java - Java toString 中的 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

我正在尝试实现一个简单的测验游戏，并且在我的 Question 中重写 toString 方法时类我得到一个 NullReferenceException。问题从何而来？这是我在问题类中的代码

    public class Question {
        private String text;
        private Answer[] answers;
        private final static int ANSWER_SIZE_ARRAY = 4;

        public Question(String text, Answer[] answers) {
          this.text = text;
          answers = new Answer[ANSWER_SIZE_ARRAY];
          this.answers = answers;
        }

        public void setText(String text) {
        this.text = text;
     }

     public void setAnswers(Answer[] answers) {
          this.answers = answers;
     }

     public String getText() {
         return this.text;
     }

     public Answer[] getAnswers() {
         return this.answers;
     }

    //test toString
    public String toString() {
         String result = this.getText() + "\n";
         for (Answer a : this.answers) {
             result += a.getText() + "\n"; // HERE COMES THE PROBLEM
         }
         return result;
     }

 }

我的主要方法是这样的:

    public class MainGameTestMethod {
      public static void main(String[] args) {
        Answer a1 = new Answer("Krisko", true);
        Answer a2 = new Answer("Beatz", false);
        Answer a3 = new Answer("Ivan", false);
        Answer a4 = new Answer("Pesho", false);
        Question q1 = new Question("Whats your name?", new Answer[] { a1, a2, a3, a4 });
        System.out.println(q1.toString());
    }
}

Answer 1

你的构造函数没有任何意义。

    public Question(String text, Answer[] answers) {
      this.text = text;
      answers = new Answer[ANSWER_SIZE_ARRAY];
      this.answers = answers;
    }

您正在传递一个数组，但您没有使用它。您只需创建一个新数组。我认为你想将它用作成员变量。因此，您必须删除构造函数中的第二行:

    public Question(String text, Answer[] answers) {
      this.text = text;
      this.answers = answers;
    }

您的数组答案始终只有空值，因此您会收到错误。

根据注释进行编辑，数组的大小应为 4:

    public Question(String text, Answer[] answers) {
      this.text = text;
      if(answers == null || answer.length != 4){
          //do some exception handling e.g. throw error
         //or do
          //if(answer != null){
// take care that this method could also lead to null values in your new array if the old arrays length is < 4
       //  this.answers = Arrays.copyOf(answers, 4); 
       // }

      }
      else{
          this.answers = answers;
      }
    }