Client.java

//type 1
Future<String> f = service.foo(s);
String s = f.get(); //blocks the thread, but at least others can run
//... do something useful with the string...

//type 2
Future<String> f = service.foo(s);
while(!f.isDone()){
    try {
        Thread.sleep(100);
    } catch (InterruptedException e) {
        ...
    }
}
String s = f.get();
//... do something useful with the string...