public static void main(String[] args) {
        double[][] A = {{2, 0, -1, -2}, {0, 1, 2, -1}, {1, -1, 0, -1}, {-1, 3, -2, 0}}; //Коэф. уравнений
        double[] B = {-8, -1, -6, 7}; //Значения

        ArrayList<Double> xValues = new ArrayList<>(4);

        for(int i = 0; i < 4; i++) {
            xValues.add((double) 0);
        }

        int k = 0;
        double prevX = 0;

        while(xValues.get(4-1) - prevX > 0.000001 || k == 0) {

            prevX = xValues.get(4-1);

            System.out.println("k = " + k);
            System.out.println();

            double x = 0;

            for(int i = 0; i < 4; i++) {

                for(int j = 0; j < 4; j++) {
                    x += xValues.get(j) * A[i][j];
                }

                x += B[i];
                xValues.set(i,x);
                System.out.print("X" + (i+1) +": " + x);
                System.out.println();

                x = 0;
            }

            k++;
        }
        System.out.println(xValues);
    }