A NavigableSet that uses an internal sorted CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties:

• It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
• It is thread-safe.
• Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array.
• Iterators do not support the mutative remove operation.
• Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

The following code sketch uses a copy-on-write navigable set to maintain a set of ordered Handler objects that perform some action upon state updates until one of the handlers returns true indicating that the update has been handled.

import java.util.concurrent.CopyOnWriteArrayNavigableSet;

abstract class Handler implements Comparable<Handler> {
    final int priority;
    
    protected Handler(int priority) {
        this.priority = priority;
    }
    
    // returns true if update has been handled
    abstract boolean handle();
    
    // ordered from highest to lowest
    public final int compareTo(Handler other) { 
        return -Integer.compare(priority, other.priority);
    }
}
    
class X {
    // Will use "Natural Order" of Comparables
    private final CopyOnWriteArrayNavigableSet<Handler> handlers
      = new CopyOnWriteArrayNavigableSet<>();
    public void addHandler(Handler h) { handlers.add(h); }
    
    private long internalState;
    private synchronized void changeState() { internalState = ...; }
    
    public void update() {
        changeState();
        for (Handler handler : handlers)
             if(handler.handle()) break;
   }
}

Include the project as a dependency in your pom.xml file:

<dependency>
  <groupId>org.bondolo</groupId>
  <artifactId>cowans7</artifactId>
  <version>2</version>
</dependency>

COWANS7 is distributed on Maven Central and should be automatically be found in most cases.

Because the CopyOnWriteArrayNavigableSet class is defined in the java.util.concurrent package the cowans7-2.jar must be placed on the bootclasspath. The java.util.concurrent package is used by CopyOnWriteArrayNavigableSet because the implementation requires access to package private fields in CopyOnWriteArrayList and also for future compatibility if CopyOnWriteArrayNavigableSet is included in the JDK.

Public Domain : CC0 1.0 Universal