Task: Reduced row echelon form
Go: package main

import "fmt"

type matrix [][]float64

func (m matrix) print() {
    for _, r := range m {
        fmt.Println(r)
    }
    fmt.Println("")
}

func main() {
    m := matrix{
        { 1, 2, -1,  -4},
        { 2, 3, -1, -11},
        {-2, 0, -3,  22},
    }
    m.print()
    rref(m)
    m.print()
}

func rref(m matrix) {
    lead := 0
    rowCount := len(m)
    columnCount := len(m[0])
    for r := 0; r < rowCount; r++ {
        if lead >= columnCount {
            return
        }
        i := r
        for m[i][lead] == 0 {
            i++
            if rowCount == i {
                i = r
                lead++
                if columnCount == lead {
                    return
                }
            }
        }
        m[i], m[r] = m[r], m[i]
        f := 1 / m[r][lead]
        for j, _ := range m[r] {
            m[r][j] *= f
        }
        for i = 0; i < rowCount; i++ {
            if i != r {
                f = m[i][lead]
                for j, e := range m[r] {
                    m[i][j] -= e * f
                }
            }
        }
        lead++
    }
}

Java: import java.util.*;
import java.lang.Math;
import org.apache.commons.math.fraction.Fraction;
import org.apache.commons.math.fraction.FractionConversionException;


class Matrix {
	LinkedList<LinkedList<Fraction>> matrix;
	int numRows;
	int numCols;	
	
	static class Coordinate {
		int row;
		int col;

		Coordinate(int r, int c) {
			row = r;
			col = c;
		}

		public String toString() {
			return "(" + row + ", " + col + ")";
		}
	}

	Matrix(double [][] m) {
		numRows = m.length;	
		numCols = m[0].length;

		matrix = new LinkedList<LinkedList<Fraction>>();

		for (int i = 0; i < numRows; i++) {
			matrix.add(new LinkedList<Fraction>());
			for (int j = 0; j < numCols; j++) {
				try {
					matrix.get(i).add(new Fraction(m[i][j]));
				} catch (FractionConversionException e) {
					System.err.println("Fraction could not be converted from double by apache commons . . .");
				}
			}
		}
	}

	public void Interchange(Coordinate a, Coordinate b) {
		LinkedList<Fraction> temp = matrix.get(a.row);
		matrix.set(a.row, matrix.get(b.row));		
		matrix.set(b.row, temp);

		int t = a.row;
		a.row = b.row;
		b.row = t;
	} 

	public void Scale(Coordinate x, Fraction d) {
		LinkedList<Fraction> row = matrix.get(x.row);
		for (int i = 0; i < numCols; i++) {
			row.set(i, row.get(i).multiply(d));
		}
	}

	public void MultiplyAndAdd(Coordinate to, Coordinate from, Fraction scalar) {
		LinkedList<Fraction> row = matrix.get(to.row);
		LinkedList<Fraction> rowMultiplied = matrix.get(from.row);

		for (int i = 0; i < numCols; i++) {
			row.set(i, row.get(i).add((rowMultiplied.get(i).multiply(scalar))));
		}
	}

	public void RREF() {
		Coordinate pivot = new Coordinate(0,0);

		int submatrix = 0;
		for (int x = 0; x < numCols; x++) {
			pivot = new Coordinate(pivot.row, x);
			
				
				for (int i = x; i < numCols; i++) {
					if (isColumnZeroes(pivot) == false) {
						break;	
					} else {
						pivot.col = i;
					}
				}
			
				
				pivot = findPivot(pivot);
			
				if (getCoordinate(pivot).doubleValue() == 0.0) {
					pivot.row++;
					continue;
				}

				
				
				if (pivot.row != submatrix) {
					Interchange(new Coordinate(submatrix, pivot.col), pivot);
				}
		
				
				if (getCoordinate(pivot).doubleValue() != 1) {
					
					Fraction scalar = getCoordinate(pivot).reciprocal();
					Scale(pivot, scalar);
				}
			
