字符串在追加时返回null

Question

我的任务是创造一个猜字游戏。你只有5次试着猜这个词。用户一次输入一个字母，试图找出我的秘密单词“果汁”，但你应该使用一个提示词结构，如果正确的话，每个猜测用正确的字母替换星号。

--这是输出应该是什么样子：

欢迎来到猜字游戏！你有5次试着猜这个秘密词！当前的提示是

猜小写字母

使用

*u*

猜这个秘密词

小管

继续努力！

猜小写字母

T

*u*

猜这个秘密词

突变体

继续努力！

..。继续到5次，然后你要么赢要么输。

，我的输出不允许您一直猜测，提示词显然是混乱的，因为它有5个星号的空前。我不知道怎么修好它。

当前的提示是 空* 猜小写字母 J 杰尔* 猜这个秘密词 朱特 继续努力！ 游戏结束!再试试?

，这是我的课：

public class SecretWord {
private String secretWord;
private String hintWord;
private int numberOfTurns;

//Default Constructors
public SecretWord()
{
    this.secretWord = "juice";
    for (int i = 0; i < secretWord.length(); i++)
    {
        this.hintWord+="*";
    }
    this.numberOfTurns = 0;
}
//Accessors
public String getSecretWord()
{
    return this.secretWord;
}
public String getHintWord()
{
    return this.hintWord;
}
public int getNumberOfTurns()
{
    return this.numberOfTurns;
}
//Mutators
public void setSecretWord ()
{
    this.secretWord = "juice";
}
public void setHintWord ()
{
    char[] correctLetters = new char[secretWord.length()];
    for (int i = 0; i<secretWord.length();i++)
    {
        hintWord+="*";
        correctLetters[i] += '*';
    }
}
public void setNumberOfTurns (int i)
{
    this.numberOfTurns = 5;
}
//Methods
public void guessLetter(char guess)
{
    String tempHintWord="";
    for (int i = 0; i < secretWord.length(); i++)
    {
        if (secretWord.charAt(i) == guess)
        {
            tempHintWord += guess;
        }
        else
        {
            tempHintWord += hintWord.charAt(i);
        }
    }
    hintWord = tempHintWord;
}

，这是我的驱动程序类：

public class SecretWordGame {

//Constant for number of tries
public static final int NUM_TRIES = 5;

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

    //Starts game
    boolean quit = false;
    while (quit == false)

    {
        System.out.println("Welcome to the word guessing game! You have " +
                +NUM_TRIES+" tries to guess the secret word!");

                SecretWord myWord = new SecretWord();
        System.out.println("The current hint is \n"+myWord.getHintWord());

        while (myWord.getNumberOfTurns() <NUM_TRIES)
        {
            System.out.println("Guess a lowercase letter");

            //Gets the first letter of what is entered

            char guess = keyboard.nextLine().charAt(0);

            //Updates the hint by calling guess letter method

            myWord.guessLetter(guess);

            System.out.println(myWord.getHintWord());

            System.out.println("Guess the secret word");

            String myGuess = keyboard.nextLine();
            //Checks correct guess
            if (myGuess.equals(myWord.getSecretWord()))
            {
                System.out.println("You win!");
                break;
            }
            else
            {
                System.out.println("Keep trying!");
            }

            myWord.setNumberOfTurns(myWord.getNumberOfTurns()+1);
        }
        //Prompts user to play again

        System.out.println("Game over! Try again?");

        String userInput = keyboard.nextLine();
        if(userInput.equalsIgnoreCase("no"))
        {
            quit = true;
        }
        else
        {
            System.out.println("Let's go again!");
        }
    }
    System.out.println("Goodbye!");
}

Answer 1

默认情况下，SecretWord的hintword被初始化为null。因此，当您附加+= "*"时，它会将当前值转换为"null"，然后添加"*"。如果初始化向""声明的隐藏词，它将修复该问题。

Answer 2

在SecretWord类中，不初始化hintWord。

因此，在执行hintWord += "*";时，hintWord是null，然后隐式地转换为字符串"null"与"*".*连接。