In this question you are to write a Barrier class with a constructor and one other method, checkpoint: public class Barrier { public Barrier(int n) {...} public boolean checkpoint() {...} } After a Barrier object has been created, every time a thread invokes checkpoint () on that Barrier object it will pause until n threads have invoked checkpoint () at which time all n threads will continue. The checkpoint () method returns true if the thread successfully synchronized, and false otherwise (e.g. this thread was the n+1 the thread). a) Write the Barrier class. Ensure each Barrier object is left in a state so that it can be reused. b) Draw a class diagram for the Barrier class