mat

a somewhat functional matrix implementation, supports adding subtracting and multiplying (and throws errors too!)
to use: just build as a jar

cd Mat
javac *.java
jar cfv mat.jar *

double[][] array = new double[][] {{1., 3.}, {2., 5.}};
double[][] array2 = new double[][] {{0., 1.}, {3., 2.}};
Matrix d = new Matrix(array);
Matrix e = new Matrix(array2);
System.out.println(d.multiply(e));
System.out.println(d.add(e));

docs

public Matrix(int rows, int columns) throws MatrixException
public Matrix(double[] array) throws MatrixException
public Matrix(double[][] array) throws MatrixException
public int getRows()
public int getColumns()
public double getValue(int row, int col) throws MatrixException
public void forceSet(int row, int col, double n) throws MatrixException
public Matrix add(Matrix b) throws MatrixException
public Matrix subtract(Matrix b) throws MatrixException
public Matrix multiply(double scalar) throws MatrixException
public Matrix multiply(Matrix b) throws MatrixException
public Matrix transpose() throws MatrixException
public String toString()