				
				
				for (int i = pivot.row; i < numRows; i++) {
					if (i == pivot.row) {
						continue;
					}
					Coordinate belowPivot = new Coordinate(i, pivot.col);
					Fraction complement = (getCoordinate(belowPivot).negate().divide(getCoordinate(pivot)));
					MultiplyAndAdd(belowPivot, pivot, complement);
				}
			
				
				
					
				for (int i = pivot.row; i >= 0; i--) {
					if (i == pivot.row) {
						if (getCoordinate(pivot).doubleValue() != 1.0) {
							Scale(pivot, getCoordinate(pivot).reciprocal());	
						}
						continue;
					}
					if (i == pivot.row) {
						continue;
					}
				
					Coordinate abovePivot = new Coordinate(i, pivot.col);
					Fraction complement = (getCoordinate(abovePivot).negate().divide(getCoordinate(pivot)));
					MultiplyAndAdd(abovePivot, pivot, complement);
				}
			
				
				
				if ((pivot.row + 1) >= numRows || isRowZeroes(new Coordinate(pivot.row+1, pivot.col))) {
					break;
				}

				submatrix++;
				pivot.row++;
		}
	}
	
	public boolean isColumnZeroes(Coordinate a) {
		for (int i = 0; i < numRows; i++) {
			if (matrix.get(i).get(a.col).doubleValue() != 0.0) {
				return false;
			}
		}

		return true;
	}

	public boolean isRowZeroes(Coordinate a) {
		for (int i = 0; i < numCols; i++) {
			if (matrix.get(a.row).get(i).doubleValue() != 0.0) {
				return false;
			}
		}

		return true;
	}

	public Coordinate findPivot(Coordinate a) {
		int first_row = a.row;
		Coordinate pivot = new Coordinate(a.row, a.col);
		Coordinate current = new Coordinate(a.row, a.col);	

		for (int i = a.row; i < (numRows - first_row); i++) {
			current.row = i;
			if (getCoordinate(current).doubleValue() == 1.0) {
				Interchange(current, a);
			}
		}

		current.row = a.row;
		for (int i = current.row; i < (numRows - first_row); i++) {
			current.row = i;
			if (getCoordinate(current).doubleValue() != 0) {
				pivot.row = i;
				break;
			}
		}
	
		
		return pivot;	
	}	

	public Fraction getCoordinate(Coordinate a) {
		return matrix.get(a.row).get(a.col);
	}

	public String toString() {
		return matrix.toString().replace("], ", "]\n");
	}

	public static void main (String[] args) {
        	double[][] matrix_1 = {
			{1, 2, -1, -4},
			{2, 3, -1, -11},
			{-2, 0, -3, 22}
		};

		Matrix x = new Matrix(matrix_1);
		System.out.println("before\n" + x.toString() + "\n");
		x.RREF();
		System.out.println("after\n" + x.toString() + "\n");

		double matrix_2 [][] = {
			{2, 0, -1, 0, 0},
			{1, 0, 0, -1, 0},
			{3, 0, 0, -2, -1},
			{0, 1, 0, 0, -2},
			{0, 1, -1, 0, 0}
		};
	
		Matrix y = new Matrix(matrix_2);
		System.out.println("before\n" + y.toString() + "\n");
		y.RREF();
		System.out.println("after\n" + y.toString() + "\n");

		double matrix_3 [][] = {
			{1, 2, 3, 4, 3, 1},
			{2, 4, 6, 2, 6, 2},
			{3, 6, 18, 9, 9, -6},
			{4, 8, 12, 10, 12, 4},
			{5, 10, 24, 11, 15, -4}
		};

		Matrix z = new Matrix(matrix_3);
		System.out.println("before\n" + z.toString() + "\n");
		z.RREF();
		System.out.println("after\n" + z.toString() + "\n");

		double matrix_4 [][] = {
			{0, 1},
			{1, 2},
			{0,5}
		};

		Matrix a = new Matrix(matrix_4);
		System.out.println("before\n" + a.toString() + "\n");
		a.RREF();
		System.out.println("after\n" + a.toString() + "\n");
	}	
}