java - Java中如何解决“线程“main”中的异常java.lang.StringIndexOutOfBoundsException:字符串索引超出范围:5”的问题

Question

我正在尝试创建一个代码，如果输入以“ Today”开头并以“ MLIA”结尾（大小写无关），则打印“ VALID ENTRY”。如果不是，则打印“格式不正确，请尝试其他提交”。由于某种原因，该程序不断给我一个越界错误，我不知道为什么。

当我将子字符串代码更改为sub.substring(0,1)进行测试时，仍然给我一个错误，所以这不是问题。我还尝试为每个字母添加char值，以确定单词是什么，但这也不起作用。

public class submit{
   public static void main(String[] args) throws IOException{

   Scanner scanner = new Scanner(new File("submit.txt"));

   int trials = scanner.nextInt();
   int total = 0;

   for(int x = 0; x <= trials; x++){
       String sub = scanner.nextLine();
       sub = sub.toLowerCase();
       if(sub.substring(0,5) == "today") //I only have it set up to find "today"
         System.out.println("VALID ENTRY");
       else
         System.out.println("INCORRECT FORMATTING, TRY ANOTHER SUBMISSION");

      }  
   }//end of main
}//end of class

输入：

5  
ToDAY, I went to school. mlia  
Hehehe today mlia this shouldn't work  
Today, I went to a programming contest. Hehe. MLIA  
TODAYMLIA  
T0day is a brand new day! MLIA  

预期输出应为：

VALID ENTRY  
INCORRECT FORMATTING, TRY ANOTHER SUBMISSION  
VALID ENTRY  
VALID ENTRY  
INCORRECT FORMATTING, TRY ANOTHER SUBMISSION  

实际输出：

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 5  
    at java.lang.String.substring(String.java:1963)   
    at submit.main(submit.java:15)

Answer 1

代码有两个问题。首先，您should compare Strings using equals, not ==。否则，即使对于相同的String，它也可能返回false。

其次，nextLine将从Scanner读取直到下一个换行符之后。但是nextInt只会读到该换行符之前，因此您第一次调用nextLine只是将Scanner前进一个字符并返回一个空的String。您需要在nextLine之后调用一个额外的nextInt，以前进到整数之后的下一行。有关更多信息，请参见this question。

因此，您应该对代码进行以下更改：

Scanner scanner = new Scanner(new File("submit.txt"));

int trials = scanner.nextInt();
scanner.nextLine(); //Add this line, to consume the newline character after "5"
// alternatively, you could replace both of the above lines with this:
// int trials = Integer.parseInt(scanner.nextLine());
int total = 0;

for(int x = 0; x <= trials; x++){
   String sub = scanner.nextLine();
   sub = sub.toLowerCase();
   if(sub.substring(0,5).equals("today")) //compare strings using equals, not ==
     System.out.println("VALID ENTRY");
   else
     System.out.println("INCORRECT FORMATTING, TRY ANOTHER SUBMISSION");

}