为什么白屏啊，Android stuio中运行的，求大佬指点

Question

package arrayadapter.listview.immoc.myapplication001;

import android.app.Activity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class MainActivity extends Activity{
    private ListView listView1;
    private ArrayAdapter<String> arr_adapter;
    private SimpleAdapter sim_adpter;
    private List<Map<String , Object>>  datalist;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        listView1=(ListView)findViewById(R.id.listview);
//        新建一个数据源适配器
//        新建一个数据源
        String[]arr_data ={"慕课1","慕课2","慕课3","慕课4","慕课5","慕课6"};
       arr_adapter =new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,arr_data);
//        视图加载适配器
//        listView1.setAdapter(arr_adapter);
         sim_adpter=new SimpleAdapter(this,getData(),R.layout.item ,new String [] {"pic","txt"},new int [] {R.id.pic,R.id.txt});
         listView1.setAdapter(sim_adpter);
    }
    private List<Map<String,Object>> getData()
    {
        for(int i=0;i<20;i++)
        {
            Map<String,Object>map = new HashMap<String,Object>();
            map.put("pic",R.mipmap.ic_launcher);
            map.put("txt","慕课网"+i+1);
            datalist.add(map);
        }
        return datalist;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">

    <ImageView
        android:id="@+id/pic"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:layout_margin="15dp"
        android:src="@mipmap/ic_launcher"
        />
    <TextView
        android:id="@+id/txt"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:text="demo"
        />
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical"
    >

    <ListView
        android:id="@+id/listview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        ></ListView>

</LinearLayout>

Answer 1

for(int i=0;i<20;i++)         {             Map<String,Object>map = new HashMap<String,Object>();             map.put("pic",R.mipmap.ic_launcher);             map.put("txt","慕课网"+i+1);             datalist.add(map);         }         return datalist;没初始化吧，for前先datalist=new List();