you have to use the capabilities like
caps.setCapability("deviceName", "Android Emulator");
caps.setCapability("deviceOrientation", "portrait");
caps.setCapability("platformVersion", "5.1");
caps.setCapability("platformName", "Android");
caps.setCapability("appActivity", androidConfig.getProperty("appActivity"));
caps.setCapability("appPackage", androidConfig.getProperty("appPakcage"));
//give the absolute path of your apk file
capabilities.setCapability("app", appdir.getAbsolutePath());
driver = new AndroidDriver(new URL("http://127.0.0.1:4723/wd/hub"), capabilities);