There is HashMap 'regularKeyCodesMap' in sun.awt.ExtendedKeyCodes class. It contains only non-null keys and values.
t means we can replace containsKey+get with get+null check. Or with single getOrDefault call.
It's clearer and a bit faster.

     public static final int getExtendedKeyCodeForChar( int c ) {
         int uc = Character.toUpperCase( c );
         int lc = Character.toLowerCase( c );
         if (regularKeyCodesMap.containsKey( c )) {
             if(regularKeyCodesMap.containsKey(uc)) {
                 return regularKeyCodesMap.get( uc );
             }
             return regularKeyCodesMap.get( c );
         }
         uc += 0x01000000;
         lc += 0x01000000;
         if (extendedKeyCodesSet.contains( uc )) {
             return uc;
         }else if (extendedKeyCodesSet.contains( lc )) {
             return lc;
         }
         return KeyEvent.VK_UNDEFINED;
     }