Exercise

Complete a timer task scheduler in Java without using built-in timers.

Solution

The solution executes a runnable task exactly once after the specified time interval has elapsed. The implemented interface to the timer executor is as follows:

public interface Timer {
/**
* Creates and executes a one-shot action that becomes enabled
* after the given delay.
* @param command the task to execute
* @param delay the time from now to delay execution
* @param unit the time unit of the delay parameter
*/
void schedule(Runnable command, long delay, TimeUnit unit);
}

I took the following constraints into consideration:

• The solution must run in a multi-threaded environment and must be thread-safe.
• The solution must support the scenario where up to a million of timers get scheduled rapidly.
• Using high-level primitives that turn this exercise into a simple wrapper is avoided. In other words, I am allowed to use Java-based frameworks or classes from Core Java, except those that have built-in timers and solve the problem for you.

Please follow my comments in code for the known gaps and deficiencies of implementation.

Requirements

Java 8, JUnit 4.12.