嘗試運行代碼時出現異常。字符串索引超出範圍

Question

每當我嘗試運行代碼時，在輸入我想要購買的票數後，它會給我一個錯誤。這是一個彩票計劃。下面是代碼

import java.util.Scanner; 
public class Lottery { 
public static void main(String[] args) { 
    //local data 
    String guess; 
    int numTickets; 
    int counter; 
    Scanner in = new Scanner(System.in); 

    //output 
    System.out.println("How many tickets would you like to buy?"); 
    numTickets = in.nextInt(); 
    for (counter = 0; counter < numTickets; counter++) { 
     System.out.println("Please enter your three numbers (e.g. 123): "); 
     guess = in.nextLine(); 
     int randNum = (int) (Math.random() * 1000); 
     String randNumb; 
     randNumb = Integer.toString(randNum); 
     char ch1 = randNumb.charAt(0); 
     char ch2 = randNumb.charAt(1); 
     char ch3 = randNumb.charAt(2); 

     if (ch1 == guess.charAt(0) && ch2 == guess.charAt(1) && ch3 == guess.charAt(2)) { 
      System.out.print("Winner: " + randNumb); 
      System.out.print("Congratulations, both pairs matched. /n"); 
     } else if (ch3 == guess.charAt(2) && ch2 == guess.charAt(1)) { 
      System.out.print("Winner: " + randNumb); 
      System.out.print("Congratulations, the end pair matched."); 
     } else if (ch1 == guess.charAt(0) && ch2 == guess.charAt(1)) { 
      System.out.print("Winner: " + randNumb); 
      System.out.print("Congratulations, the first pair matched."); 
     } else { 
      System.out.print("The Correct Number: " + randNumb); 
     } 
     System.out.println("\t Sorry, no matches, You only had"); 
     System.out.println("\t one chance out of 100 to win anyway."); 

     } 
    } 
} 

例外：

異常螺紋\ 「主」 java.lang.StringIndexOutOfBoundsException： 字符串索引超出範圍：在 java.lang.String.charAt 0（ String.java:658）at lottery.Lottery.main（Lottery.java:32） C：\ Users \ Ben \ AppData \ Local \ NetBeans \ Cache \ 8.1 \ executor-snippets \ run.xml：53： Java返回：1

這是我嘗試運行該程序時顯示的異常錯誤。 幫助將不勝感激！

Answer 1

該錯誤是由Scanner的nextInt方法的行爲引起的。另外，你的代碼也有其他錯誤。

nextInt方法只讀取下一個整數，沒有別的。這意味着不會讀取新行字符（\ n或\ r或\ r \ n，具體取決於操作系統）。在您致電nextLine之前，這很好。 nextLine會讀取每個字符，直到遇到新的行字符。所以nextLine看到這個未讀的新行字符，並停止讀取任何進一步，它返回一個空字符串，然後將其放入您的guess變量。當您嘗試使用charAt(0)訪問guess的第一個字符時，它會崩潰，因爲guess沒有任何字符。

我建議你使用Integer.parseInt(in.nextLine())從用戶讀取一個整數。

我已經固定的代碼爲您提供：

String guess; 
int numTickets; 
int counter; 
Scanner in = new Scanner(System.in); 

//output 
System.out.println("How many tickets would you like to buy?"); 
numTickets = Integer.parseInt(in.nextLine()); 
Random rand = new Random(); 
for (counter = 0; counter < numTickets; counter++) { 
    System.out.println("Please enter your three numbers (e.g. 123): "); 
    guess = in.nextLine(); 
    int randNum = rand.nextInt(1000); 
    String randNumb = String.format("%03d", randNum); 
    char ch1 = randNumb.charAt(0); 
    char ch2 = randNumb.charAt(1); 
    char ch3 = randNumb.charAt(2); 

    if (ch1 == guess.charAt(0) && ch2 == guess.charAt(1) && ch3 == guess.charAt(2)) { 
     System.out.print("Winner: " + randNumb); 
     System.out.print("Congratulations, both pairs matched. /n"); 
    } else if (ch3 == guess.charAt(2) && ch2 == guess.charAt(1)) { 
     System.out.print("Winner: " + randNumb); 
     System.out.print("Congratulations, the end pair matched."); 
    } else if (ch1 == guess.charAt(0) && ch2 == guess.charAt(1)) { 
     System.out.print("Winner: " + randNumb); 
     System.out.print("Congratulations, the first pair matched."); 
    } else { 
     System.out.print("The Correct Number: " + randNumb); 
    } 
    System.out.println("\t Sorry, no matches, You only had"); 
    System.out.println("\t one chance out of 100 to win anyway."); 

} 

正如你所看到的，我做了其他改進。首先，Math.random() * 1000可以返回非三位數字，因此我將其更改爲使用Random對象。使用rand.nextInt(900) + 100只會生成三位數字。

其次，我使用String.valueOf(randNum)作爲XtremeBaumer在評論中說過。