java - 为什么我的字符串数组有空值？ (java)

Question

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how to compare elements in a string array in java?

我试图将数组中的单词设置为字符串，然后计算不包括重复项的单词数，但是当我尝试使用 if 语句检查重复项时，它显示“NullPointerException”，这意味着存在一个数组中为空。为什么数组有空值？

下面是设置字符串数组的代码，输入为“DO UNTO OTHERS AS YOU WOULD HAVE THEM DO UNTO YOU”:

   String[] stringArray = new String[wordCount];
   while (!line.equals("DONE"))
   {
       for ( int k = 0 ; k < wordCount ; k++)
       {
           //put tokens into string array
           StringTokenizer tokens = new StringTokenizer(line);
           stringArray[k] = tokens.nextToken();
       }
   }

以下是导致 NullPointerException 的比较和 if 语句的代码:

   for ( int j = 0 ; j < wordCount ; j++)
   {    
       for (int i = j+1 ; i < wordCount ; i++)
       {       
            if (stringArray[i] == null)
            {
                stringArray[i] = "null";
            } 
            else if (stringArray[i].compareTo(stringArray[j]) == 0 && i!=j) 
            {
                //duplicate
                System.out.print("\n" + stringArray[j]);
                duplicates++;
            }
       }
   }
   wordCount -= duplicates;
   System.out.print("\nNumber of words, not including duplicates: " + wordCount);

我正在尝试进行 null 检查，但结果仍然很差，因为它会增加更多重复项，因为当我将 stringArray[i] 更改为“null”时，它也会更改 stringArray[j]

请帮忙!我长期以来一直在尝试解决这个问题

Answer 1

您应该使用equals()而不是compareTo()。如果传递 null，则 compareTo() 引发 NullPointerException。

Comparable 的 Java 文档

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

          if (stringArray[i] == null) {
                continue;
            } else if (stringArray[i].equals(stringArray[j]) && i != j) {
                // duplicate
                duplicates++;
            }

完整的源代码应该是这样的。 我没有更改 for 循环内的任何逻辑。

    String line = "DO UNTO OTHERS AS YOU WOULD HAVE THEM DO UNTO YOU";
    String[] stringArray = line.split("\\s");//Use split StringTokenizer use is discouraged in new code
    int duplicates = 0;
    int wordCount = stringArray.length;
    for (int j = 0; j < stringArray.length; j++) {
        for (int i = j + 1; i < stringArray.length; i++) {
            if (stringArray[i] == null) {
                stringArray[i] = "null";
            } else if (stringArray[i].equals(stringArray[j]) && i != j) {
                // duplicate
                System.out.print("\n" + stringArray[j]);
                duplicates++;
            }
        }
    }
    wordCount -= duplicates;
    System.out.print("\nNumber of words, not including duplicates: "
            + wordCount);