为什么面板(JPanel)不能响应键盘事件

Question

InputMap mapaEntrada;
ActionMap mapaAction ;
...
 mapaEntrada= getInputMap(WHEN_IN_FOCUSED_WINDOW);
 mapaAction = getActionMap();
 mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_LEFT,0),"LeftKey");
 mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT,0),"RightKey");
 mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_PAGE_UP,0),"PageUp");
 mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_PAGE_DOWN,0),"PageDown");
 mapaAction.put("LeftKey",hkLeft);
 mapaAction.put("RightKey",hkRight);
 mapaAction.put("PageUp",hkPageUp);
 mapaAction.put("PageDown",hkPageDown);