Question

我是一名高中生，目前正在修读计算机科学课程。在课堂上，我们被分配了创建一个程序，该程序将用户输入卡片表示法并返回卡片的完整描述。例如，输入“AS”的用户将在终端窗口中看到“黑桃王牌”。但是，当我的代码执行时，我得到一个“null of null”而不是“Ace of Spades”或另一种卡符号。

公共类卡

{

private String rank; 

private String suit;

private String fullRank;

private String fullSuit;

public Card(String rankAndSuit)
{
    rank = rankAndSuit.substring(0,1);


    if (rank == "A")
    {    
       fullRank = "Ace";
    } 
    else
    if  (rank == "2")
    {
        fullRank = "2"; 
    }
    else
    if (rank == "3")
    {    
        fullRank = "3";
    } 
    else
    if  (rank == "4")
    { 
        fullRank = "4"; 
    }
    else
    if (rank == "5")
    {    
        fullRank = "5";
    } 
    else
    if  (rank == "6")
    { 
        fullRank = "6"; 
    }
    else
    if (rank == "7")
    {    
        fullRank = "7";
    } 
    else
    if  (rank == "8")
    {
        fullRank = "8"; 
    }
    else
    if (rank == "9")
    {    
        fullRank = "9";
    } 
    else
    if  (rank == "10")
    {
        fullRank = "10"; 
    }
    else
    if (rank == "J")
    {    
        fullRank = "Jack";
    } 
    else
    if  (rank == "Q")
    { 
        fullRank = "Queen"; 
    }
    else
    if (rank == "K")
    {    
        fullRank = "King";
    } 


    suit = rankAndSuit.substring(1,2);


    if (suit == "D")
    {    
       fullSuit = "Diamonds";
    } 
    else
    if  (suit == "H")
    {
        fullSuit = "Hearts"; 
    }
    else
    if (suit == "S")
    {    
        fullSuit = "Spades";
    } 
    else
    if  (suit == "C")
    {      fullSuit = "Clubs"; 
    }
}

public String getCardDescription()
{
    return fullRank + " of " + fullSuit;
}

}

我的测试员课程是：

公共类CardTester

{

public static void main(String[] args)
{
    Card testCard = new Card("AS");
    String cardDescription = testCard.getCardDescription();

    System.out.print(cardDescription);
}

}

是什么导致我得到空？

Answer 1

您正在使用==比较字符串：

rank == "5"

别。使用：

if("5".equals(rank)){

代替。顺序（与if(rank.equals("5"))相对）确保如果您比较的字符串始终为null，则不存在空指针异常。

Answer 2

你在Java中犯了字符串比较的主要罪。

请参阅此问题：How do I compare strings in Java?

执行if语句后出现“Null”逻辑错误

2 个答案: