Java中ArrayList引用传递的陷阱与解决方案：避免数据意外修改

ArrayList<String> c = new ArrayList<String>();
// ... 添加选项 ...
q.add(new Question("Geography","Which ocean is the largest?", c, "Pacific", "..."));
// 此时，Question对象内部存储的是对变量c所指向的ArrayList实例的引用。

c.removeAll(c); // 清空了变量c所指向的那个ArrayList实例中的所有元素。
// 由于Question对象持有的是同一个ArrayList实例的引用，
// 因此它的choices列表也会被清空。

// ... 添加新的选项 ...
q.add(new Question("Geography", "How many countries are in the world?", c, "195", "..."));
// 此时，第一个Question对象的choices列表，也变成了第二个Question对象的choices。

import java.util.ArrayList;
import java.util.List; // 假设Question构造函数接受List<String>

// 假设Question类定义如下（简化示例）
class Question {
    String genre;
    String questionText;
    List<String> choices; // 使用List接口更通用
    String answer;
    String funFact;

    public Question(String genre, String questionText, List<String> choices, String answer, String funFact) {
        this.genre = genre;
        this.questionText = questionText;
        // 注意：这里为了防止外部修改，通常会进行防御性复制
        // 但在这个特定场景下，我们已经确保了传入的是新实例，所以直接赋值也无妨
        this.choices = choices; 
        this.answer = answer;
        this.funFact = funFact;
    }

    // Getter方法（省略）
    public List<String> getChoices() {
        return choices;
    }

    @Override
    public String toString() {
        return "Question{" +
               "genre='" + genre + '\'' +
               ", questionText='" + questionText + '\'' +
               ", choices=" + choices +
               ", answer='" + answer + '\'' +
               ", funFact='" + funFact + '\'' +
               '}';
    }
}


public class Database { // 假设这是包含allInitialQuestions方法的类

    public static ArrayList<Question> allInitialQuestions(ArrayList<Question> q) {

        ArrayList<String> c; // 声明变量，每次使用前重新实例化

        // 第一个问题：海洋
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("Pacific");
        c.add("Atlantic");
        c.add("Arctic");
        c.add("Indian");
        q.add(new Question("Geography","Which ocean is the largest?", c, "Pacific", "The Pacific Ocean stretches to an astonishing 63.8 million square miles!"));

        // 第二个问题：国家数量
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("192");
        c.add("195");
        c.add("193");
        c.add("197");
        q.add(new Question("Geography", "How many countries are in the world?", c, "195", "Africa has the most countries of any continent with 54."));

        // 第三个问题：最长河流
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("Mississippi");
        c.add("Nile");
        c.add("Congo");
        c.add("Amazon");
        q.add(new Question("Geography", "What is the name of the longest river in the world?", c, "Nile","Explorer John Hanning Speke discovered the source of the Nile on August 3rd, 1858."));

        // 第四个问题：人口最多国家
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("United States");
        c.add("China");
        c.add("Japan");
        c.add("India");
        q.add(new Question("Geography","Which country has the largest population?" ,c, "China", "Shanghai is the most populated city in China with a population of 24,870,895."));

        // 第五个问题：距离地球最近的行星
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("Mars");
        c.add("Mercury");
        c.add("Venus");
        c.add("Jupiter");
        q.add(new Question("Geography","Which planet is closest to Earth?",c,"Venus","Even though Venus is the closest, the planet it still ~38 million miles from Earth!"));

        // 第六个问题：马里奥的创造者
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("Sega");
        c.add("Nintendo");
        c.add("Sony");
        c.add("Atari");
        q.add(new Question("Video Games", "Which company created the famous plumber Mario?", c, "Nintendo", "Nintendo created Mario in 1981 for the arcade game Donkey Kong."));

        // 第七个问题：蓝色刺猬角色
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("Sonic");
        c.add("Tales");
        c.add("Knuckles");
        c.add("Amy");
        q.add(new Question("Video Games", "What is the name of the famous video character who is a blue hedgehog?",c,"Sonic", "In some official concept art, Sonic was originally meant to be a rabbit."));

        // 第八个问题：最畅销游戏
        c = new ArrayList<String>(); // 创建新的ArrayList实例
        c.add("Wii Sports");
        c.add("Grand Theft Auto V");
        c.add("Tetris");
        c.add("Minecraft");
        q.add(new Question("Video Games","As of 2022, which of the following is the best selling video game of all time?",c,"Minecraft","As of 2022, Minecraft has sold over 238 million units."));

        return q;
    }
}