'contains' 方法对 ArrayList<int[]> 不起作用，还有其他方法吗? [英] The &#39;contains&#39; method does not work for ArrayList&lt;int[]&gt;, is there another way?

问题描述

如果 ArrayList 还没有 int[]，我想向它添加一个 int[]，但由于某种原因，它不起作用.在这种情况下，arrlist 是 ArrayList 而 arr 是 int[].此代码位于 for 循环中，其中 arr 在循环中定义，因此 arr 中的值会发生变化.即使我打印出 arrlist 并且它有 arr，代码总是会说 arrlist 不包含 arr.是否有另一种方法来检查 ArrayList 是否包含 int[]?

I want to add an int[] to an ArrayList if it doesn't already have that int[], but for some reason, it's not working. In this case, arrlist is the ArrayList<int[]> and arr is the int[]. This code is in a for loop where arr is defined in the loop, so the values in arr changes. Even though I printed out arrlist and it had arr, the code would always say that arrlist didn't contain arr. Is there another way to check if an ArrayList contains an int[]?

int n = scan.nextInt();
ArrayList<int[]> arrlist = new ArrayList<>();
int[][] coordinates = new int[n][2];
boolean[] isTrue = new boolean[n];
for (int j = 0; j < n; j++) {
    int[] arr = new int[2];
    arr[0] = coordinates[j][0];
    arr[1] = coordinates[j][1];
    if (arrlist.contains(arr)) {
        isTrue[j] = true;
    } else {
        arrlist.add(arr);
    }
}

推荐答案

考虑这段代码:

int[] a = {1, 2};
int[] b = {1, 2};
System.out.println(a.equals(b));

你明白为什么输出是false"吗?

Do you understand why the output is "false"?

这就是您的问题的原因:根据'equals'方法，内容相同的两个数组不相等，该方法在此处显式调用，并在您的 List<> 示例中隐式调用.

This is the cause of your problems: two arrays with equal content are not equal according to the 'equals' method, which is explicitly invoked here, and implicitly invoked in your List<> example.

只要您想使用 int[] 数组，我看不到一个简单的解决方法.您不能定义 equals 方法.我认为，您最好的方法是根本不使用数组来包含坐标.

I don't see an easy fix as long as you want to use int[] arrays. You cannot define an equals method. Your best approach, I think, is to not use an array at all to contain the coordinates.

定义一个类，其实例包含所需的两个整数:

Define a class whose instances hold the required two integers:

class Point {
    int x, y;

    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public boolean equals(Object o) {
        return o instanceof Point
                && x == ((Point) o).x
                && y == ((Point) o).y;
    }
}

然后保留一个ArrayList.包含"现在将按预期工作.

and then keep an ArrayList<Point>. 'contains' will now work as expected.

(您还应该在 Point 类中定义 hashCode，但我跳过了它，它与答案并不直接相关).

(You should also define hashCode in class Point, but I skipped that, it is not immediately relevant to the answer).

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