android程序 点击两次返回键程序退出

private static Boolean isExit = false;  
private static Boolean hasTask = false;  
    Timer tExit = new Timer();  
    TimerTask task = new TimerTask() {  

        @Override  
public void run() {  
            isExit = false;  
            hasTask = true;  
        }  
    };  


public boolean onKeyDown(int keyCode, KeyEvent event) {  
// TODO Auto-generated method stub  
                if(keyCode == KeyEvent.KEYCODE_BACK){  
//                        System.out.println("user back down");  
                        if(isExit == false ) {  
                                isExit = true;  
                                Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();  
if(!hasTask) {  
                                        tExit.schedule(task, 2000);  
                                }} else {  
                                                                                }  
                                finish();  
                                System.exit(0);  
                        }  
                }                          
return false;  
        }