Return values in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java method call?

Consider the following Java method:

public static int multiplyByTwo(int x) {
    return x * 2;
}

public static void main(String[] args) {
    System.out.println(multiplyByTwo(5));
}

What will be printed when main runs?

Java

public static int multiplyByTwo(int x) {
    return x * 2;
}

public static void main(String[] args) {
    System.out.println(multiplyByTwo(5));
}

A 25

B 5

C Compilation error

D 10

💡 Hint

Think about what the method returns when given 5.

The method multiplies the input by 2 and returns it. So 5 * 2 = 10.

❓ Predict Output

intermediate

What does this method return?

Look at this Java method:

public static String greet(String name) {
    if (name == null) {
        return "Hello, Guest!";
    }
    return "Hello, " + name + "!";
}

public static void main(String[] args) {
    System.out.println(greet(null));
}

What will be printed when main runs?

Java

public static String greet(String name) {
    if (name == null) {
        return "Hello, Guest!";
    }
    return "Hello, " + name + "!";
}

public static void main(String[] args) {
    System.out.println(greet(null));
}

A Hello, null!

B Hello, Guest!

C Hello, !

D NullPointerException

💡 Hint

Check what happens when the input is null.

If the input is null, the method returns "Hello, Guest!" directly.

❓ Predict Output

advanced

What is the output of this recursive method?

Consider this Java method:

public static int factorial(int n) {
    if (n <= 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

public static void main(String[] args) {
    System.out.println(factorial(4));
}

What will be printed when main runs?

Java

public static int factorial(int n) {
    if (n <= 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

public static void main(String[] args) {
    System.out.println(factorial(4));
}

A 10

B 1

C 24

D StackOverflowError

💡 Hint

Recall how factorial is calculated recursively.

factorial(4) = 4 * factorial(3) = 4 * 3 * factorial(2) = 4 * 3 * 2 * factorial(1) = 4 * 3 * 2 * 1 = 24.

❓ Predict Output

advanced

What is the value of x after this method call?

Given this Java code:

public static int addFive(int num) {
    num = num + 5;
    return num;
}

public static void main(String[] args) {
    int x = 10;
    addFive(x);
    System.out.println(x);
}

What will be printed when main runs?

Java

public static int addFive(int num) {
    num = num + 5;
    return num;
}

public static void main(String[] args) {
    int x = 10;
    addFive(x);
    System.out.println(x);
}

A 10

B 15

C 5

D Compilation error

💡 Hint

Remember Java passes primitives by value.

The method changes only its local copy of num. The original x remains 10.

🧠 Conceptual

expert

Which option causes a compilation error due to return type mismatch?

Which of the following Java methods will cause a compilation error because the return type does not match the returned value?

A public static int getNumber() { return 3.14; }

B public static double getDouble() { return 3.14; }

C public static String getText() { return "Hello"; }

D public static boolean isTrue() { return true; }

💡 Hint

Check if the returned value type matches the declared return type exactly.

Method getNumber declares return type int but returns a double literal 3.14, causing a compilation error.

Practice

1. What does the return keyword do in a Java method?

easy

A. It stops the program completely.

B. It sends a value back from the method to where it was called.

C. It prints a value to the screen.

D. It declares a new variable inside the method.

Solution

1. Step 1: Understand the purpose of return

   The return keyword is used inside a method to send a value back to the caller.

2. Step 2: Differentiate from other actions

   Stopping the program, printing, or declaring variables are different actions and not what return does.

3. Final Answer:

   It sends a value back from the method to where it was called. -> Option B

4. Quick Check:

   Return sends value back [OK]

Hint: Return sends data back from method to caller [OK]

Common Mistakes:

• Thinking return stops the whole program
• Confusing return with print statements
• Assuming return declares variables

2. Which of the following method signatures correctly declares a method that returns an integer?

easy

A. public String getNumber() { return 5; }

B. public void getNumber() { return 5; }

C. public int getNumber() { return 5; }

D. public int getNumber() { System.out.println(5); }

Solution

1. Step 1: Check method return type and return statement

   The method must declare int as return type to return an integer value.

2. Step 2: Match return type with returned value

   public int getNumber() { return 5; } declares int and returns 5, which is an integer, so it is correct.

3. Final Answer:

   public int getNumber() { return 5; } -> Option C

4. Quick Check:

   Return type matches returned value [OK]

Hint: Return type must match returned value type [OK]

Common Mistakes:

• Using void but returning a value
• Returning wrong type value
• Missing return statement in non-void method

3. What is the output of the following code?

public class Test {
  public static int add(int a, int b) {
    return a + b;
  }
  public static void main(String[] args) {
    System.out.println(add(3, 4));
  }
}

medium

A. 7

B. 34

C. Error

D. 0

Solution

1. Step 1: Understand the add method

   The method adds two integers and returns their sum.

2. Step 2: Calculate add(3, 4)

   3 + 4 equals 7, so the method returns 7.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Add numbers, return sum, print result [OK]

Common Mistakes:

• Concatenating numbers as strings
• Expecting error due to return
• Confusing return with print

4. Identify the error in this method:

public int multiply(int x, int y) {
  int result = x * y;
}

medium

A. Method name is invalid.

B. Method return type should be void.

C. Variables x and y are not declared.

D. Method is missing a return statement.

Solution

1. Step 1: Check method return type

   The method declares it returns an int, so it must return an int value.

2. Step 2: Verify return statement presence

   The method calculates result but does not return it, so it misses a return statement.

3. Final Answer:

   Method is missing a return statement. -> Option D

4. Quick Check:

   Non-void methods must return a value [OK]

Hint: Non-void methods must have return statement [OK]

Common Mistakes:

• Forgetting to return value in non-void method
• Changing return type to void without reason
• Assuming variable declaration is error

5. Given this method:

public static String checkNumber(int n) {
  if (n > 0) return "Positive";
  else if (n < 0) return "Negative";
  else return "Zero";
}

What will checkNumber(0) return?

hard

A. "Zero"

B. "Negative"

C. "Positive"

D. null

Solution

1. Step 1: Analyze the if-else conditions

   If n > 0 returns "Positive", if n < 0 returns "Negative".

2. Step 2: Check the else case for n = 0

   Since 0 is neither greater nor less than 0, else returns "Zero".

3. Final Answer:

   "Zero" -> Option A

4. Quick Check:

   0 triggers else return "Zero" [OK]

Hint: Check all conditions including else for exact return [OK]

Common Mistakes:

• Assuming 0 is positive or negative
• Missing else return case
• Expecting null when no return