quick-cocos2d-x想制作一个退出游戏的按钮，怎么实现

function MainScene:addBackEvent()
if device.platform == "android" then
self.touchLayer = display.newLayer()
self.touchLayer:addNodeEventListener(cc.KEYPAD_EVENT, function(event)
if event.key == "back" then
--CCDirector:sharedDirector():endToLua()
local javaClassName = "com/cocos2dx/testgame/Testgame"
local javaMethodName = "exit"
luaj.callStaticMethod(javaClassName, javaMethodName)
end
end)
self.touchLayer:setKeypadEnabled(true)
self:addChild(self.touchLayer)
end
end