Unary operators in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pre-increment and post-increment

What is the output of the following Java code?

Java

int x = 5;
System.out.println(x++);
System.out.println(++x);

A 5\n7

B 6\n7

C 5\n6

D 6\n6

💡 Hint

Remember that x++ returns the value before incrementing, while ++x increments first then returns.

The first print outputs 5 because x++ returns the current value then increments. After that, x becomes 6. The second print uses ++x, which increments x to 7 first, then prints 7.

❓ Predict Output

intermediate

Output of unary minus and plus operators

What is the output of this Java code snippet?

Java

int a = 10;
int b = -a;
int c = +b;
System.out.println(b);
System.out.println(c);

A 10\n10

B -10\n-10

C -10\n10

D 10\n-10

💡 Hint

Unary minus changes the sign, unary plus returns the value as is.

Variable b is assigned -a, so b is -10. Variable c is +b, which keeps the value -10. So both print -10.

❓ Predict Output

advanced

Output with mixed unary operators and expressions

What will be printed by this Java code?

Java

int x = 3;
int y = x++ + ++x + --x + x--;
System.out.println(y);

A 13

B 15

C 16

D 12

💡 Hint

Evaluate each part step-by-step, tracking x's value changes carefully.

Step by step: Initial x=3 - x++ returns 3, x becomes 4 - ++x increments to 5, returns 5 - --x decrements to 4, returns 4 - x-- returns 4, x becomes 3 Sum: 3 + 5 + 4 + 4 = 16

❓ Predict Output

advanced

Output of unary logical complement operator

What is the output of this Java code?

Java

boolean flag = false;
System.out.println(!flag);
System.out.println(!!flag);

A false\ntrue

B true\ntrue

C false\nfalse

D true\nfalse

💡 Hint

The ! operator flips the boolean value.

First print: !false is true. Second print: !!false is !(!false) which is !true = false.

🧠 Conceptual

expert

Effect of unary operators on variable values

Given the following Java code, what is the final value of variable 'a' after execution?

Java

int a = 1;
int b = a++ + ++a + a-- + --a;

A 1

B 4

C 3

D 2

💡 Hint

Track the value of 'a' carefully after each unary operation.

Initial a=1 Evaluate b: a++ returns 1, a=2 ++a increments a to 3, returns 3 a-- returns 3, a=2 --a decrements a to 1, returns 1 Sum for b: 1+3+3+1=8 (not asked) Final a after all operations is 1.

Practice

1.

What does the unary operator ++ do in Java?

easy

A. Negates the value of a variable

B. Increases the value of a variable by 1

C. Decreases the value of a variable by 1

D. Checks if a value is true or false

Solution

1. Step 1: Understand the meaning of ++

   The ++ operator adds 1 to the current value of a variable.

2. Step 2: Compare with other unary operators

   -- subtracts 1, ! flips boolean, and negation changes sign.

3. Final Answer:

   Increases the value of a variable by 1 -> Option B

4. Quick Check:

   ++ means add one [OK]

Hint: ++ means add one to a number quickly [OK]

Common Mistakes:

• Confusing ++ with --
• Thinking ++ negates the value
• Mixing ++ with boolean negation !

2.

Which of the following is the correct **postfix** syntax to decrement a variable x by 1 using a unary operator?

int x = 5;
// Your code here

easy

A. x-1;

B. --x;

C. x--;

D. x=!1;

Solution

1. Step 1: Identify decrement unary operators

   Both x-- and --x decrement by 1, but x-1 is an expression, not an operator.

2. Step 2: Choose correct syntax for decrement

   x--; correctly decreases x by 1 after its current use.

3. Final Answer:

   x--; -> Option C

4. Quick Check:

   Use x-- to subtract one [OK]

Hint: Use x-- or --x to subtract one [OK]

Common Mistakes:

• Using x-1; which doesn't change x
• Confusing ! operator with decrement
• Writing invalid syntax like x=!1;

3.

What is the output of the following Java code?

int a = 3;
System.out.println(++a);
System.out.println(a++);
System.out.println(a);

medium

A. 4 4 5

B. 3 4 5

C. 4 3 4

D. 3 3 4

Solution

1. Step 1: Evaluate ++a

   ++a increases a before printing, so prints 4 and a becomes 4.

2. Step 2: Evaluate a++

   a++ prints current a (4) then increases it to 5.

3. Step 3: Print final value of a

   After previous steps, a is 5, so prints 5.

4. Final Answer:

   4 4 5 -> Option A

5. Quick Check:

   Prefix ++ adds before print, postfix ++ adds after print [OK]

Hint: Prefix ++ changes before print, postfix ++ after print [OK]

Common Mistakes:

• Mixing prefix and postfix effects
• Forgetting that ++ changes the variable
• Assuming no change in variable after print

4.

Find the error in the following code snippet:

boolean flag = true;
flag = !flag++;
System.out.println(flag);

medium

A. No error, code runs fine

B. Missing semicolon after flag declaration

C. Logical NOT operator ! cannot be used here

D. Cannot apply ++ to a boolean variable

Solution

1. Step 1: Check variable type and operators

   flag is boolean; ++ operator works only on numeric types, not boolean.

2. Step 2: Identify invalid operation

   Using flag++ causes a compile-time error because booleans cannot be incremented.

3. Final Answer:

   Cannot apply ++ to a boolean variable -> Option D

4. Quick Check:

   ++ works only on numbers, not booleans [OK]

Hint: ++ cannot be used on boolean variables [OK]

Common Mistakes:

• Trying to increment boolean values
• Confusing logical NOT ! with increment ++
• Assuming code compiles without error

5.

Given the code below, what will be the final value of count after execution?

int count = 0;
for (int i = 0; i < 3; i++) {
    count = count++ + ++count;
}
System.out.println(count);

hard

A. 14

B. 6

C. 12

D. 9

Solution

1. Step 1: Understand count = count++ + ++count; behavior

   Binary + evaluates left operand first (count++: old value then increment), then right (++count: increment then new value).

2. Step 2: Trace loop iterations

   Iteration 1 (count=0): count++ value=0 (count=1), ++count value=2, sum=2 -> count=2.
   Iteration 2 (count=2): count++=2 (count=3), ++count=4, sum=6 -> count=6.
   Iteration 3 (count=6): count++=6 (count=7), ++count=8, sum=14 -> count=14.

3. Step 3: Final value after loop

   After 3 iterations, count is 14.

4. Final Answer:

   14 -> Option A

5. Quick Check:

   Left-to-right: postfix old + prefix new [OK]

Hint: Left operand postfix (old then inc), right prefix (inc then new) [OK]

Common Mistakes:

• Assuming both increments add fully
• Ignoring postfix increment side effects
• Calculating sum without assignment effect