You can visit West Java in the dry season from April to the end of October/beginning of November is not as wet as the rainy season that is brought by the west monsoon from December to March.
Temperatures range between 21 and 33 degrees centigrade. Average is 60-90%. The rainy season is no reason not to visit as it rarely pours for days on end and in fact it is very unusual that it rains all day. It tends to rain in the afternoon almost at fixed hours so visitors can easily plan an exciting schedule no matter what the weather.