diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index abb0fcb..b75dc6a 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -58,6 +58,7 @@
 class NativeHandle;
 class Region;
 class String8;
+class TestableSurfaceFlinger;
 
 namespace Hwc2 {
 class Composer;
@@ -170,6 +171,9 @@
 
     std::optional<hwc2_display_t> getHwcDisplayId(int32_t displayId) const;
 private:
+    // For unit tests
+    friend TestableSurfaceFlinger;
+
     static const int32_t VIRTUAL_DISPLAY_ID_BASE = 2;
 
     bool isValidDisplay(int32_t displayId) const;