Return values in Java - Mini Project: Build & Apply

Return values

📖 Scenario: You are creating a simple calculator program that adds two numbers and returns the result.

🎯 Goal: Build a Java program with a method that takes two numbers, adds them, and returns the sum. Then print the returned value.

📋 What You'll Learn

Create a method called addNumbers that takes two int parameters and returns their sum as an int.

Call the addNumbers method from main with the numbers 7 and 5.

Store the returned value in a variable called result.

Print the value of result.

💡 Why This Matters

🌍 Real World

Returning values from methods is how programs get results from calculations or data processing.

💼 Career

Understanding return values is essential for writing reusable code and building functions that communicate results in software development.

1

Create the main class and method

Create a public class called Calculator with a main method inside it.

Java

public class Calculator {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Start by writing public class Calculator { and then add the main method inside.

2

Create the addNumbers method

Inside the Calculator class but outside the main method, create a public static method called addNumbers that takes two int parameters named a and b, and returns their sum as an int.

Java

public class Calculator {
    public static void main(String[] args) {
    }

    // Your code here
}

Hint

Write a method with public static int addNumbers(int a, int b) and return the sum of a and b.

3

Call addNumbers and store the result

Inside the main method, call the addNumbers method with the arguments 7 and 5. Store the returned value in an int variable called result.

Java

public class Calculator {
    public static void main(String[] args) {
        // Your code here
    }

    public static int addNumbers(int a, int b) {
        return a + b;
    }
}

Hint

Inside main, write int result = addNumbers(7, 5); to call the method and save the sum.

4

Print the result

Add a line inside the main method to print the value of the result variable using System.out.println.

Java

public class Calculator {
    public static void main(String[] args) {
        int result = addNumbers(7, 5);
        // Your code here
    }

    public static int addNumbers(int a, int b) {
        return a + b;
    }
}

Hint

Use System.out.println(result); to show the sum on the screen.

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