diff --git a/cmd/comparetzpb/main.go b/cmd/comparetzpb/main.go
index 3c7afeb..cc85675 100644
--- a/cmd/comparetzpb/main.go
+++ b/cmd/comparetzpb/main.go
@@ -13,8 +13,8 @@ import (
 )
 
 var (
-	defaultfinder *tzf.DefaultFinder
-	fullFinder    *tzf.Finder
+	defaultfinder tzf.F
+	fullFinder    tzf.F
 )
 
 func init() {
diff --git a/cmd/tzf/main.go b/cmd/tzf/main.go
index a03877d..965a6f0 100644
--- a/cmd/tzf/main.go
+++ b/cmd/tzf/main.go
@@ -11,7 +11,7 @@ import (
 	"google.golang.org/protobuf/proto"
 )
 
-var finder *tzf.Finder
+var finder tzf.F
 
 func init() {
 	input := &pb.CompressedTimezones{}
diff --git a/f.go b/f.go
new file mode 100644
index 0000000..a1c5c68
--- /dev/null
+++ b/f.go
@@ -0,0 +1,7 @@
+package tzf
+
+type F interface {
+	GetTimezoneName(lng float64, lat float64) string
+	GetTimezoneNames(lng float64, lat float64) ([]string, error)
+	TimezoneNames() []string
+}
diff --git a/tzf.go b/tzf.go
index a17f35d..42ef70a 100644
--- a/tzf.go
+++ b/tzf.go
@@ -98,7 +98,7 @@ type Finder struct {
 	opt     *Option
 }
 
-func NewFinderFromRawJSON(input *convert.BoundaryFile, opts ...OptionFunc) (*Finder, error) {
+func NewFinderFromRawJSON(input *convert.BoundaryFile, opts ...OptionFunc) (F, error) {
 	timezones, err := convert.Do(input)
 	if err != nil {
 		return nil, err
@@ -106,7 +106,7 @@ func NewFinderFromRawJSON(input *convert.BoundaryFile, opts ...OptionFunc) (*Fin
 	return NewFinderFromPB(timezones, opts...)
 }
 
-func NewFinderFromPB(input *pb.Timezones, opts ...OptionFunc) (*Finder, error) {
+func NewFinderFromPB(input *pb.Timezones, opts ...OptionFunc) (F, error) {
 	now := time.Now()
 	items := make([]*tzitem, 0)
 	names := make([]string, 0)
@@ -184,7 +184,7 @@ func NewFinderFromPB(input *pb.Timezones, opts ...OptionFunc) (*Finder, error) {
 	return finder, nil
 }
 
-func NewFinderFromCompressed(input *pb.CompressedTimezones, opts ...OptionFunc) (*Finder, error) {
+func NewFinderFromCompressed(input *pb.CompressedTimezones, opts ...OptionFunc) (F, error) {
 	tzs, err := reduce.Decompress(input)
 	if err != nil {
 		return nil, err
diff --git a/tzf_default_finder.go b/tzf_default_finder.go
index 8369d0b..237035b 100644
--- a/tzf_default_finder.go
+++ b/tzf_default_finder.go
@@ -12,12 +12,12 @@ import (
 //
 // It's designed for performance first and allow some not so correct return at some area.
 type DefaultFinder struct {
-	fuzzyFinder *FuzzyFinder
-	finder      *Finder
+	fuzzyFinder F
+	finder      F
 }
 
-func NewDefaultFinder() (*DefaultFinder, error) {
-	fuzzyFinder, err := func() (*FuzzyFinder, error) {
+func NewDefaultFinder() (F, error) {
+	fuzzyFinder, err := func() (F, error) {
 		input := &pb.PreindexTimezones{}
 		if err := proto.Unmarshal(tzfrel.PreindexData, input); err != nil {
 			panic(err)
@@ -28,7 +28,7 @@ func NewDefaultFinder() (*DefaultFinder, error) {
 		return nil, err
 	}
 
-	finder, err := func() (*Finder, error) {
+	finder, err := func() (F, error) {
 		input := &pb.CompressedTimezones{}
 		if err := proto.Unmarshal(tzfrel.LiteCompressData, input); err != nil {
 			panic(err)
diff --git a/tzf_default_finder_test.go b/tzf_default_finder_test.go
index 70b0669..27bc89c 100644
--- a/tzf_default_finder_test.go
+++ b/tzf_default_finder_test.go
@@ -14,7 +14,7 @@ import (
 )
 
 var (
-	defaultFinder *tzf.DefaultFinder
+	defaultFinder tzf.F
 )
 
 func init() {
diff --git a/tzf_fuzzy.go b/tzf_fuzzy.go
index 2205e71..7436e4f 100644
--- a/tzf_fuzzy.go
+++ b/tzf_fuzzy.go
@@ -18,7 +18,7 @@ type FuzzyFinder struct {
 	names   []string
 }
 
-func NewFuzzyFinderFromPB(input *pb.PreindexTimezones) (*FuzzyFinder, error) {
+func NewFuzzyFinderFromPB(input *pb.PreindexTimezones) (F, error) {
 	f := &FuzzyFinder{
 		m:       make(map[maptile.Tile][]string),
 		idxZoom: int(input.IdxZoom),
diff --git a/tzf_fuzzy_test.go b/tzf_fuzzy_test.go
index 0677cd3..efb8401 100644
--- a/tzf_fuzzy_test.go
+++ b/tzf_fuzzy_test.go
@@ -14,7 +14,7 @@ import (
 )
 
 var (
-	fuzzyFinder *tzf.FuzzyFinder
+	fuzzyFinder tzf.F
 )
 
 func init() {
diff --git a/tzf_test.go b/tzf_test.go
index ca2fe44..f6993db 100644
--- a/tzf_test.go
+++ b/tzf_test.go
@@ -20,8 +20,8 @@ import (
 )
 
 var (
-	finder     *tzf.Finder
-	fullFinder *tzf.Finder
+	finder     tzf.F
+	fullFinder tzf.F
 )
 
 func init() {
@@ -127,7 +127,7 @@ func ExampleFinder_GetTimezoneLoc() {
 		panic(err)
 	}
 	finder, _ := tzf.NewFinderFromPB(input)
-	fmt.Println(finder.GetTimezoneLoc(116.6386, 40.0786))
+	fmt.Println(finder.(*tzf.Finder).GetTimezoneLoc(116.6386, 40.0786))
 	// Output: Asia/Shanghai <nil>
 }
 
@@ -141,7 +141,7 @@ func ExampleFinder_GetTimezoneShapeByName() {
 		panic(err)
 	}
 	finder, _ := tzf.NewFinderFromPB(input)
-	pbtz, err := finder.GetTimezoneShapeByName("Asia/Shanghai")
+	pbtz, err := finder.(*tzf.Finder).GetTimezoneShapeByName("Asia/Shanghai")
 	fmt.Printf("%v %v\n", pbtz.Name, err)
 	// Output: Asia/Shanghai <nil>
 }
@@ -156,7 +156,7 @@ func ExampleFinder_GetTimezoneShapeByShift() {
 		panic(err)
 	}
 	finder, _ := tzf.NewFinderFromPB(input)
-	pbtzs, _ := finder.GetTimezoneShapeByShift(28800)
+	pbtzs, _ := finder.(*tzf.Finder).GetTimezoneShapeByShift(28800)
 
 	pbnames := make([]string, 0)
 	for _, pbtz := range pbtzs {