diff --git a/containers/enumerable.go b/containers/enumerable.go
index 89ca07e..5824b23 100644
--- a/containers/enumerable.go
+++ b/containers/enumerable.go
@@ -36,10 +36,12 @@ type EnumerableWithIndex interface {
 
 	// Invokes the given function once for each element and returns a
 	// container containing the values returned by the given function.
-	Map(func(index int, value interface{}) interface{}) Container
+	// TODO need help on how to enforce this in containers (don't want to type assert when chaining)
+	// Map(func(index int, value interface{}) interface{}) Container
 
 	// Returns a new container containing all elements for which the given function returns a true value.
-	Select(func(index int, value interface{}) bool) Container
+	// TODO need help on how to enforce this in containers (don't want to type assert when chaining)
+	// Select(func(index int, value interface{}) bool) Container
 
 	// Passes each element of the container to the given function and
 	// returns true if the function ever returns true for any element.
@@ -62,10 +64,12 @@ type EnumerableWithKey interface {
 
 	// Invokes the given function once for each element and returns a container
 	// containing the values returned by the given function as key/value pairs.
-	Map(func(key interface{}, value interface{}) (interface{}, interface{})) Container
+	// TODO need help on how to enforce this in containers (don't want to type assert when chaining)
+	// Map(func(key interface{}, value interface{}) (interface{}, interface{})) Container
 
 	// Returns a new container containing all elements for which the given function returns a true value.
-	Select(func(key interface{}, value interface{}) bool) Container
+	// TODO need help on how to enforce this in containers (don't want to type assert when chaining)
+	// Select(func(key interface{}, value interface{}) bool) Container
 
 	// Passes each element of the container to the given function and
 	// returns true if the function ever returns true for any element.
diff --git a/lists/arraylist/arraylist.go b/lists/arraylist/arraylist.go
index d577385..cdcd2d9 100644
--- a/lists/arraylist/arraylist.go
+++ b/lists/arraylist/arraylist.go
@@ -218,7 +218,7 @@ func (list *List) Each(f func(index int, value interface{})) {
 
 // Invokes the given function once for each element and returns a
 // container containing the values returned by the given function.
-func (list *List) Map(f func(index int, value interface{}) interface{}) containers.Container {
+func (list *List) Map(f func(index int, value interface{}) interface{}) *List {
 	newList := &List{}
 	iterator := list.Iterator()
 	for iterator.Next() {
@@ -228,7 +228,7 @@ func (list *List) Map(f func(index int, value interface{}) interface{}) containe
 }
 
 // Returns a new container containing all elements for which the given function returns a true value.
-func (list *List) Select(f func(index int, value interface{}) bool) containers.Container {
+func (list *List) Select(f func(index int, value interface{}) bool) *List {
 	newList := &List{}
 	iterator := list.Iterator()
 	for iterator.Next() {
diff --git a/lists/arraylist/arraylist_test.go b/lists/arraylist/arraylist_test.go
index 68c499a..c3f0c58 100644
--- a/lists/arraylist/arraylist_test.go
+++ b/lists/arraylist/arraylist_test.go
@@ -162,7 +162,7 @@ func TestArrayListEnumerableAndIterator(t *testing.T) {
 	// Map
 	mappedList := list.Map(func(index int, value interface{}) interface{} {
 		return "mapped: " + value.(string)
-	}).(*List)
+	})
 	if actualValue, _ := mappedList.Get(0); actualValue != "mapped: a" {
 		t.Errorf("Got %v expected %v", actualValue, "mapped: a")
 	}
@@ -179,7 +179,7 @@ func TestArrayListEnumerableAndIterator(t *testing.T) {
 	// Select
 	selectedList := list.Select(func(index int, value interface{}) bool {
 		return value.(string) >= "a" && value.(string) <= "b"
-	}).(*List)
+	})
 	if actualValue, _ := selectedList.Get(0); actualValue != "a" {
 		t.Errorf("Got %v expected %v", actualValue, "value: a")
 	}
diff --git a/lists/doublylinkedlist/doublylinkedlist.go b/lists/doublylinkedlist/doublylinkedlist.go
index 2fa22d4..73f7e59 100644
--- a/lists/doublylinkedlist/doublylinkedlist.go
+++ b/lists/doublylinkedlist/doublylinkedlist.go
@@ -350,7 +350,7 @@ func (list *List) Each(f func(index int, value interface{})) {
 
 // Invokes the given function once for each element and returns a
 // container containing the values returned by the given function.
-func (list *List) Map(f func(index int, value interface{}) interface{}) containers.Container {
+func (list *List) Map(f func(index int, value interface{}) interface{}) *List {
 	newList := &List{}
 	iterator := list.Iterator()
 	for iterator.Next() {
@@ -360,7 +360,7 @@ func (list *List) Map(f func(index int, value interface{}) interface{}) containe
 }
 
