diff --git a/src/main/java/org/apache/commons/collections4/FluentIterable.java b/src/main/java/org/apache/commons/collections4/FluentIterable.java
index 503a4e0a7..2b4a545f2 100644
--- a/src/main/java/org/apache/commons/collections4/FluentIterable.java
+++ b/src/main/java/org/apache/commons/collections4/FluentIterable.java
@@ -148,7 +148,7 @@ public class FluentIterable<E> implements Iterable<E> {
      * @param other  the other iterable to combine, may be null
      * @return a new iterable, combining this iterable with other
      */
-    public FluentIterable<E> append(final Iterable<E> other) {
+    public FluentIterable<E> append(final Iterable<? extends E> other) {
         return of(IterableUtils.chainedIterable(iterable, other));
     }
 
@@ -171,7 +171,7 @@ public class FluentIterable<E> implements Iterable<E> {
      * @return a new iterable, collating this iterable with the other in natural order
      * @see {@link org.apache.commons.collections4.iterators.CollatingIterator CollatingIterator}
      */
-    public FluentIterable<E> collate(final Iterable<E> other) {
+    public FluentIterable<E> collate(final Iterable<? extends E> other) {
         return of(IterableUtils.collatedIterable(iterable, other, null));
     }
 
@@ -197,7 +197,8 @@ public class FluentIterable<E> implements Iterable<E> {
      * @return a new iterable, collating this iterable with the other in natural order
      * @see {@link org.apache.commons.collections4.iterators.CollatingIterator CollatingIterator}
      */
-    public FluentIterable<E> collate(final Iterable<E> other, Comparator<? super E> comparator) {
+    public FluentIterable<E> collate(final Iterable<? extends E> other,
+                                     final Comparator<? super E> comparator) {
         return of(IterableUtils.collatedIterable(iterable, other, comparator));
     }
 
@@ -226,7 +227,7 @@ public class FluentIterable<E> implements Iterable<E> {
      * @return a new iterable, providing a filtered view of this iterable
      * @throws NullPointerException if predicate is null
      */
-    public FluentIterable<E> filter(final Predicate<E> predicate) {
+    public FluentIterable<E> filter(final Predicate<? super E> predicate) {
         return of(IterableUtils.filteredIterable(iterable, predicate));
     }
 
@@ -300,26 +301,25 @@ public class FluentIterable<E> implements Iterable<E> {
 
     /**
      * Returns a new FluentIterable whose iterator will traverse
-     * the elements of this iterable and the provided elements in
+     * the elements of this iterable and the other iterable in
      * alternating order.
      *
-     * @param elements  the elements to interleave
-     * @return a new iterable, interleaving this iterable with the elements
+     * @param other  the other iterable to interleave
+     * @return a new iterable, interleaving this iterable with others
      */
-    @SuppressWarnings("unchecked")
-    public FluentIterable<E> zip(final E... elements) {
-        return zip(Arrays.asList(elements));
+    public FluentIterable<E> zip(final Iterable<? extends E> other) {
+        return of(IterableUtils.zippingIterable(iterable, other));
     }
 
     /**
      * Returns a new FluentIterable whose iterator will traverse
-     * the elements of this iterable and the other iterable in
+     * the elements of this iterable and the other iterables in
      * alternating order.
      *
      * @param others  the iterables to interleave
      * @return a new iterable, interleaving this iterable with others
      */
-    public FluentIterable<E> zip(final Iterable<E>... others) {
+    public FluentIterable<E> zip(final Iterable<? extends E>... others) {
         @SuppressWarnings("unchecked")
         Iterable<E>[] iterables = new Iterable[1 + others.length];
         iterables[0] = iterable;
diff --git a/src/main/java/org/apache/commons/collections4/IterableUtils.java b/src/main/java/org/apache/commons/collections4/IterableUtils.java
index 66ac47c2f..fd4edfba8 100644
--- a/src/main/java/org/apache/commons/collections4/IterableUtils.java
+++ b/src/main/java/org/apache/commons/collections4/IterableUtils.java
@@ -53,7 +53,8 @@ public class IterableUtils {
      * @return a new iterable, combining the provided iterables
      */
     @SuppressWarnings("unchecked")
-    public static <E> Iterable<E> chainedIterable(final Iterable<? extends E> a, final Iterable<? extends E> b) {
+    public static <E> Iterable<E> chainedIterable(final Iterable<? extends E> a,
+                                                  final Iterable<? extends E> b) {
         return chainedIterable(new Iterable[] {a, b});
     }
 
@@ -155,8 +156,8 @@ public class IterableUtils {
      *   may be null, in which case natural ordering will be used
      * @return a filtered view on the specified iterable
      */
-    public static <E> Iterable<E> collatedIterable(final Iterable<E> a,
-                                                   final Iterable<E> b,
+    public static <E> Iterable<E> collatedIterable(final Iterable<? extends E> a,
+                                                   final Iterable<? extends E> b,
                                                    final Comparator<? super E> comparator) {
         return new FluentIterable<E>() {
             @Override
@@ -395,7 +396,8 @@ public class IterableUtils {
      * @return a new iterable, interleaving the provided iterables
      */
     @SuppressWarnings("unchecked")
-    public static <E> Iterable<E> zippingIterable(final Iterable<E> a, final Iterable<E> b) {
+    public static <E> Iterable<E> zippingIterable(final Iterable<? extends E> a,
+                                                  final Iterable<? extends E> b) {
         return zippingIterable(new Iterable[] {a, b});
     }
 
@@ -413,12 +415,12 @@ public class IterableUtils {
      * @param iterables  the array of iterables to interleave
      * @return a new iterable, interleaving the provided iterables
      */
-    public static <E> Iterable<E> zippingIterable(final Iterable<E>... iterables) {
+    public static <E> Iterable<E> zippingIterable(final Iterable<? extends E>... iterables) {
         return new FluentIterable<E>() {
             @Override
             public Iterator<E> iterator() {
                 @SuppressWarnings("unchecked")
-                Iterator<E>[] iterators = new Iterator[iterables.length];
+                Iterator<? extends E>[] iterators = new Iterator[iterables.length];
                 for (int i = 0; i < iterables.length; i++) {
                     iterators[i] = emptyIteratorIfNull(iterables[i]);
                 }