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Ace the AP Computer Science Exam for High Schoolers

Common Mistakes To Avoid

Understand the common mistakes and errors in order to avoid them.

We'll cover the following...

❌ Common errors

Some common errors with methods that return values are:

  • Forgetting to write a return type before the method name
  • Forgetting to use the return keyword to return a value at the end of the method
  • Forgetting to store or print the return value after returning from a method
  • Adding a return type before the constructor name (like void)

🚨 An important alert

Look at the code below.

Java
Files
public class ShoppingCart

{

    // Instance variable

    private Item product;



    // Constructor

    public ShoppingCart(Item item)

    {

        product = item;

    }



    public void print()

    {

        product.print();

    }



    // Entry point: main menthod for testing purposes

    public static void main (String args[])

    {

        Item i = new Item("Pringles", 100);

        ShoppingCart cart= new ShoppingCart(i);

        cart.print();

        System.out.println("\n");



        i.updatePrice(200); // Updating the price 

        ShoppingCart newCart= new ShoppingCart(i);

        newCart.print();

        System.out.println("\n");



        cart.print();       // Here's the problem!

    }

}

The code is a bit tricky. Don’t panic. Let’s go through it.

Open the Item.java file. We make an Item class. It has two variables: name and price. Next, we make its constructor to set the initial values. At line 12, we make a print() function that prints the name and price of an Item object.

In a supermarket, the prices of items change over time, so we make a void method, updatePrice(), at line 18 that takes an int type value ...