Google maps use wgs84 coordinate system, but sometimes you want to convert to world coordinate system. And you may also want to convert it back.

Convert latitude to a Y value in World Coordinates.

public static final double lat2y(double aLat) {
    return ((1 - Math.log(Math.tan(aLat * Math.PI / 180) + 1 / Math.cos(aLat * Math.PI / 180)) / Math.PI) / 2 * Math.pow(2, 0)) * 256;
}

Convert longitude to a X value in World Coordinates.

public static final double lon2x(double lon) {
    return (lon + 180f) / 360f * 256f;
}

Convert Y value in World Coordinates to latitude

public static final double y2lat(double y) {
    double z = Math.pow(Math.E, (2 * Math.PI * (1 - y / 128)));
    return Math.asin((z - 1) / (z + 1)) * 180 / Math.PI;
}

Convert X value in World Coordinates to longitude

public static final double x2lng(double x) {
    return x * 360 / 256 - 180;
}

References:

Geographic coordinate conversion

Google IO 2013 App Mystery Values

Google Map Hack

Coordinate conversions made easy

UTM Coordinates