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();