Added an implemention of ArrayDeque, as well as unit tests

I also used this opportunity to reduce code duplication around other queue/deque implementations.

classpath/java/util/AbstractDeque.java

@@ -0,0 +1,90 @@
+/* Copyright (c) 2008-2014, Avian Contributors
+
+   Permission to use, copy, modify, and/or distribute this software
+   for any purpose with or without fee is hereby granted, provided
+   that the above copyright notice and this permission notice appear
+   in all copies.
+
+   There is NO WARRANTY for this software.  See license.txt for
+   details. */
+
+package java.util;
+
+public abstract class AbstractDeque<T> extends AbstractQueue<T> 
+                                       implements Deque<T> {
+
+  @Override
+  public void push(T e) {
+    addFirst(e);
+  }
+  
+  @Override
+  public void addLast(T element) {
+    if (! offerLast(element)) {
+      throw new IllegalStateException();
+    }
+  }
+
+  @Override
+  public void addFirst(T element) {
+    if (! offerFirst(element)) {
+      throw new IllegalStateException();
+    }
+  }
+
+  @Override
+  public boolean offer(T element) {
+    return offerLast(element);
+  }
+
+  @Override
+  public T poll() {
+    return pollFirst();
+  }
+
+  @Override
+  public T pop() {
+    return removeFirst();
+  }
+
+  @Override
+  public T removeFirst() {
+    return remove();
+  }
+
+  @Override
+  public boolean remove(Object element) {
+    return removeFirstOccurrence(element);
+  }
+
+  @Override
+  public T removeLast() {
+    T result = pollLast();
+    if (result == null) {
+      throw new NoSuchElementException();
+    } else {
+      return result;
+    }
+  }
+
+  @Override
+  public T getFirst() {
+    return element();
+  }
+
+  @Override
+  public T getLast() {
+    T result = peekLast();
+    
+    if (result == null) {
+      throw new NoSuchElementException();
+    }
+    
+    return result;
+  }
+
+  @Override
+  public T peek() {
+    return peekFirst();
+  }
+}