Java Native Access (JNA) class Vs interface

private interface User32 extends StdCallLibrary {
 static final User32 INSTANCE = (User32)Native.load (
  "user32", User32.class, W32APIOptions.DEFAULT_OPTIONS
 );
 class WINDOWPLACEMENT extends Structure {
  public int length;
  public int flags;
  public int showCmd;
  public POINT ptMinPosition;
  public POINT ptMaxPosition;
  public RECT rcNormalPosition;
  private WINDOWPLACEMENT(){
   length = size();
   
  // Lignes totalement inutiles, juste pour ne pas avoir de @SuppressWarnings("unused")
   List<Object>_null = Arrays.asList (
    length, flags, showCmd, ptMinPosition, ptMaxPosition, rcNormalPosition
   );
   _null.isEmpty();
  //===================================================================================
  }
  @Override
  protected List<String>getFieldOrder(){
   return Arrays.asList (
    "length", "flags", "showCmd", "ptMinPosition", "ptMaxPosition", "rcNormalPosition"
   );
  }
 }
 int GetWindowRect(HWND h_wnd, int _rect[]);
 int GetWindowTextA(HWND h_wnd, byte lp_string[], int n_max_count);
 boolean EnumWindows(WNDENUMPROC lp_enum_func, Pointer _pointer);
 boolean IsWindowVisible(HWND h_wnd);
 UINT SendInput(DWORD cInputs, INPUT pInputs[], int cbSize);
 BOOL SetForegroundWindow(HWND h_wnd);
 BOOL GetWindowPlacement(HWND h_wnd, WINDOWPLACEMENT lp_wnd_pl);
 BOOL SetWindowPlacement(HWND h_wnd, WINDOWPLACEMENT lp_wnd_pl);
 BOOL SetWindowPos (
  HWND h_wnd, HWND h_wnd_insert_after, int _x, int _y, int _cx, int _cy, int u_flag
 );
}

class

interface

private static class User32 {
 static {
  Native.register("user32");
 }
 public static class WINDOWPLACEMENT extends Structure {
  public int length;
  public int flags;
  public int showCmd;
  public POINT ptMinPosition;
  public POINT ptMaxPosition;
  public RECT rcNormalPosition;
  public WINDOWPLACEMENT(){
   length = size();
   
   // Lignes totalement inutiles, juste pour ne pas avoir de @SuppressWarnings("unused")
   List<Object>_null = Arrays.asList (
    length, flags, showCmd, ptMinPosition, ptMaxPosition, rcNormalPosition
    );
   _null.isEmpty();
   //===================================================================================
  }
  @Override
  protected List<String>getFieldOrder(){
   return Arrays.asList (
    "length", "flags", "showCmd", "ptMinPosition", "ptMaxPosition", "rcNormalPosition"
   );
  }
 }
 private static native BaseTSD.LONG_PTR SetWindowLongPtrA(HWND hWnd, int nIndex, BaseTSD.LONG_PTR dwNewLongPtr);
 private static native HWND GetForegroundWindow();
 //private static native HHOOK SetWindowsHookEx(int idHook, HOOKPROC lpfn, HINSTANCE hMod, int dwThreadId);
 //private static native UINT SendInput(DWORD cInputs, INPUT pInputs[], int cbSize);
 private static native BOOL GetWindowPlacement(HWND h_wnd, WINDOWPLACEMENT lp_wnd_pl);
 private static native BOOL SetWindowPlacement(HWND h_wnd, WINDOWPLACEMENT lp_wnd_pl);
 private static native Pointer SetWindowLongA(HWND hWnd, int nIndex, Pointer dwNewLong);
 private static native Pointer SetWindowLongPtrA(HWND hWnd, int nIndex, Pointer dwNewLongPtr);
 private static native LRESULT CallNextHookEx(HHOOK hhk, int nCode, WPARAM wParam, Pointer lParam);
 private static native LRESULT CallNextHookEx(HHOOK hhk, int nCode, WPARAM wParam, LPARAM lParam);
 private static native LRESULT DefWindowProcA(HWND hWnd, int Msg, WPARAM wParam, LPARAM lParam);
// private static native LRESULT DispatchMessage(MSG lpMsg);
 private static native void PostMessageA(HWND hWnd, int msg, WPARAM wParam, LPARAM lParam);
 private static native void PostQuitMessage(int nExitCode);
// private static native int GetMessage(MSG lpMsg, HWND hWnd, int wMsgFilterMin, int wMsgFilterMax);
 private static native int GetWindowTextLengthA(HWND hWnd);
 private static native int GetWindowTextA(HWND hWnd, char[] lpString, int nMaxCount);
 private static native int GetWindowThreadProcessId(HWND hWnd, IntByReference lpdwProcessId);
 private static native int SetWindowLongA(HWND hWnd, int nIndex, int dwNewLong);
 private static native boolean EnumWindows(WNDENUMPROC lpEnumFunc, Pointer data);
 private static native boolean GetWindowRect(HWND hWnd, RECT rect);
 private static native boolean IsWindowVisible(HWND hWnd);
 private static native boolean PeekMessageA(MSG lpMsg, HWND hWnd, int wMsgFilterMin, int wMsgFilterMax, int wRemoveMsg);
 private static native boolean SetForegroundWindow(HWND hWnd);
 private static native boolean SetWindowPos(HWND hWnd, HWND hWndInsertAfter, int X, int Y, int cx, int cy, int uFlags);
 private static native boolean ShowWindow(HWND hWnd, int nCmdShow);
 private static native boolean TranslateMessage(MSG lpMsg);
 private static native boolean UnhookWindowsHookEx(HHOOK hhk);
}

SetWindowLongPtr

DefWindowProc

PostMessage

GetWindowTextLength

GetWindowText

SetWindowLong

PeekMessage

class

HHOOK

HHOOK SetWindowsHookEx(int idHook, HOOKPROC lpfn, HINSTANCE hMod, int dwThreadId)

LRESULT DispatchMessage(MSG lpMsg)

int GetMessage(MSG lpMsg, HWND hWnd, int wMsgFilterMin, int wMsgFilterMax)

interface

class

UINT SendInput(DWORD cInputs, INPUT pInputs[], int cbSize)

SetForegroundWindow(HWND hWnd)

INPUT