Java 如何在android sdk中创建简单类？

我在Eclipse中借助android SDK创建了一个简单的android项目，我的代码是：

package com.example.deomanapp;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.location.Location;
import android.location.LocationManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import android.os.Build;
import android.provider.SyncStateContract.Helpers;

public class MainActivity extends ActionBarActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }


    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }


        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            final View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            Button b = (Button) rootView.findViewById(R.id.button1);
            b.setOnClickListener(new OnClickListener() {

                Mhelper mhelper = new Mhelper();


            @Override
            public void onClick(View v) 
            {
                mhelper.mContext();
                TextView tx = (TextView) rootView.findViewById(R.id.textView1);
                tx.setText("Hi");

                Toast.makeText(getActivity(), "ssss", Toast.LENGTH_SHORT).show();
            }
         });
            return rootView;
        }
    }

}

我想在其他类中编写更多代码，因此我在项目中创建了helper类： 包com.example.deomanapp

import com.example.deomanapp.MainActivity.PlaceholderFragment;

import android.app.Activity;
import android.content.Context;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.widget.Toast;

public class Mhelper {


    public void mContext(Context context)
    {
        LocationManager lm;
        lm   = (LocationManager)getSystemService(context.LOCATION_SERVICE);
        Location location = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
        double longitude = location.getLongitude();
        double latitude = location.getLatitude();

    }
}

我的问题是，我无法将上下文传递到类的任何部分或mhelper.mContext（）中（它需要Contex参数）

我怎样才能把button onClick带到其他班级？或者我怎样才能使这个代码工作？我的新类是否应该从其他类扩展

编辑：

日志：

你能行

 mhelper.mContext(getActivity());

然后

  lm   = (LocationManager)context.getSystemService(context.LOCATION_SERVICE); 

---

它没有给我任何错误，但是程序在使用nullpointexeption单击按钮时崩溃。@Shombol shagol发布stacktrace。它将指示原因04-20 01:18:29.526:E/AndroidRuntime（1130）：java.lang。NullPointerException@Shombol-shagol需要完整的stacktrace。my guess

getLastKnownLocation

返回空值。由你来纠正that@Shombol-shagol

Mhelper.java

中的第22行是什么？请遵循java惯例并以大写字母开头类名。小写字母表示变量名和方法名。我把它改为大写，但还是有同样的错误。@Shombol shagol崩溃不是因为这个。建议在命名类时遵循一些惯例。那是一件和这件事无关的事crash@Shombol-shagol在

Mhelper.java

@Shombol shagol中，第22行是什么？您是否进行了此更改

lm=（LocationManager）context.getSystemService（context.LOCATION\u SERVICE）？？。您仍然有
lm=（LocationManager）getSystemService（context.LOCATION\u服务）
  lm   = (LocationManager)context.getSystemService(context.LOCATION_SERVICE); 

public final Activity getActivity ()

Added in API level 11
Return the Activity this fragment is currently associated with.