如何在实例化Question类时保留ArrayList选项，避免被修改？

问题本质 #

Java里ArrayList是引用类型，你所有的Question对象都接收了同一个ArrayList实例的引用。也就是说，不管创建多少个Question，它们的choices字段指向的都是内存里同一个列表。后续清空、修改这个列表时，所有已创建的Question的选项都会跟着变化。

两种解决办法 #

方法1：每次创建Question时，使用新的ArrayList实例

不用复用同一个c，每次准备新选项前直接新建一个ArrayList，这样每个Question拿到的都是独立的列表，互相不影响。

修改后的代码如下：

public static ArrayList<Question> allInitialQuestions(ArrayList<Question> q)
{
    // 第一个问题的选项，新建列表
    ArrayList<String> c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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，新建列表
    c = new ArrayList<String>();
    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;
}

方法2：修改Question类的构造方法，内部复制传入的列表

如果不想每次新建列表，可以在Question的构造方法里，把传入的ArrayList复制一份，这样Question内部保存的是自己的副本，外部修改原列表不会影响它。

修改Question的构造方法示例：

public class Question {
    private String genre;
    private String question;
    private ArrayList<String> choices;
    private String answer;
    private String funFact;

    public Question(String genre, String question, ArrayList<String> choices, String answer, String funFact) {
        this.genre = genre;
        this.question = question;
        // 复制传入的列表，创建独立副本
        this.choices = new ArrayList<>(choices);
        this.answer = answer;
        this.funFact = funFact;
    }

    // 其他方法...
}

这样你的原代码不用改动，直接复用c也没问题，因为每个Question都会自己复制一份选项列表。

内容的提问来源于stack exchange，提问作者EMAN-GAMING