accessing android sdk in unity3d

I need to use android sdk in my u3d game. here's my java code:

 package com.fy.game;
 …………
 public class MainActivity extends UnityPlayerActivity {
     public void InitCharge(final int _charge_num){
         runOnUiThread(new Runnable(){
             public void run(){
             //init
             }
         }
     }
 }

here's my AndroidMainifest.xml

 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
 package="com.fy.game"
 android:versionCode="1"
 android:versionName="1.0"
 android:installLocation="preferExternal">
 …………
 <activity android:name="com.fy.game.MainActivity"
 android:label="@string/app_name"
 android:screenOrientation="landscape"
 android:configChanges="fontScale|keyboard|keyboardHidden|locale|mnc|mcc|navigation|orientation|screenLayout|screenSize|smallestScreenSize|uiMode|touchscreen">
 <intent-filter>
 <action android:name="android.intent.action.MAIN" />
 <category android:name="android.intent.category.LAUNCHER" />
 </intent-filter>
 </activity>
 …………

my c# code:

 AndroidJavaClass ajc = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
 AndroidJavaObject activity = ajc.GetStatic("currentActivity");
 activity.Call("InitCharge", new int[]{100});

and I've already put jar file,res folder,libs folder into Plugins/Android/ but when it run,there comes Exception: Exception: java.lang.NoSuchMethodException

then I changed my c# code:

 AndroidJavaObject activity = new AndroidJavaObject("com.fy.game.MainActivity");
 activity.Call("InitCharge", new int[]{100});

and Exception become: Exception: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

please tell me how can I solve this problem.thx a lot!