在Java中表示可变长度的矩阵（二维数组）的最佳方法？

Question

I want to create a matrix with a variable number of rows which is filled during runtime. The number of rows depends on the data and is not known before the program starts. What I tried is this:

    ArrayList<HashMap<Integer, Double>> eMatrix = new ArrayList<HashMap<Integer, Double>>();
    HashMap<Integer, Double> row = new HashMap<Integer, Double>();
    while (i < foo.size() - 1) {
        if (foo.get(i) == 0) {
            row.put(0, foo.get(i));
        }
        if (foo.get(i) == 1) {
            ro.put(1, foo.get(i));
        }
        if (foo.get(i) == 2) {
            ro.put(2, foo.get(i));
        }
        if (!row.isEmpty()) {
            eMatrix.add(row);
        }
        row.clear();
        i++;
    }

The idea was that as ArrayLists and HashMaps can have variable size to create rows with the HashMap, and add these rows to the ArrayList resulting in a matrix. Now the problem is that row.clear clears the entries in eMatrix as well. Do you have a better idea how to do it?

Answer 1

Your specific implementation problem can be solved by creating a new HashMap instance for each row of the matrix :

ArrayList<HashMap<Integer, Double>> eMatrix = new ArrayList<HashMap<Integer, Double>>();
while (i < foo.size() - 1) {
    HashMap<Integer, Double> row = new HashMap<Integer, Double>();
    if (foo.get(i) == 0) {
        row.put(0, foo.get(i));
    }
    if (foo.get(i) == 1) {
        ro.put(1, foo.get(i));
    }
    if (foo.get(i) == 2) {
        ro.put(2, foo.get(i));
    }
    if (!row.isEmpty()) {
        eMatrix.add(row);
    }
    i++;
}

That said, I would still try to use a simple 2D array ( double[][] ). You can initialize it at run time, once you know the required dimensions of the matrix.