How one extends JNA interface mappings? (Java)

User32 interface (platform library) is missing some WinAPI functions, so I tried extending it:

package myapp
import com.sun.jna.platform.win32.W32API

public interface User32 extends com.sun.jna.platform.win32.User32 {
    myapp.User32 INSTANCE
    boolean IsWindow(W32API.HWND hWnd)
}

But then calling myapp.User32.INSTANCE.FindWindow(..) results in java.lang.N开发者_如何学GoullPointerException: Cannot invoke method FindWindow() on null object

Your declaration of IsWindow is correct (I am using an identical declaration in my current project.)

Most likely you are failing to initialise INSTANCE.