Add constructor accepting collection of file alteration observers (#236)

* Add constructor accepting collection of file alteration observers

* Add missing whitespace

* Use optionals for null-safe implementation

* Add unit-tests for collection constructor

* Add missing param descriptions

Co-authored-by: Rotmann, Maksim <maksim.rotmann@soptim.de>

Author: nstdspace

src/main/java/org/apache/commons/io/monitor/FileAlterationMonitor.java

@@ -16,7 +16,10 @@
  */
 package org.apache.commons.io.monitor;
 
+import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
+import java.util.Optional;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ThreadFactory;
 
@@ -52,6 +55,21 @@ public FileAlterationMonitor(final long interval) {
         this.interval = interval;
     }
 
+    /**
+     * Wrapper constructor for {@link #FileAlterationMonitor(long, FileAlterationObserver...)}.
+     * Constructs a monitor with the specified interval and collection of observers.
+     *
+     * @param interval The amount of time in milliseconds to wait between
+     * checks of the file system
+     * @param observers The collection of observers to add to the monitor
+     */
+    public FileAlterationMonitor(final long interval, final Collection<FileAlterationObserver> observers) {
+        this(interval, Optional.ofNullable(observers)
+                .orElse(Collections.emptyList())
+                .toArray(new FileAlterationObserver[0])
+        );
+    }
+
     /**
      * Constructs a monitor with the specified interval and set of observers.
      *

src/test/java/org/apache/commons/io/monitor/FileAlterationMonitorTestCase.java

@@ -53,6 +53,31 @@ public void testDefaultConstructor() {
         assertEquals(10000, monitor.getInterval(), "Interval");
     }
 
+    @Test
+    public void testCollectionConstructorShouldDoNothingWithNullCollection() {
+        Collection<FileAlterationObserver> observers = null;
+        final FileAlterationMonitor monitor = new FileAlterationMonitor(0, observers);
+        assertFalse(monitor.getObservers().iterator().hasNext());
+    }
+
+    @Test
+    public void testCollectionConstructorShouldDoNothingWithNullObservers() {
+        Collection<FileAlterationObserver> observers = new ArrayList<>(5);
+        final FileAlterationMonitor monitor = new FileAlterationMonitor(0, observers);
+        assertFalse(monitor.getObservers().iterator().hasNext());
+    }
+
+    @Test
+    public void testCollectionConstructor() {
+        Collection<FileAlterationObserver> observers = new ArrayList<>();
+        FileAlterationObserver observer = new FileAlterationObserver("foo");
+        observers.add(observer);
+
+        FileAlterationMonitor monitor = new FileAlterationMonitor(0, observers);
+        Iterator<FileAlterationObserver> iterator = monitor.getObservers().iterator();
+        assertEquals(observer, iterator.next());
+    }
+
     /**
      * Test add/remove observers.
      */