Java：如何访问对象列表中包含的2D数组

Question

I wanted to make an array of 2D arrays, but was pointed towards an ArrayList by a member here, and following his suggestion I wrote the following code:

public class matrixArray {
    double[][] array;
    public matrixArray(double[][] initialArray){
        array = initialArray;
    }
}

I then implemented it by the following:

public static ArrayList LUDecompose(double[][] A){

    ArrayList<matrixArray> LU = new ArrayList<>();
    double[][] L = new double[A.length][A.length];
    double[][] U = new double[A.length][A.length];

    for(int j=0; j<(A.length-1);j++){
        for(int i=(j+1); i<A.length; i++){

            double lower = A[i][j]/A[j][j];
            L[i][j] = lower;

            double[] replacement = row_scalar_mult(lower,A[j]);
            replacement = vector_add(replacement, A[i]);

            row_replace(i, A, replacement);
        }
    }

    LU.add(new matrixArray(L));
    LU.add(new matrixArray(A));

    return LU;
}

So far so good. Now, I want to access the 2D arrays in my fancy new ArrayList. When I try the following code in my main method, I get the error "Incompatible types: Object cannot be converted to double[][]":

double[][] L = LUDecompose(A).get(0);
double[][] U = LUDecompose(A).get(0);

OR

ArrayList LU = LUDecompose(A);
double[][] L = LUDecompose(A).get(0);
double[][] U = LUDecompose(A).get(0);

So my question becomes: How do I access the "Object" to get out my treasured 2D Arrays?

Answer 1

You need to specify the proper return type in the method signature:

public static ArrayList<matrixArray> LUDecompose(double[][] A){

note the use of ArrayList<matrixArray> rather than just ArrayList

then use

ArrayList LU = LUDecompose(A);
double[][] L = LUDecompose(A).get(0).array;
double[][] U = LUDecompose(A).get(0).array;