How to call java function in .java from javascript?

I am currently using cordova/phonegap in eclipse. I want to call the saveToken (token) in my MyFirebaseInstanceIDService.java from the JavaScript in my html file. i created a class called JavaScriptInterface.java to get the class from MyFirebaseInstanceIDService.java. How should i do this?

MyFirebaseInstanceIDService.java

public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService {

// [START refresh_token]
@Override
public void onTokenRefresh() {
    // Get updated InstanceID token.
    String token = FirebaseInstanceId.getInstance().getToken();
    Log.d(TAG, "Refreshed token: " + token);
    saveToken(token);



}

  }

JavaScriptInterface.java

public class JavaScriptInterface {
Context mContext;


JavaScriptInterface(Context c) {
   mContext = c;
}

@JavascriptInterface
public void postToken(){
    MyFirebaseInstanceIDService service = new MyFirebaseInstanceIDService();
    service.postToken();
   }
}

MainActivity.java

public class MainActivity extends CordovaActivity
{    
   loadUrl(launchUrl);
  WebView webView = (WebView) findViewById(R.id.webView1);
    webView.addJavascriptInterface(new JavaScriptInterface(this), "Token");
    }
  }

Index.html

   <script>
    function postToken(){

        service.postToken();

    }
</script>