diff --git a/containers/iterator.go b/containers/iterator.go
index 0dcb829..db0ea7a 100644
--- a/containers/iterator.go
+++ b/containers/iterator.go
@@ -28,11 +28,28 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 package containers
 
-type Iterator interface {
+// Stateful iterator for ordered containers whose values can be fetched by an index.
+type IteratorWithIndex interface {
 	// Moves the iterator to the next element and returns true if there was a next element in the container.
+	// Modifies the state of the iterator.
 	Next() bool
 	// Returns the current element's value.
+	// Does not modify the state of the iterator.
 	Value() interface{}
-	// Returns the current element's index(key).
-	Index() interface{}
+	// Returns the current element's index.
+	// Does not modify the state of the iterator.
+	Index() int
+}
+
+// Stateful iterator for ordered containers whose elements are key value pairs.
+type IteratorWithKey interface {
+	// Moves the iterator to the next element and returns true if there was a next element in the container.
+	// Modifies the state of the iterator.
+	Next() bool
+	// Returns the current element's value.
+	// Does not modify the state of the iterator.
+	Value() interface{}
+	// Returns the current element's key.
+	// Does not modify the state of the iterator.
+	Key() interface{}
 }
diff --git a/lists/arraylist/arraylist.go b/lists/arraylist/arraylist.go
index 059ef3e..df40aa8 100644
--- a/lists/arraylist/arraylist.go
+++ b/lists/arraylist/arraylist.go
@@ -41,7 +41,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.Enumerable = (*List)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithIndex = (*Iterator)(nil)
 }
 
 type List struct {
@@ -196,7 +196,7 @@ func (iterator *Iterator) Value() interface{} {
 	return iterator.list.elements[iterator.index]
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Index() int {
 	return iterator.index
 }
 
diff --git a/lists/doublylinkedlist/doublylinkedlist.go b/lists/doublylinkedlist/doublylinkedlist.go
index 69d6368..40b6d43 100644
--- a/lists/doublylinkedlist/doublylinkedlist.go
+++ b/lists/doublylinkedlist/doublylinkedlist.go
@@ -41,7 +41,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.Enumerable = (*List)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithIndex = (*Iterator)(nil)
 }
 
 type List struct {
@@ -328,7 +328,7 @@ func (iterator *Iterator) Value() interface{} {
 	return iterator.element.value
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Index() int {
 	return iterator.index
 }
 
diff --git a/lists/singlylinkedlist/singlylinkedlist.go b/lists/singlylinkedlist/singlylinkedlist.go
index 69177b3..3faed7d 100644
--- a/lists/singlylinkedlist/singlylinkedlist.go
+++ b/lists/singlylinkedlist/singlylinkedlist.go
@@ -41,7 +41,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.Enumerable = (*List)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithIndex = (*Iterator)(nil)
 }
 
 type List struct {
@@ -298,7 +298,7 @@ func (iterator *Iterator) Value() interface{} {
 	return iterator.element.value
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Index() int {
 	return iterator.index
 }
 
diff --git a/stacks/arraystack/arraystack.go b/stacks/arraystack/arraystack.go
index 4b670dd..c775e80 100644
--- a/stacks/arraystack/arraystack.go
+++ b/stacks/arraystack/arraystack.go
@@ -40,7 +40,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ stacks.Stack = (*Stack)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithIndex = (*Iterator)(nil)
 }
 
 type Stack struct {
@@ -115,7 +115,7 @@ func (iterator *Iterator) Value() interface{} {
 	return value
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Index() int {
 	return iterator.index
 }
 
diff --git a/stacks/linkedliststack/linkedliststack.go b/stacks/linkedliststack/linkedliststack.go
index 14b720f..7bf5d03 100644
--- a/stacks/linkedliststack/linkedliststack.go
+++ b/stacks/linkedliststack/linkedliststack.go
@@ -41,7 +41,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ stacks.Stack = (*Stack)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithIndex = (*Iterator)(nil)
 }
 
 type Stack struct {
@@ -111,7 +111,7 @@ func (iterator *Iterator) Value() interface{} {
 	return value
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Index() int {
 	return iterator.index
 }
 
diff --git a/trees/binaryheap/binaryheap.go b/trees/binaryheap/binaryheap.go
index c5b2e39..ab7452b 100644
--- a/trees/binaryheap/binaryheap.go
+++ b/trees/binaryheap/binaryheap.go
@@ -42,7 +42,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ trees.Tree = (*Heap)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithIndex = (*Iterator)(nil)
 }
 
 type Heap struct {
@@ -130,7 +130,7 @@ func (iterator *Iterator) Value() interface{} {
 	return value
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Index() int {
 	return iterator.index
 }
 
diff --git a/trees/redblacktree/redblacktree.go b/trees/redblacktree/redblacktree.go
index 35d1d56..da000ad 100644
--- a/trees/redblacktree/redblacktree.go
+++ b/trees/redblacktree/redblacktree.go
@@ -41,7 +41,7 @@ import (
 
 func assertInterfaceImplementation() {
 	var _ trees.Tree = (*Tree)(nil)
-	var _ containers.Iterator = (*Iterator)(nil)
+	var _ containers.IteratorWithKey = (*Iterator)(nil)
 }
 
 type color bool
@@ -311,7 +311,7 @@ func (iterator *Iterator) Value() interface{} {
 	return iterator.left.Value
 }
 
-func (iterator *Iterator) Index() interface{} {
+func (iterator *Iterator) Key() interface{} {
 	return iterator.left.Key
 }
 
diff --git a/trees/redblacktree/redblacktree_test.go b/trees/redblacktree/redblacktree_test.go
index 90f1fe0..eee8955 100644
--- a/trees/redblacktree/redblacktree_test.go
+++ b/trees/redblacktree/redblacktree_test.go
@@ -234,7 +234,7 @@ func TestRedBlackTreeIterator(t *testing.T) {
 	count := 0
 	for it.Next() {
 		count += 1
-		index := it.Index()
+		index := it.Key()
 		switch index {
 		case count:
 			if actualValue, expectedValue := index, count; actualValue != expectedValue {
@@ -259,7 +259,7 @@ func TestRedBlackTreeIterator(t *testing.T) {
 	count = 0
 	for it.Next() {
 		count += 1
-		index := it.Index()
+		index := it.Key()
 		switch index {
 		case count:
 			if actualValue, expectedValue := index, count; actualValue != expectedValue {
@@ -282,7 +282,7 @@ func TestRedBlackTreeIterator(t *testing.T) {
 	count = 0
 	for it.Next() {
 		count += 1
-		index := it.Index()
+		index := it.Key()
 		switch index {
 		case count:
 			if actualValue, expectedValue := index, count; actualValue != expectedValue {