访问 ArrayList<ArrayList<SomeObject>>元素 [英] Accessing ArrayList&lt;ArrayList&lt;SomeObject&gt;&gt; elements

问题描述

我有一个 ArrayList 的 ArrayList ，我需要在其中找到一种访问元素的简单方法，以便更容易理解我已经制定了我的目标我想实现:

I have an ArrayList of ArrayLists where i need to find a simple way of accessing the elements, to make it easier to understand i have drawn my goal of what i want to achieve:

如上图所示，主 ArrayList 由 m ArrayList 组成，我希望在其中使用 get 方法，从 0 到 N 个元素，其中 N 是 ArrayList1 和 ArrayList2<的总元素/代码>.可能会出现更多的 ArrayList.

As seen on the image above the main ArrayList consists of m ArrayList, where i wish to get an element by using a get method which goes from 0 to N elements, where N is the total elements of ArrayList1 and ArrayList2. More ArrayList's may occur.

我当然可以通过使用两个 for 循环来遍历元素，这不是我在这种情况下搜索的内容.

I can of course iterate through the elements by using two for-loops, which is not what im searching for in this case.

推荐答案

您基本上需要在新包装类上拥有 ArrayList 成员并以不同的方式实现它们.我创建了一个示例，演示了在 get() 中计算的正确索引.

You would need to basically have the the ArrayList members on your new wrapper class and implement them in a different manner. I whipped up an example that demonstrates the correct index being calculated in get().

import java.util.ArrayList;

public class ListHolder<T> {
    public ArrayList<ArrayList<T>> list = new ArrayList<ArrayList<T>>();

    public int size() {
        int size = 0;
        for (int i = 0; i < list.size(); i++) {
            size += list.get(i).size();
        }
        return size;
    }

    public T get(int i) {
        if (i >= size())
            return null;

        int listIndex = 0;
        int valueIndex = i;

        while (valueIndex >= list.get(listIndex).size()) {
            valueIndex -= list.get(listIndex++).size();
        }

        return list.get(listIndex).get(valueIndex);
    }
}

我用来验证我的方法的内容:

What I used to verify my methods:

public static void main(String[] args)
{
    ListHolder<Object> listHolder = new ListHolder<Object>();

    listHolder.list.add(new ArrayList<Object>());
    listHolder.list.get(0).add("hello");
    listHolder.list.get(0).add("world");

    listHolder.list.add(new ArrayList<Object>());
    listHolder.list.get(1).add("a");
    listHolder.list.get(1).add("b");
    listHolder.list.get(1).add("c");

    System.out.println("Size: " + listHolder.size());
    System.out.println("listHolder[0]: " + listHolder.get(0)); // "hello"
    System.out.println("listHolder[1]: " + listHolder.get(1)); // "world"
    System.out.println("listHolder[2]: " + listHolder.get(2)); // "a"
    System.out.println("listHolder[3]: " + listHolder.get(3)); // "b"
    System.out.println("listHolder[4]: " + listHolder.get(4)); // "c"
    System.out.println("listHolder[5]: " + listHolder.get(5)); // "null"
}

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