屬性null當它看起來不應該

Question

我有一個類TrueFalse，它擴展了一個類問題。

問題有一個屬性答案這是類型回答的ArrayList

protected ArrayList<Answer> answers = new ArrayList<Answer>(); 

TrueFalse有一個稱爲取（詢問Q），其試圖將回答添加方法答案 ArrayList。

這是我的問題 - 答案 ArrayList拋出一個NullPointerException並顯示爲null。

public void take(Question question) { 
    input = new Input(); 
    answer = new Answer(); 

    System.out.print("Enter Option: "); 
    int response = input.getIntInput(question.options.size()); 
    answer.setAnswerNumber(response); 

    System.out.println(question.answers); // PRINTS NULL 
    question.answers.add(answer); 
} 

有沒有人有一個想法，爲什麼？謝謝您的幫助！

編輯：

我相信這是堆棧跟蹤？我是相當新的，所以我認爲這就是我們正在尋找：

null 
Exception in thread "main" java.lang.NullPointerException 
at Question.TrueFalse.take(TrueFalse.java:53) 
at Survey.Survey.take(Survey.java:239) 
at Survey.Main.mainMenuSelection(Main.java:74) 
at Survey.Survey.mainMenu(Survey.java:320) 
at Survey.Main.mainMenuSelection(Main.java:41) 
at Survey.Survey.mainMenu(Survey.java:320) 
at Survey.Main.main(Main.java:196) 

EDIT2：

這就是take方法被調用，從我調查類。

public void take(Survey survey) { 

    if (survey.name == null && survey.questions.isEmpty()) { 
     System.out.println("MESSAGE: Please create/load first."); 
     mainMenu(); 
    } 

    System.out.print("Name: "); 
    System.out.println(survey.name); 

    for (int i = 0; i < survey.questions.size(); i++) { 
     System.out.print("\n" + (i+1) + ". " + survey.questions.get(i).questionType + ") "); 
     System.out.println(survey.questions.get(i).prompt); 

     for(int j = 0; j < survey.questions.get(i).options.size(); j++) { 
      System.out.print(j+1 + ". "); 
      System.out.println(survey.questions.get(i).options.get(j)); 
     } 

     survey.questions.get(i).take(survey.questions.get(i)); // CALLED HERE 

    } 
    System.out.println("Survey complete!"); 
} 

EDIT3：

這裏是我的問題類

public abstract class Question implements Serializable { 

protected static final long serialVersionUID = 1L; 

protected Input input; 
protected Answer answer; 
protected int optionCount = 0; 

public String prompt; 
public String questionType; 
public ArrayList<String> options; 
public ArrayList<String> correctOptions; 
public ArrayList<String> matchingOptions; 
public ArrayList<Answer> answers; 

public Question() { 
    options = new ArrayList<String>(); 
    correctOptions = new ArrayList<String>(); 
    matchingOptions = new ArrayList<String>(); 
    answers = new ArrayList<Answer>(); 
} 

注：我不是最好的程序員，而不是新的，所以它可能會很邋遢:(

EDIT4：

我的TrueFalse類：

public class TrueFalse extends Question implements Serializable { 

private static final long serialVersionUID = 1L; 
public TrueFalse() { 
    questionType = "True/False"; 
} 

@Override 
public void addPrompt() { 
    input = new Input(); 

    System.out.println("Enter the prompt for your True/False question: "); 
    prompt = input.getStringInput(); 
} 

@Override 
public void addOption() { 
    options.add("True"); 
    options.add("False"); 
} 

@Override 
public void addCorrectOption() { 
    System.out.println("Select correct option: T/F"); 
    String response = input.getStringInput(); 

    while ((!response.toLowerCase().equals("t")) && (!response.toLowerCase().equals("f"))) { 
     System.out.println("MESSAGE: Enter a valid input."); 
    } 
    if (response.toLowerCase().equals("t")) { 
     correctOptions.add("True"); 
    } else { 
     correctOptions.add("False"); 
    } 
} 

public void take(Question question) { 
    input = new Input(); 
    answer = new Answer(); 

    System.out.print("Enter Option: "); 
    int response = input.getIntInput(question.options.size()); 
    answer.setAnswerNumber(response); 

    System.out.println(question.answers); 
    question.answers.add(answer); 
} 

}

Answer 1

很好，看不出有什麼明顯的問題在這裏。

不過，我想用「保護」屬性在這裏不看對我好，建議在問題類說你有：

private ArrayList<Answer> answers = new ArrayList<Answer>(); 
protected ArrayList<Answer> getAnswers(){ 
    return answers; 
} 

在子類TrueFalse，是指通過getter方法

System.out.println(question.getAnswers()); 
question.getAnswers().add(answer); 

Answer 2

您確定在調查中正確初始化了您的問題對象嗎？

survey.questions.get(i).take(survey.questions.get(i)); 

一般情況下，我不能確定這行的含義是什麼。對於我的調查，從我的問題中選擇一個特定的問題，然後讓這個問題激活一個方法..本身？這就是爲什麼這關鍵字是爲首位......也許你應該嘗試：

public void take() { 
input = new Input(); 
answer = new Answer(); 

System.out.print("Enter Option: "); 
int response = input.getIntInput(this.options.size()); 
answer.setAnswerNumber(response); 

System.out.println(this.answers); // NO NEED TO SEND THE QUESTION TO ITSELF 
this.answers.add(answer); 

}

Answer 3

我拿出答案的ArrayList的分配在我問題類，並把它在我取（）方法，因爲這樣的：

public void take(Question question) { 
    input = new Input(); 
    answer = new Answer(); 
    answers = new ArrayList<Answer>(); 

    System.out.print("Enter Option: "); 
    int response = input.getIntInput(question.options.size()); 
    answer.setAnswerNumber(response); 

    System.out.println(question.answers); 
    question.answers.add(answer); 
} 

在問題，它現在看起來像：

public ArrayList<Answer> answers; 

有沒有人有一個想法，爲什麼這解決了它？