如何将 Arraylist 转换为 2D 双[][] 数组

Question

我有一个具有s的对象，它们是x和y：ArrayListCoord

ArrayList<Coords> positionsArrayList = new ArrayList<>(values);

class Coords {

    int x;

    int y;

    public boolean equals(Object o) {

        Coords c = (Coords) o;

        return c.x == x && c.y == y;

    }

    public Coords(int x, int y) {

        super();

        this.x = x;

        this.y = y;

    }

    public int hashCode() {

        return new Integer(x + "0" + y);

    }

}

现在我想以2D数组的形式提取这个x和y。就像double[][]

double[][] vector={{0,0},{1,1},{2,2}}

我试过这个代码：

for (Coords value : positionsArrayList) {

    positions = new double[][]{{value.x, value.y}};

}

但它确实只输入最后一个条目。Java新手请帮忙

Answer 1

声明：positions&nbsp;=&nbsp;new&nbsp;double[][]{{value.x,&nbsp;value.y}};每次重新分配引用。最后，将“指向”（包含）最后一个元组，这就是为什么你把它看作是结果。positions

Answer 2

您可以做的是使用传统的 for 循环来实现结果：positions = new double[positionsArrayList.size()][2];for(int i =0; i < positionsArrayList.size(); i++){&nbsp; &nbsp; positions[i][0] = positionsArrayList.get(i).getX();&nbsp; &nbsp; positions[i][1] = positionsArrayList.get(i).getY();}

Answer 3

您可以为每个索引分配每个坐标值，例如：double[][] positions = new double[positionsArrayList.size()][2];for (int i = 0; i < positionsArrayList.size(); i++) {&nbsp; &nbsp; positions[i][0] = positionsArrayList.get(i).x;&nbsp; &nbsp; positions[i][1] = positionsArrayList.get(i).y;}或者，如果您熟悉直播：double[][] positions= positionsArrayList.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .map(coords -> new double[]{coords.x, coords.y})&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .toArray(double[][]::new);