diff --git a/android/onceper.go b/android/onceper.go
index 6160506..5fefb71 100644
--- a/android/onceper.go
+++ b/android/onceper.go
@@ -26,7 +26,7 @@
 
 // Once computes a value the first time it is called with a given key per OncePer, and returns the
 // value without recomputing when called with the same key.  key must be hashable.
-func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {
+func (once *OncePer) Once(key OnceKey, value func() interface{}) interface{} {
 	// Fast path: check if the key is already in the map
 	if v, ok := once.values.Load(key); ok {
 		return v
@@ -50,7 +50,7 @@
 
 // Get returns the value previously computed with Once for a given key.  If Once has not been called for the given
 // key Get will panic.
-func (once *OncePer) Get(key interface{}) interface{} {
+func (once *OncePer) Get(key OnceKey) interface{} {
 	v, ok := once.values.Load(key)
 	if !ok {
 		panic(fmt.Errorf("Get() called before Once()"))
@@ -60,12 +60,12 @@
 }
 
 // OnceStringSlice is the same as Once, but returns the value cast to a []string
-func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {
+func (once *OncePer) OnceStringSlice(key OnceKey, value func() []string) []string {
 	return once.Once(key, func() interface{} { return value() }).([]string)
 }
 
 // OnceStringSlice is the same as Once, but returns two values cast to []string
-func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, []string)) ([]string, []string) {
+func (once *OncePer) Once2StringSlice(key OnceKey, value func() ([]string, []string)) ([]string, []string) {
 	type twoStringSlice [2][]string
 	s := once.Once(key, func() interface{} {
 		var s twoStringSlice