 // Returns a new container containing all elements for which the given function returns a true value.
-func (list *List) Select(f func(index int, value interface{}) bool) containers.Container {
+func (list *List) Select(f func(index int, value interface{}) bool) *List {
 	newList := &List{}
 	iterator := list.Iterator()
 	for iterator.Next() {
diff --git a/lists/doublylinkedlist/doublylinkedlist_test.go b/lists/doublylinkedlist/doublylinkedlist_test.go
index 6d580c8..5e8331d 100644
--- a/lists/doublylinkedlist/doublylinkedlist_test.go
+++ b/lists/doublylinkedlist/doublylinkedlist_test.go
@@ -164,7 +164,7 @@ func TestDoublyLinkedListEnumerableAndIterator(t *testing.T) {
 	// Map
 	mappedList := list.Map(func(index int, value interface{}) interface{} {
 		return "mapped: " + value.(string)
-	}).(*List)
+	})
 	if actualValue, _ := mappedList.Get(0); actualValue != "mapped: a" {
 		t.Errorf("Got %v expected %v", actualValue, "mapped: a")
 	}
@@ -181,7 +181,7 @@ func TestDoublyLinkedListEnumerableAndIterator(t *testing.T) {
 	// Select
 	selectedList := list.Select(func(index int, value interface{}) bool {
 		return value.(string) >= "a" && value.(string) <= "b"
-	}).(*List)
+	})
 	if actualValue, _ := selectedList.Get(0); actualValue != "a" {
 		t.Errorf("Got %v expected %v", actualValue, "value: a")
 	}
diff --git a/lists/singlylinkedlist/singlylinkedlist.go b/lists/singlylinkedlist/singlylinkedlist.go
index 52e834e..e71a36f 100644
--- a/lists/singlylinkedlist/singlylinkedlist.go
+++ b/lists/singlylinkedlist/singlylinkedlist.go
@@ -320,7 +320,7 @@ func (list *List) Each(f func(index int, value interface{})) {
 
 // Invokes the given function once for each element and returns a
 // container containing the values returned by the given function.
-func (list *List) Map(f func(index int, value interface{}) interface{}) containers.Container {
+func (list *List) Map(f func(index int, value interface{}) interface{}) *List {
 	newList := &List{}
 	iterator := list.Iterator()
 	for iterator.Next() {
@@ -330,7 +330,7 @@ func (list *List) Map(f func(index int, value interface{}) interface{}) containe
 }
 
 // Returns a new container containing all elements for which the given function returns a true value.
-func (list *List) Select(f func(index int, value interface{}) bool) containers.Container {
+func (list *List) Select(f func(index int, value interface{}) bool) *List {
 	newList := &List{}
 	iterator := list.Iterator()
 	for iterator.Next() {
diff --git a/lists/singlylinkedlist/singlylinkedlist_test.go b/lists/singlylinkedlist/singlylinkedlist_test.go
index c5ce378..d5879c3 100644
--- a/lists/singlylinkedlist/singlylinkedlist_test.go
+++ b/lists/singlylinkedlist/singlylinkedlist_test.go
@@ -164,7 +164,7 @@ func TestSinglyLinkedListEnumerableAndIterator(t *testing.T) {
 	// Map
 	mappedList := list.Map(func(index int, value interface{}) interface{} {
 		return "mapped: " + value.(string)
-	}).(*List)
+	})
 	if actualValue, _ := mappedList.Get(0); actualValue != "mapped: a" {
 		t.Errorf("Got %v expected %v", actualValue, "mapped: a")
 	}
@@ -181,7 +181,7 @@ func TestSinglyLinkedListEnumerableAndIterator(t *testing.T) {
 	// Select
 	selectedList := list.Select(func(index int, value interface{}) bool {
 		return value.(string) >= "a" && value.(string) <= "b"
-	}).(*List)
+	})
 	if actualValue, _ := selectedList.Get(0); actualValue != "a" {
 		t.Errorf("Got %v expected %v", actualValue, "value: a")
 	}
diff --git a/maps/treemap/treemap.go b/maps/treemap/treemap.go
index c8aee76..b18f293 100644
--- a/maps/treemap/treemap.go
+++ b/maps/treemap/treemap.go
@@ -163,7 +163,7 @@ func (m *Map) Each(f func(key interface{}, value interface{})) {
 
