Laplace.java

import java.awt.Color;

public class Laplace {

    public static void main(String[] args) {
        int N = Integer.parseInt(args[0]);
        StdDraw.create(N+1, N+1);
        StdDraw.setScale(0, 0, N+1, N+1);
        double[][] V = new double[N+1][N+1];

        // precompute colors
        Color[] colors = new Color[101];
        for (int i = 0; i <= 100; i++) {
            int red   = (255 * i) / 100;
            int green = 0;
            int blue  = 255 - red;
            colors[i] = new Color(red, green, blue);
        }
        for (int i = 0; i <= 100; i = i + 10)
            colors[i] = Color.WHITE;
       

        // initialize all points with reasonable starting values for the potential
        for (int i = 1; i < N; i++)
            for (int j = 1; j < N; j++)
                V[i][j] = 51.0;
                
        // boundary conditions
        for (int j = 0; j <= N; j++) V[0][j] =  30;   // left
        for (int j = 0; j <= N; j++) V[N][j] =  70;   // right
        for (int i = 0; i <= N; i++) V[i][0] =   0;   // bottom
        for (int i = 0; i <= N; i++) V[i][N] = 100;   // top
                


        // numerically solve Laplace's equation	
        for (int t = 0; true; t++) {

            // draw
            for (int i = 1; i < N; i++) {
                for (int j = 1; j < N; j++) {
                    int c = (int) Math.round(V[i][j]);
                    StdDraw.setColor(colors[c]);
                    StdDraw.moveTo(i, j);
                    StdDraw.spot();
                }
            }
	    StdDraw.show();

            // repeat 100 times before drawing to screen
            for (int r = 0; r < 500; r++)
                for (int i = 1; i < N; i++)
                    for (int j = 1; j < N; j++)
                        V[i][j] = 0.25* (V[i-1][j] + V[i+1][j] + V[i][j-1] + V[i][j+1]);

	}
    }
}