Android AlertDialog 实例

public boolean onKeyDown(int keyCode, KeyEvent event)
    {
        if(keyCode == KeyEvent.KEYCODE_BACK)
        {
            AlertDialog.Builder alert = new AlertDialog.Builder(cxt);
            alert.setTitle("退出提示");
            alert.setMessage("确认退出吗？");
            alert.setNegativeButton("取消", new DialogInterface.OnClickListener() {
                
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    
                }
            });
            alert.setPositiveButton("确认", new DialogInterface.OnClickListener() {
                
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    System.exit(0);                    
                }
            });
            alert.show();
        }
        return false;
    }

xtc在类里面定义private Context cxt = null;