How To Minimize CPU Usage For Sockets And Threads

I created an instant messenger using java. When I have the Server that communicates between the clients and one client running on my Computer the CPU Usage is at 100%. It really slows down everything else I'm doing and I figure this might be an issue if I gave this to people to use. I don't want the client taking up a lot of CPU Usage if they're just running it in the background while doing other things on their computer. The program utilizes multithreading. Each thread is constantly being polled for input.

The Server, as seen below, has two threads. I explain what the threads do before the code. There is also another while loop running constantly in the server that is waiting for sockets to connect. The loop does not run constantly at the line socket.accept(); it stops and just waits.

The User, split into a menu and chat window, has two threads as well. I explain what the threads do before the code. After I originally posted I put a 100 ms sleep in all my threads. CPU Usage is still at 100%*

This thread listens for input from the user. The input tells the server what action to take. There is a thread running for every user currently connected to the server.

public void run()
{
try
{
input = new DataInputStream(user.getSocket().getInputStream());
output = new DataOutputStream(user.getSocket().getOutputStream());

[code]....