在 Java 中编写许多 if 语句的更好方法？

Question

So I have this homework where I need to build a vending machine, and I assigned a coordinate to every product (A1, A2, A3..) and when the user enters the coin value I have to calculate if he can buy the product that he chose and if yes calculate the change, since i'm still new to programming I now have ended up with many statements like this

if ("a1".equals(choice)) {
    System.out.println("You chose SNICKERS!");
    if (money < 50) {
        System.out.println("This is not enough money to buy this product");
    } else if (money >= 50) {
        System.out.println(" Price = 50  Your change = " + (money - 50));
    }
}

where the only things changing are the coordinates (a1, a2, a3, a4, b1, b2 and so on) and the prices. What would be a better way to do this?

Answer 1

You could use a more OO approach.

Make a class called Product :

class Product {
    private String name;
    private int price;

    public String getName() { return name; }
    public int getPrice() { return price; }

    public Product(String name, int price) {
        this.name = name;
        this.price = price;
    }
}

Then, create a HashMap<String, Product> and add all your products and their coordinates in:

HashMap<String, Product> productMap = new HashMap<>();
productMap.put("A1", new Product("Snickers", 50));
productMap.put("A2", new Product("Something else", 40));
// do this for every coordinate...

Now, you can just use this code:

Product chosenProduct = productMap.get(choice);
System.out.println("You chose " + chosenProduct.getName() + "!");
if (money < chosenProduct.getPrice()) {
    System.out.println("This is not enough money to buy this product");
} else {
    System.out.println(" Price = " + chosenProduct.getPrice() + "  Your change = " + (money - chosenProduct.getPrice()));
}

Answer 2

This is a common moment in programming and you're right to think there's a better way to go about it.

In your particular case, where you have a similar structure of code in many places, you should try using a function. Try writing a private function in the same class that takes in some of the parameters that change. Depending on the level of complexity you end up with you might end up with a large function with a lot of parameters which isn't great either. Regardless, writing a function is probably the first place you should go when you encounter this situation.

Secondarily, consider what this segment of code is doing. Making a generic function to replace the whole code segment might not be ideal but could you easily write a function to do a small part of it? If so, is it now easy to make the rest into another function? Keep track of any variances you have across your many similar code segments, if any, and try to create functions that address those variations.

Finally, depending on what is in scope for your current level of programming, you might be able to create data structure to help with the identification of the choice. For example, maybe you could make a map where you could store each of the choices and an associated object that contains all of the data you need to respond to the user (ie cost, item name, etc.). With that kind of approach you can pre-populate all of those options in a straightforward manner and have your code simply look up the set of data associated with the choice and call of function to print the necessary output.

Ultimately, how you go about this and future situations like it is highly dependent upon what your level of experience with programming, how many choices there are, and how complex the data associated with each choice is.

Continue to trust your instincts though, they will serve you well.

Answer 3

Switch case is the desired functionality.

 
 
 
  
  switch(choice) { case 'a' : <some code> break; case 'b': <some other code> break; default: <if none of the choices matched> }
 
 

The only caveat is that your choice value has to be a char or an int

---

EDIT :

• This was a bad answer from my side. Clearly, you need a more cleaner way to write your vending machine.
• A good way to structure it would be to first create a Product class which can store the name and the price of the product.

public static Main(String[] args) {
    // ...
    private Product processUserInput(String productName) {
        // Add logic to process the string and return a Product
    }
    // ...
}

• You could then create a function (which could be used in your main() ) which takes the text input from your user and map it to an actual product.

public static Main(String[] args) {
    // ...
    private int processUserProductChoice(Product product, int amountAvailable) {
        // Add logic to process the product and amount
        // Probably use switch case now.
    }
    // ...
}

• Then you could add a function that would process the Product and the amount the user has and return the change remaining or maybe -1 if the product costs more than the available amount.

 public static Main(String[] args) { // ... private int processUserProductChoice(Product product, int amountAvailable) { // Add logic to process the product and amount // Probably use switch case now. } // ... }