Logical operators in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of combined logical operators

What is the output of the following Java code snippet?

Java

public class Test {
    public static void main(String[] args) {
        boolean a = true;
        boolean b = false;
        boolean c = true;
        if (a && b || c) {
            System.out.println("YES");
        } else {
            System.out.println("NO");
        }
    }
}

A Compilation error

B NO

C YES

D Runtime exception

💡 Hint

Remember the precedence of && and || operators in Java.

The expression a && b || c is evaluated as (a && b) || c. Since a && b is false, but c is true, the overall expression is true, so "YES" is printed.

❓ Predict Output

intermediate

Logical operator short-circuit behavior

What will be the output of this Java program?

Java

public class Test {
    public static void main(String[] args) {
        int x = 0;
        if (false && (++x > 0)) {
            System.out.println("Inside if");
        }
        System.out.println(x);
    }
}

A 1

B Runtime exception

C Compilation error

D 0

💡 Hint

Consider if the increment happens when the first condition is false in &&.

The && operator short-circuits. Since the first condition is false, ++x is not evaluated, so x remains 0.

❓ Predict Output

advanced

Output with mixed logical operators and precedence

What is the output of this Java code?

Java

public class Test {
    public static void main(String[] args) {
        boolean a = false;
        boolean b = true;
        boolean c = false;
        if (a || b && c) {
            System.out.println("TRUE");
        } else {
            System.out.println("FALSE");
        }
    }
}

A FALSE

B TRUE

C Compilation error

D Runtime exception

💡 Hint

Remember that && has higher precedence than ||.

The expression is evaluated as a || (b && c). Since b && c is false, and a is false, the whole expression is false, so "FALSE" is printed.

❓ Predict Output

advanced

Logical NOT operator effect

What will be printed by this Java program?

Java

public class Test {
    public static void main(String[] args) {
        boolean flag = true;
        if (!flag || flag && false) {
            System.out.println("YES");
        } else {
            System.out.println("NO");
        }
    }
}

A NO

B YES

C Compilation error

D Runtime exception

💡 Hint

Evaluate the expression step by step, considering operator precedence and NOT operator.

The expression !flag || flag && false is (!flag) || (flag && false). Since flag is true, !flag is false and flag && false is false, so the whole expression is false || false = false. So the output should be "NO". The correct answer is B.

❓ Predict Output

expert

Complex logical expression evaluation

What is the output of this Java program?

Java

public class Test {
    public static void main(String[] args) {
        boolean a = true;
        boolean b = false;
        boolean c = true;
        boolean d = false;
        if ((a || b) && !(c && d) || b && !a) {
            System.out.println("PASS");
        } else {
            System.out.println("FAIL");
        }
    }
}

A Runtime exception

B PASS

C Compilation error

D FAIL

💡 Hint

Break down the expression into parts and evaluate carefully.

Evaluate stepwise:
1. (a || b) = true
2. (c && d) = false
3. !(c && d) = true
4. (a || b) && !(c && d) = true && true = true
5. b && !a = false && false = false
6. Overall: true || false = true
So "PASS" is printed.

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