java - 将二维 ArrayList 转换为二维数组

Question

我已经查看了几个答案，但发现我找到的答案有误。我正在尝试将 Doubles[] 的 ArrayList 转换为普通的 double 二维数组。我的代码:

    public ArrayList<Double[]>  ec;
    public double[][]           ei;
    ...
    encogCorpus = new ArrayList<Double[]>();
    ...
    ec.add(inputs);
    ...
    ei = new double[ec.size()][];

    for (int i = 0; i < ec.size(); i++) {
        ArrayList<Double> row = ec.get(i);
        ei[i] = row.toArray(new double[row.size()]);
    }

我收到错误提示

Type mismatch: cannot convert from Double[] to ArrayList

和

The method toArray(T[]) in the type ArrayList is not applicable for the arguments (double[])

Answer 1

问题

1. First of all, ec here is of type ArrayList<Double[]>, which means ec.get(i) should return Double[] and not ArrayList<Double>.
2. Second, double and Double are completely different types. You can't simply use row.toArray(new double[row.size()]) on your code.

---

解决方案

1.

如果你想要一个真正的 2D ArrayList的 Doubles然后是 ec 的类型应该是 ArrayList<ArrayList<Double>> .但是因为我们不能使用toArray() ，我们改为手动循环。

public ArrayList<ArrayList<Double>> ec;  // line changed here
public double[][]                   ei;
...
encogCorpus = new ArrayList<ArrayList<Double>>(); // and here also
...
ec.add(inputs); // `inputs` here should be of type `ArrayList<Double>`
...
ei = new double[ec.size()][];

for (int i = 0; i < ec.size(); i++) {
    ArrayList<Double> row = ec.get(i);

    // Perform equivalent `toArray` operation
    double[] copy = new double[row.size()];
    for (int j = 0; j < row.size(); j++) {
        // Manually loop and set individually
        copy[j] = row.get(j);
    }

    ei[i] = copy;
}

2.

但是如果你坚持使用ArrayList<Double[]> ，我们只需要改变主要部分:

public ArrayList<Double[]>  ec;
public double[][]           ei;
...
encogCorpus = new ArrayList<Double[]>();
...
ec.add(inputs);
...
ei = new double[ec.size()][];

for (int i = 0; i < ec.size(); i++) {
    // Changes are only below here

    Double[] row = ec.get(i);
    double[] copy = new double[row.length];

    // Still, manually loop...
    for (int j = 0; j < row.length; j++) {
        copy[j] = row[j];
    }

    ei[i] = copy;
}

3.

最后，如果你能改变Double[]至 double[] , 解 2 会变成,

public ArrayList<double[]>  ec; // Changed type
public double[][]           ei;
...
...
for (int i = 0; i < ec.size(); i++) {
    // Simpler changes here
    ei[i] = ec.get(i).clone();
}
...