 // Invokes the given function once for each element and returns a container
 // containing the values returned by the given function as key/value pairs.
-func (m *Map) Map(f func(key1 interface{}, value1 interface{}) (interface{}, interface{})) containers.Container {
+func (m *Map) Map(f func(key1 interface{}, value1 interface{}) (interface{}, interface{})) *Map {
 	newMap := &Map{tree: rbt.NewWith(m.tree.Comparator)}
 	iterator := m.Iterator()
 	for iterator.Next() {
@@ -174,7 +174,7 @@ func (m *Map) Map(f func(key1 interface{}, value1 interface{}) (interface{}, int
 }
 
 // Returns a new container containing all elements for which the given function returns a true value.
-func (m *Map) Select(f func(key interface{}, value interface{}) bool) containers.Container {
+func (m *Map) Select(f func(key interface{}, value interface{}) bool) *Map {
 	newMap := &Map{tree: rbt.NewWith(m.tree.Comparator)}
 	iterator := m.Iterator()
 	for iterator.Next() {
diff --git a/maps/treemap/treemap_test.go b/maps/treemap/treemap_test.go
index 3ad688c..4335b7f 100644
--- a/maps/treemap/treemap_test.go
+++ b/maps/treemap/treemap_test.go
@@ -213,7 +213,7 @@ func TestTreeMapEnumerableAndIterator(t *testing.T) {
 	// Map
 	mappedMap := m.Map(func(key1 interface{}, value1 interface{}) (key2 interface{}, value2 interface{}) {
 		return key1, "mapped: " + key1.(string)
-	}).(*Map)
+	})
 	if actualValue, _ := mappedMap.Get("a"); actualValue != "mapped: a" {
 		t.Errorf("Got %v expected %v", actualValue, "mapped: a")
 	}
@@ -230,7 +230,7 @@ func TestTreeMapEnumerableAndIterator(t *testing.T) {
 	// Select
 	selectedMap := m.Select(func(key interface{}, value interface{}) bool {
 		return key.(string) >= "a" && key.(string) <= "b"
-	}).(*Map)
+	})
 	if actualValue, _ := selectedMap.Get("a"); actualValue != 1 {
 		t.Errorf("Got %v expected %v", actualValue, "value: a")
 	}
diff --git a/sets/treeset/treeset.go b/sets/treeset/treeset.go
index b9ca141..750e264 100644
--- a/sets/treeset/treeset.go
+++ b/sets/treeset/treeset.go
@@ -144,7 +144,7 @@ func (set *Set) Each(f func(index int, value interface{})) {
 
 // Invokes the given function once for each element and returns a
 // container containing the values returned by the given function.
-func (set *Set) Map(f func(index int, value interface{}) interface{}) containers.Container {
+func (set *Set) Map(f func(index int, value interface{}) interface{}) *Set {
 	newSet := &Set{tree: rbt.NewWith(set.tree.Comparator)}
 	iterator := set.Iterator()
 	for iterator.Next() {
@@ -154,7 +154,7 @@ func (set *Set) Map(f func(index int, value interface{}) interface{}) containers
 }
 
 // Returns a new container containing all elements for which the given function returns a true value.
-func (set *Set) Select(f func(index int, value interface{}) bool) containers.Container {
+func (set *Set) Select(f func(index int, value interface{}) bool) *Set {
 	newSet := &Set{tree: rbt.NewWith(set.tree.Comparator)}
 	iterator := set.Iterator()
 	for iterator.Next() {
diff --git a/sets/treeset/treeset_test.go b/sets/treeset/treeset_test.go
index e725d82..db30724 100644
--- a/sets/treeset/treeset_test.go
+++ b/sets/treeset/treeset_test.go
@@ -111,7 +111,7 @@ func TestTreeSetEnumerableAndIterator(t *testing.T) {
 	// Map
 	mappedSet := set.Map(func(index int, value interface{}) interface{} {
 		return "mapped: " + value.(string)
-	}).(*Set)
+	})
 	if actualValue, expectedValue := mappedSet.Contains("mapped: a", "mapped: b", "mapped: c"), true; actualValue != expectedValue {
 		t.Errorf("Got %v expected %v", actualValue, expectedValue)
 	}
@@ -125,7 +125,7 @@ func TestTreeSetEnumerableAndIterator(t *testing.T) {
 	// Select
 	selectedSet := set.Select(func(index int, value interface{}) bool {
 		return value.(string) >= "a" && value.(string) <= "b"
-	}).(*Set)
+	})
 	if actualValue, expectedValue := selectedSet.Contains("a", "b"), true; actualValue != expectedValue {
 		fmt.Println("A: ", mappedSet.Contains("b"))
 		t.Errorf("Got %v (%v) expected %v (%v)", actualValue, selectedSet.Values(), expectedValue, "[a b]")