java - 线程 "main"中出现异常错误，我需要输入一条消息

Question

我没有看到问题或如何输入消息。我不断收到的完整错误如下。我看不出代码有任何问题，也找不到将消息写入代码的方法。

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of ran
ge: 30
    at java.base/java.lang.StringLatin1.charAt (String.java:47)
    at java.base/java.lang.String.charAt (String.java:693
    at Main.main (Main.java:15)

import java.util.Scanner;

public class Main
{
    public static void main( String[] args )
    {
        Scanner kb = new Scanner(System.in);

        System.out.println("What is your message? ");
        String message ="" + "\n";

        System.out.println("\nYour message is " +  " characters long.");
        System.out.println("The first character is at position 0 and is '" +  "'.");
        int lastpos = 30;
        System.out.println("The last character is at position " + lastpos + " and is '" + message.charAt(lastpos) + "'.");
        System.out.println("\nHere are all the characters, one at a time:\n");

        for ( int i=0; i<message.length(); i++ )
        {
            System.out.println("\t" + i + " - '" + message.charAt(i) + "'");
        }

        int a_count = 0;

        for ( int i=0; i<message.length(); i++ )
        {
            char letter = message.charAt(i);
            if ( letter == 'a' || letter == 'A' );
            {
                a_count++;
            }
        }

        System.out.println("\nYour message contains the letter 'a' " + a_count + " times. Isn't that interesting?");

    }
}

Answer 1

您实际上从未从扫描仪捕获消息 - 像这样:

message = kb.nextLine();

此外，如果您尝试获取消息的最后一个索引/位置，您需要执行以下操作:

int lastpos = message.length();
OR
int lastpos = message.length()-1;

两者之间的区别在于，第一个给出字符串的大小，第二个给出字符串的最后一个索引(例如，“abcdefg”，lastpos 1 将是 7，lastpos 2 将是6).

最后，请记住，您可以执行类似的操作

message.charAt(0);

获取字符串的第一个索引字符。