility result

💡 Why This Matters

🌍 Real World

Logical operators help decide if conditions are met, like checking eligibility for discounts or access.

💼 Career

Understanding logical operators is essential for writing decision-making code in software development.

1

Create variables for age and membership status

Create an int variable called age and set it to 25. Create a boolean variable called isMember and set it to true.

Java

// Your code here

Hint

Use int for numbers and boolean for true/false values.

2

Create a boolean variable for discount eligibility

Create a boolean variable called eligibleForDiscount that is true if age is greater than or equal to 18 and isMember is true. Use the logical AND operator &&.

Java

int age = 25;
boolean isMember = true;
// Your code here

Hint

Use parentheses to group conditions and && to combine them.

3

Use if-else to print eligibility result

Write an if statement that checks eligibleForDiscount. If true, print "You get a discount!". Otherwise, print "No discount available.".

Java

int age = 25;
boolean isMember = true;
boolean eligibleForDiscount = (age >= 18) && isMember;
// Your code here

Hint

Use if and else blocks to print different messages.

4

Run the program to see the output

Run the program and print the output. The program should print You get a discount!.

Java

int age = 25;
boolean isMember = true;
boolean eligibleForDiscount = (age >= 18) && isMember;
if (eligibleForDiscount) {
    System.out.println("You get a discount!");
} else {
    System.out.println("No discount available.");
}
// Your code here

Hint

Check the console output matches exactly.

Practice

1. Which logical operator in Java means "both conditions must be true"?

easy

A. && (AND)

B. || (OR)

C. ! (NOT)

D. ^ (XOR)

Solution

1. Step 1: Understand the meaning of logical AND

   The operator && means both conditions must be true for the whole expression to be true.

2. Step 2: Compare with other operators

   || means one or both true, ! reverses true/false, ^ means exclusive or (one true only).

3. Final Answer:

   && (AND) -> Option A

4. Quick Check:

   Both true = && [OK]

Hint: AND means both true, use && [OK]

Common Mistakes:

• Confusing || (OR) with && (AND)
• Thinking ! means AND
• Mixing up ^ with &&

2. Which of the following is the correct syntax to negate a boolean variable flag in Java?

easy

A. flag = not flag;

B. flag = !flag;

C. flag = ~flag;

D. flag = flag !=;

Solution

1. Step 1: Recall the NOT operator syntax

   In Java, the NOT operator is ! and placed before the boolean variable to reverse its value.

2. Step 2: Check each option

   flag = !flag; uses ! correctly. flag = not flag; uses 'not' which is invalid in Java. flag = ~flag; uses ~ which is bitwise NOT, not for booleans. flag = flag !=; is invalid syntax.

3. Final Answer:

   flag = !flag; -> Option B

4. Quick Check:

   Negate boolean with ! [OK]

Hint: Use ! before boolean to negate it [OK]

Common Mistakes:

• Using 'not' instead of !
• Using bitwise ~ on booleans
• Incorrect syntax like 'flag !=;'

3. What is the output of the following Java code?

boolean a = true;
boolean b = false;
System.out.println(a && b || !b);

medium

A. false

B. Compilation error

C. true

D. Runtime exception

Solution

1. Step 1: Evaluate the expression step-by-step

   a && b is true && false = false. !b is !false = true.

2. Step 2: Apply OR operator

   Now false || true = true.

3. Final Answer:

   true -> Option C

4. Quick Check:

   (true && false) || !false = true [OK]

Hint: Evaluate && first, then ||, then ! [OK]

Common Mistakes:

• Ignoring operator precedence
• Evaluating || before &&
• Confusing !b value

4. Identify the error in this Java code snippet:

boolean x = true;
boolean y = false;
if (x & | y) {
    System.out.println("Yes");
}

medium

A. Invalid use of & | operators together

B. Missing semicolon after if condition

C. Boolean variables cannot be used in if

D. No error, code runs fine

Solution

1. Step 1: Check the if condition syntax

   The condition uses '& |' which is not a valid operator combination in Java.

2. Step 2: Understand valid logical operators

   Valid operators are &&, ||, &, | but not combined as '& |'. This causes a syntax error.

3. Final Answer:

   Invalid use of & | operators together -> Option A

4. Quick Check:

   Operators & and | cannot be combined like '& |' [OK]

Hint: Use && or ||, not mixed & | [OK]

Common Mistakes:

• Confusing bitwise & with logical &&
• Combining & and | without operand
• Thinking no error occurs

5. You want to check if a number n is between 10 and 20 inclusive in Java. Which condition correctly uses logical operators?

hard

A. if (n > 10 && n < 20)

B. if (n >= 10 || n <= 20)

C. if (!(n < 10 && n > 20))

D. if (n >= 10 && n <= 20)

Solution

1. Step 1: Understand the range condition

   To check if n is between 10 and 20 inclusive, both conditions n >= 10 and n <= 20 must be true.

2. Step 2: Analyze each option

   if (n >= 10 || n <= 20) uses || which means either condition true, so it allows numbers outside the range. if (n >= 10 && n <= 20) uses && which correctly requires both conditions true. if (!(n < 10 && n > 20)) negates an impossible condition and is incorrect. if (n > 10 && n < 20) excludes 10 and 20 by using > and <.

3. Final Answer:

   if (n >= 10 && n <= 20) -> Option D

4. Quick Check:

   Both limits true = && [OK]

Hint: Use && to require both conditions true [OK]

Common Mistakes:

• Using || instead of && for range
• Excluding boundary values by using > or <
• Misusing negation for range check