Android空指针异常问题，新手求教

package com.stu.sqlite;



import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;



import com.stu.bean.Student;

import com.stu.service.StudentService;



import android.app.Activity;

import android.os.Bundle;

import android.widget.ListView;

import android.widget.SimpleAdapter;



public class MainActivity extends Activity {

	private StudentService studentService=null;

	private ListView listView=null;

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        studentService=new StudentService(getApplicationContext());

        

        listView=(ListView)findViewById(R.id.listView);

        

        show();

    }



	private void show() {

		List<Student> students=studentService.getScrollData(0,20);

		

		List<Map<String,Object>> data=new ArrayList<Map<String,Object>>();

		

		for(Student student:students)

		{

			Map<String,Object> map=new HashMap<String, Object>();

			map.put("name",student.getName());

			map.put("age",student.getAge());

			map.put("money",student.getMoney());

			map.put("id",student.getId());

			data.add(map);

		}

	

		//创建SimpleAdapter对象

		SimpleAdapter adapter=new SimpleAdapter(getApplicationContext(), data,R.layout.item,

				new String[]{"name","age","money"},new int[]{R.id.name,R.id.age,R.id.money});

		

		listView.setAdapter(adapter);//这个地方提示空指针异常

		

	}

}