Using Return - One

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Return Values vs. Console Output in Java

Learning to distinguish between methods that return values and methods that simply output text is a fundamental concept in programming. Methods that return values from some set of arguments are easier to test, cleaner to write, and much more flexible when you need to re-use some logic. Let's explore this distinction with some explanations and exercises.

Understanding the Difference

When you're new to programming, it's easy to confuse these two concepts:

Returning a value

A method computes something and gives that value back to wherever it was called from, allowing further use of that value.

Printing to console

A method displays text in the console using System.out.println(), but doesn't make the value available for further computation.

Think of it this way: A method that returns a value is like a calculator that shows you the answer on its screen AND lets you use that answer in another calculation. A method that just prints is like a calculator that shows the answer but doesn't let you use it for anything else.

Key Differences

// Method that RETURNS a value
public static int add(int a, int b) {
    return a + b;  // Returns the sum to the caller
}

// Method that PRINTS a value
public static void printAdd(int a, int b) {
    System.out.println(a + b);  // Just displays the sum, returns nothing
}

With the first method, you can:

int sum = add(5, 3);  // sum now equals 8
int doubled = sum * 2;  // You can use the result in further calculations

//or even...
System.out.println(add(5, 3));

With the second method:

printAdd(5, 3);  // Displays "8" to console
// But there's no way to capture that 8 for further use

And in spite of these admonitions

Some of you will try to figure out a way to unit test a method that returns void but prints something to System.out.

BEFORE you do such a thing, you should read thru a terrible example.

Practical Exercises

Let's work through some exercises to reinforce this concept:

Student Exercises

Exercise 1: Identify the Difference

Review the square() and printSquare() methods in the code. Explain why you can use the result from square() in further calculations but not the result from printSquare().

Exercise 2: Fix the Code

Fix this broken code that tries to use a printed value:

public static void main(String[] args) {
    int base = 10;
    int exponent = 2;
    int result = printPower(base, exponent); // Error!
    int doubled = result * 2;
    System.out.println(doubled);
}

public static void printPower(int base, int exponent) {
    int result = (int)Math.pow(base, exponent);
    System.out.println(result);
}

Exercise 3: Convert Methods

Take the printPriceWithTax() method and convert it to a method that returns the total price instead of printing it.

Exercise 4: Temperature Converter

Write two versions of a temperature converter:

1. A method that takes Celsius and returns Fahrenheit
2. A method that takes Celsius and prints the Fahrenheit value

Then demonstrate how the first method is more flexible by using its return value in another calculation.

Exercise 5: Build a Calculator

Create a simple calculator program with separate methods for each operation (add, subtract, multiply, divide) that return values. Then create a main method that demonstrates chaining these operations together.

Common Mistakes and Tips

• Mistake: Printing a value and thinking it's the same as returning it.

  public static int badMethod(int a, int b) {
      System.out.println(a + b);
      // Missing return statement!
  }

• Mistake: Returning a value but forgetting to capture or use it.

square(5); // Calculates 25 but does nothing with it

• Tip: Methods that perform calculations should typically return their results rather than printing them, which makes them more reusable.
• Tip: Use meaningful names that indicate what the method returns:

  // Good: Name indicates it returns a value
  public static double calculateAreaOfCircle(double radius) {
      return Math.PI * radius * radius;
  }
  
  // Good: Name indicates it prints something
  public static void displayAreaOfCircle(double radius) {
      System.out.println("Area: " + (Math.PI * radius * radius));
  }

These exercises should help you understand the crucial difference between returning values and merely printing them, setting the foundation for more advanced programming concepts.