Logical operators in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if both conditions are true using the AND operator.

Java

boolean result = (a > 5) [1] (b < 10);

Drag options to blanks, or click blank then click option'

A|

B||

C&&

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using || which means OR instead of AND.

Using single & which is a bitwise operator here.

The AND operator in Java is &&. It returns true only if both conditions are true.

2fill in blank

medium

Complete the code to check if either condition is true using the OR operator.

Java

boolean result = (x == 0) [1] (y == 1);

Drag options to blanks, or click blank then click option'

A||

B&&

C&

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using && which means AND instead of OR.

Using single | which is a bitwise operator here.

The OR operator in Java is ||. It returns true if at least one condition is true.

3fill in blank

hard

Fix the error in the code to negate the condition correctly.

Java

boolean isFalse = [1] flag;

Drag options to blanks, or click blank then click option'

Aflag

B~

C!!

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using double negation !! which returns the original value.

Using bitwise complement ~ which works on integers, not booleans.

The NOT operator in Java is !. It negates the boolean value.

4fill in blank

hard

Fill all three blanks to create a condition that checks if number is between 10 and 20 inclusive.

Java

boolean inRange = (number [1] 10) [2] (number [3] 20);

Drag options to blanks, or click blank then click option'

A>=

B&&

C<=

D||

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR instead of AND which allows numbers outside the range.

Using > or < instead of >= or <= which excludes boundary values.

To check if a number is between 10 and 20 inclusive, use >= and <= with AND &&. Here, BLANK_1 is >= (A), BLANK_2 is && (B), and BLANK_3 is <= (C).

5fill in blank

hard

Fill all three blanks to create a condition that checks if a character is a vowel (a, e, i, o, u).

Java

boolean isVowel = (ch == 'a') [1] (ch == 'e') [2] (ch == 'i') [3] (ch == 'o') || (ch == 'u');

Drag options to blanks, or click blank then click option'

A||

B&&

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND instead of OR which requires ch to be all vowels at once.

Using != which checks for not equal.

To check if ch is any vowel, use OR || between each equality check.

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