Make API for Monitor work better with Java 8

The SafeBox example for Monitor is much more verbose than expected. We can do better:

public class SafeBox<V> {
  private V value;
  private final Monitor monitor = new Monitor();
  private final Monitor.Guard valuePresent = guard(monitor, () -> value != null);
  private final Monitor.Guard valueAbsent = guard(monitor, () -> value == null);

  public V get() throws InterruptedException {
    try (LockedMonitor ignored = monitor.autoEnterWhen(valuePresent)) {
      V result = value;
      value = null;
      return result;
    }
  }

  public void set(V newValue) throws InterruptedException {
    try (LockedMonitor ignored = monitor.autoEnterWhen(valueAbsent)) {
      value = newValue;
    }
  }
}

[jsampson]

Original author of Monitor here (no longer at Google but still occasionally contributing). Just noticed this issue.

FYI, we did at least implement “creation of Guards using a BooleanSupplier” a little while back. It’s an instance method called newGuard. @cpovirk, I wonder if it’s time to change the examples in the class javadoc to use newGuard and save 8 lines of code. 😀 There are also a couple of typos in the javadoc for the method itself that could be fixed.

Beyond that, I actually did a complete redesign/rewrite a few years ago that’s both easier to use (lambdas for all guards and tasks) and higher performance (built on primitives instead of wrapping a lock), but it’s complicated enough that no one at Google has found the time to review it. 😉 Perhaps I should just put it up on GitHub myself to crowdsource the review.

[jsampson2]

FYI, I’ve posted the API for my complete redesign/rewrite of Monitor as a feature request here: #3265