It's a java matrix problem. Could you help me solve this thank you. 

class to model a 3 x 4 matrix of doubles Supports Gaussian row operations In this version, rows and columns start at 0 

 public class Matrix { public static final int ROW = 3; public static final int COL = 4; // declare the instance variable that will hold the 2-dim array /**Instantiate a ROW x COL matrix, empty */ public Matrix() { } /** set the value of a particular element in the matrix * @param row the row of the element. 0 <= row < Matrix.ROW * @param col the column of the element. 0 <= col < Matrix.COL * @param value the value to be stored * @throws ArrayIndexOutOfBoundsException for invalid row or column */ public void setValue(int row, int col, double value) { // Why don't we have to test row/col for validity? } /** returns the value of a particular element in the matrix * @param row the row of the element. 0 <= row < Matrix.ROW * @param col the column of the element. 0 <= col < Matrix.COL * @throws ArrayIndexOutOfBoundsException for invalid row or column */ public double getValue(int row, int col) { } /** swap 2 rows of this matrix * @param r1 one of the rows to swap. 0 <= r1 < Matrix.ROW * @param r2 the other row to swap. 0 <= r2 < Matrix.ROW * @throws ArrayIndexOutOfBoundsException for invalid row or column */ public void swapRows(int r1, int r2) { } /** multiply one row by a non-zero constant * @param multiple the non-zero constant * @param row the row to change * @throws IllegalArgumentException if multiple is 0 * @throws ArrayIndexOutOfBoundsException for invalid row */ public void multiplyRow(double multiple, int row) { } /** add row r1 into row r2. Equivalent to r2 += r1 * @param r1 the row to add 0 <= r1 < Matrix.ROW * @param r2 the row to add into. 0 <= r2 < Matrix.ROW. This row will change. * @throws ArrayIndexOutOfBoundsException for invalid row */ public void addRows (int r2, int r1) { } /** * set new row in the matrix * @param row the new row to insert * @param rIdx the index of this new row 0 <= rIdx < Matrix.ROW * @return the old row * @throws IllegalArgument exception if row is not the correct length of Matrix.COL * @throws ArrayIndexOutOfBoundsException for invalid row */ public double[] replace(double[] row, int rIdx){ } /** * Add 2 Matrices together and return a new Matrix * @param m the 2nd Matrix * @return the matrix sum of this + m */ public Matrix sum(Matrix m){ } /** Return this matrix as a String of 3 rows of numbers in 4 columns */ public String toString() { } }