Cancels the request, if possible. Requests that are already complete cannot be canceled.
Schedules the request to be executed at some point in the future.
Invokes the request immediately, and blocks until the response can be processed or is in error.
Returns true if this call has been either {@linkplain #execute() executed} or {@linkplain #enqueue(Callback) enqueued}. It is an error to execute a call more than once.
Returns the original request that initiated this call.
A call is a request that has been prepared for execution. A call can be canceled. As this object represents a single request/response pair (stream), it cannot be executed twice.