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

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

Most likely you are failing to initialise INSTANCE.