(Java) 如何覆蓋我已經聲明 Null 值的二維數組

Question

您好我正在使用二維數組來模擬自動售貨機。

我想聲明一個具有設定長度的數組並使其超大，以便它可以容納我向其中添加項目。

前任：

String[][] itemsT1 = new String[][] {null, null, null, null, null, null, null, null, null};
int itemCount = 0;

// The way I've been adding things in my "addItem' method is:

if ( (items.length > 0) && (itemsCount < items.length) ) {
            items[itemsCount][0] = nameOfItem;
            items[itemsCount][1] = expirationDate;
            itemsCount++;
            System.out.println("Item added");
        }

// so that the end result of the array should be (with nameOfItem = "Water" and expDate = "15") something like this:

{ {"Water", "15"}, null, null, null, null, null, null, null, null}

// and it's scalable to be hypothetically used again:

{ {"Water", "15"}, {"Chocolate", "20"}, {"Juice", "25"}, null, null, null, null, null, null}

我可能會帶着更多問題回來，所以謝謝你的回答，如果我需要提供更多，請告訴我！

Answer 1

聲明您的 2D 字符串數組並初始化您要在自動售貨機中處理的商品數量，假設10行（商品），每行有 3 個數據列：

String[][] items = new String[10][3];

使用掃描儀 class 打開 stream 以接受鍵盤輸入：

Scanner userInput = new Scanner(System.in);

現在創建一個for循環來遍歷數組中的每個項目行，以便用戶可以填充每個項目行。 以下是最簡單的形式，沒有驗證。 如果您在詢問項目名稱時只是按Enter鍵，則輸入過程將結束：

for (int i = 0; i < items.length; i++) {
    System.out.print("Enter a name for item #" + (i + 1) + ": -> ");
    String itemName = userInput.nextLine().trim();
    if (itemName.isEmpty()) { 
        break; 
    }
    System.out.print("Enter expiry for item #" + (i + 1) + ": -> ");
    String itemExpiry = userInput.nextLine().trim();
    System.out.print("Enter price for item  #" + (i + 1) + ": -> ");
    String itemPrice = userInput.nextLine().trim();
    items[i][0] = itemName;
    items[i][1] = itemExpiry;
    items[i][2] = itemPrice;
    System.out.println();
}

現在，將數組中包含的所有輸入項顯示到控制台 Window 中：

for (String[] diffItems : items) {
    if (diffItems[0] != null) {
        System.out.println(Arrays.toString(diffItems));
    }
}

Answer 2

如果您更喜歡使用 2D arrays，首先您需要在自動售貨機中找到第一個“空”空間 -> null 元素。 （我會把它保存在一個方法中，這樣它就更清楚更容易使用了）

public int getFreeSpaceIndex(final String[][] vendingMachine) {
    for (int index = 0; index < vendingMachine.length; index++)
        if (vendingMachine[index] == null)
            return index;
    // meaning there's no empty space, you should use some static field
    // to describe this situation to avoid magic numbers in your code
    return -1;
}

找到元素后，您可以在其中插入所需的任何內容

// you've got the itemT1 array that you've created
// and let's say a user inputs the water item down below,
// I simplified it to declaration, cause it's off the topic
final String[] water = new String[] {"Water", "15"};
final int freeSpaceIndex = findFreeSpaceIndex(itemT1);
if (freeSpaceIndex == -1) {
    System.err.printf("There's not enough space for the %s item.", Arrays.toString(water));
} else {
    itemT1[freeSpaceIndex] = item;
}

最終，您可以將數組包裝到您自己的數據結構中，並簡單地創建一個addItem(String[])方法，該方法將搜索空白空間並處理溢出。

class VendingMachineStock {
    private String[][] stock = new String[][] {null, null, null, null, null, null, null, null, null};
    // constructors, api...

    public boolean hasFreeSpace() {
        return getFreeSpaceIndex() != -1;
    }

    public boolean addItem(final String[] item) {
        final int freeSpaceIndex = getFreeSpaceIndex();
        if (freeSpaceIndex == -1) {
            System.err.printf("There's not enough space for the %s item.", Arrays.toString(water));
            return false;
        }
        stock[freeSpaceIndex] = item;
        return true;
    }

    private int getFreeSpaceIndex() {
        for (int index = 0; index < stock.length; index++)
            if (stock[index] == null)
                return index;
        return -1;
    }
}

但是這樣的代碼有很多缺陷。 任何人都可以搞砸並添加String[] {"15", "Water"}甚至new String[] {"15", "30", "I'm Foo Bar:)"} 。

解決您的問題的正確數據結構實際上是帶有Item對象的鏈表（如@DevilsHnd 所述）和您自己的限制結構。

class Item {
    private final String name;
    private int price;
    // default getters, setters, consturctor...
}

class VendingMachineStock {
    private final LinkedList<Item> stock;
    // this will limit your stock list size
    private int maxSlots; 

    // I omit API and constructor intentionally, it's off the
    // topic, but you must write it

    public void addItem(final Item item) {
        // you can either throw exceptions or return boolean
        // flag indicating the result, it's up to you.
        // in this case I prefer a detailed explanation on why
        // the method did not add the item.
        if (stock.size() >= maxSlots) {
            throw new IllegalStateException("Stock overflow, can't add more items");
        }
        stock.add(item);
    }
}