为我的一个实例变量设置值不起作用

Question

我试图写一个程序，本质上评估一个5卡扑克手，是用户生成的。程序的一部分是用户可以选择一个变量来随机改变。问题在于为我的一个实例变量设置值，现在，我的setCards、getCards和changeOne方法是：

public void setCards(String str) {
    this.cards = str;
    calculateScore();
    history = history + cards + score + changes;
    changes++;
}

public String getCards() {
    return this.cards;
}

public void changeOne(int pos) {
    
    getCards();
    
    calculateScore();
    history = history + cards + score + changes;
    
    randomChar = allCards.charAt((int)(Math.random()*cards.length()));
    this.cards = cards.substring(0, pos) + randomChar + cards.substring(pos + 1, cards.length());
    
    changes++;
    
} 

在另一个类中，我使用：

cards = in.nextLine().toUpperCase();
myCards.setCards(cards);

我不知道为什么，但是每当我尝试使用changeOne方法时，总是给我一个错误：线程“main”java.lang.StringIndexOutOfBoundsException中的Exception：Begin0,End3,length 0。我不知道发生了什么，为什么它没有得到适当的价值卡，帮助将非常感谢。

完整代码：第一类：

import java.util.Scanner;
public class Assignment4{

public static void main(String[] args) {
    
    Scanner in = new Scanner(System.in);
    FiveCards myCards = new FiveCards();
    
    int position;
    String choice, cards;
    char charChoice;
    
    final char NEW = 'A';
    final char CHANGE = 'B';
    final char DISPLAY = 'C';
    final char QUIT = 'Q';
    
    do {
        
        System.out.println("Choose (A: Make New Cards), (B: Change One Card), (C: Display Data), or (Q: Quit)");
        choice = in.next();
        charChoice = choice.toUpperCase().charAt(0);
        
        switch(charChoice) {
            case NEW:
                System.out.println("*** Make New FiveCards ***");
                System.out.println("Type five letters without space: ");
                in.next();
                cards = in.nextLine().toUpperCase();
                myCards.setCards(cards);
                System.out.println("[Cards] [Score] [Changes]");
                myCards.displayData();
                break;
            case CHANGE:
                System.out.println("*** Change One Card ***");
                System.out.println("Type one position to change (0-4): ");
                position = in.nextInt();
                myCards.changeOne(position);
                System.out.println("[Cards] [Score] [Changes]");
                myCards.displayData();
                break;
            case DISPLAY:
                System.out.println("[Cards] [Score] [Changes]");
                myCards.displayData();
                break;
            case QUIT:
                System.out.println("*** End of Program ***");
                break;
            default:
                System.out.println("Invalid input. Try Again");
                break;
        }
        
    }while(charChoice!=QUIT);
    
}

}

第二类是：

public class FiveCards {
private String cards, history;
private int score, changes, counter;
private String allCards = "1234567890JQK";
private char randomChar;

public FiveCards() {
}

public void setCards(String str) {
    this.cards = str;
    calculateScore();
    history = history + cards + score + changes;
    changes++;
}

public String getCards() {
    return this.cards;
}

public void changeOne(int pos) {
    

    
    calculateScore();
    history = history + cards + score + changes;
    
    randomChar = allCards.charAt((int)(Math.random()*cards.length()));
    this.cards = cards.substring(0, pos) + randomChar + cards.substring(pos + 1, cards.length());
    
    System.out.println(cards);
    
    changes++;
    
}

public void calculateScore() {
    for(int i = 0; i<cards.length(); i++) {
        for(int j = 0; j<cards.length(); j++) {
            if((cards.charAt(i) == cards.charAt(j)) && (i != j)) {
                counter++;
            }
        }
    }
    if(counter == 2) {
        score = 1;
    }
    else if(counter == 4) {
        score = 2;
    }
    else if(counter == 6) {
        score = 3;
    }
    else if(counter == 8) {
        score = 4;
    }
    else {
        score = 0;
    }
}

public String displayData() {
    
    calculateScore();
    history = history + cards + score + changes;
    
    if(cards.length()<=1) {
    return cards + "    " + score + "   " + changes;
    }
    else {
        return "Empty" + "  " + score + "   " + changes;
    }
    
}

}

Answer 1

第一个问题：

allcards.charat(...)。在setcards中，您为cards分配字符串，但在这里，您从allcards中选择字符。allcards的内容是什么？什么时候分配？我被分配了吗？（您的代码没有显示这一点）。

第二个问题：

Math.random()*cards.length()

字符串中字符的有效索引是从0到length（）-1包含的，但是通过生成随机索引，可以给出从0到length（）包含的索引。将其更改为math.random()*(cards.length()-1)。