I want to use the datas in japplet for constructing a tree...
In that case the DOM is what you need since it will hold all the data and may be very close to the "tree" you want to show.
There are plenty of excellent tutorials - dont charge into this project until you have done some simple samples. Client - server is hard enough to debug without adding xml parsing problems. Work with an example of your data format as a local file first.
Bill