diff --git a/lists/lists.go b/lists/lists.go
index 1f6bb08..55bd619 100644
--- a/lists/lists.go
+++ b/lists/lists.go
@@ -30,4 +30,5 @@ type List interface {
 	// Size() int
 	// Clear()
 	// Values() []interface{}
+	// String() string
 }
diff --git a/maps/maps.go b/maps/maps.go
index 93c64c9..cdce9f7 100644
--- a/maps/maps.go
+++ b/maps/maps.go
@@ -29,6 +29,7 @@ type Map interface {
 	// Size() int
 	// Clear()
 	// Values() []interface{}
+	// String() string
 }
 
 // BidiMap interface that all bidirectional maps implement (extends the Map interface)
diff --git a/sets/sets.go b/sets/sets.go
index d96801c..9641951 100644
--- a/sets/sets.go
+++ b/sets/sets.go
@@ -25,4 +25,5 @@ type Set interface {
 	// Size() int
 	// Clear()
 	// Values() []interface{}
+	// String() string
 }
diff --git a/stacks/stacks.go b/stacks/stacks.go
index b959e19..e9ae56e 100644
--- a/stacks/stacks.go
+++ b/stacks/stacks.go
@@ -22,4 +22,5 @@ type Stack interface {
 	// Size() int
 	// Clear()
 	// Values() []interface{}
+	// String() string
 }
diff --git a/trees/trees.go b/trees/trees.go
index a5a7427..8d1b868 100644
--- a/trees/trees.go
+++ b/trees/trees.go
@@ -18,4 +18,5 @@ type Tree interface {
 	// Size() int
 	// Clear()
 	// Values() []interface{}
+	// String() string
 }