无法启动活动：java.lang.NullPointerException

Question

我试图从数据库中检索员工数据。我在数据库连接期间遇到这些错误。点击按钮后，出现这些错误：

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.brs, PID: 15799
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.brs/com.example.myhospital.AboutUs}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException
        at java.util.Objects.requireNonNull(Objects.java:220)
        at java.util.Arrays$ArrayList.<init>(Arrays.java:3738)
        at java.util.Arrays.asList(Arrays.java:3725)
        at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:165)
        at com.example.myhospital.AboutUsListView.<init>(AboutUsListView.java:39)
        at com.example.myhospital.AboutUs.onCreate(AboutUs.java:46)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
I/Process: Sending signal. PID: 15799 SIG: 9

这是 about us 活动的 Java 代码，用于在列表视图中显示图像和一些数据。

关于我们.java

package com.example.myhospital;
import android.os.Bundle;
import android.os.StrictMode;
import android.widget.ListView;
import androidx.appcompat.app.AppCompatActivity;
import org.json.JSONArray;
import org.json.JSONObject;
import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
public class AboutUs extends AppCompatActivity {
    String[] name;
    String[] email;
    String[] imagepath;
    ListView listView;
    BufferedInputStream is;
    String line=null;
    String result=null;

    String  urladdress = "http://192.168.1.66/brs/team.php";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_about_us);


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


        StrictMode.setThreadPolicy((new StrictMode.ThreadPolicy.Builder().permitNetwork().build()));
        collectData();

        AboutUsListView aboutUsListView=new AboutUsListView(this,name,email,imagepath);
        listView.setAdapter(aboutUsListView);


    }


    private void collectData()
    {
        try{

            URL url=new URL(urladdress);
            HttpURLConnection con=(HttpURLConnection)url.openConnection();
            con.setRequestMethod("GET");
            is=new BufferedInputStream(con.getInputStream());

        }
        catch (Exception ex)
        {
            ex.printStackTrace();
        }
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(is));
            StringBuilder sb=new StringBuilder();
            while ((line=br.readLine())!=null){
                sb.append(line+"\n");
            }
            is.close();
            result=sb.toString();

        }
        catch (Exception ex)
        {
            ex.printStackTrace();

        }

        try{
            JSONArray ja=new JSONArray(result);
            JSONObject jo=null;
            name=new String[ja.length()];
            email=new String[ja.length()];
            imagepath=new String[ja.length()];

            for(int i=0;i<=ja.length();i++){
                jo=ja.getJSONObject(i);
                name[i]=jo.getString("name");
                email[i]=jo.getString("email");
                imagepath[i]=jo.getString("photo");
            }
        }
        catch (Exception ex)
        {

            ex.printStackTrace();
        }




    }
}

关于我们ListView.java

package com.example.myhospital;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;

import java.io.InputStream;

public class AboutUsListView extends ArrayAdapter<String> {
    private String[] name;
    private String[] email;
    private String[] imagepath;
    private Activity context;
    Bitmap bitmap;


   

    public AboutUsListView (Activity context, String[] name, String[] email, String[] imagepath) {
        super(context, R.layout.layout, name);
        this.context = context;
        this.name = name;
        this.email = email;
        this.imagepath = imagepath;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent){
        View r=convertView;

        ViewHolder viewHolder=null;
        if(r==null){
            LayoutInflater layoutInflater=context.getLayoutInflater();
            r=layoutInflater.inflate(R.layout.layout,null,true);
            viewHolder=new ViewHolder(r);
            r.setTag(viewHolder);
        }
        else {
            viewHolder=(ViewHolder)r.getTag();

        }

        viewHolder.tvw1.setText(name[position]);
        viewHolder.tvw2.setText(email[position]);
        new GetImageFromURL(viewHolder.ivw).execute(imagepath[position]);

        return r;
    }
    class ViewHolder{

        TextView tvw1;
        TextView tvw2;
        ImageView ivw;

        ViewHolder(View v){
            tvw1=(TextView)v.findViewById(R.id.dname);
            tvw2=(TextView)v.findViewById(R.id.dphone);
            ivw=(ImageView)v.findViewById(R.id.imageView);
        }

    }

    public class GetImageFromURL extends AsyncTask<String,Void, Bitmap>
    {
        ImageView imgView;
        public GetImageFromURL(ImageView imgv)
        {
            this.imgView=imgv;
        }
        @Override
        protected Bitmap doInBackground(String... url) {


                String urldisplay = url[0];
                bitmap = null;
            try{

                InputStream ist=new java.net.URL(urldisplay).openStream();
                bitmap= BitmapFactory.decodeStream(ist);
            }
            catch (Exception ex)
            {
                ex.printStackTrace();
            }

                return bitmap;
            }

        @Override
        protected void onPostExecute(Bitmap bitmap){

            super.onPostExecute(bitmap);
            imgView.setImageBitmap(bitmap);
        }

        }
    }

这是我的关于我们页面的 XML 代码

关于我们.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:background="#DFD8D8"
    tools:context=".AboutUs">

    <TextView
        android:id="@+id/textView"
        android:layout_width="418dp"
        android:layout_height="62dp"
        android:background="@color/red"
        android:text="Available Ambulance Details"
        android:textAlignment="center"
        android:textColor="@android:color/background_light"
        android:textSize="30sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0" />

    <ListView
        android:id="@+id/lview"
        android:layout_width="368dp"
        android:layout_height="495dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.488"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:layout_constraintVertical_bias="0.477" />

</androidx.constraintlayout.widget.ConstraintLayout>

这是我用于布局的 XML 代码。

布局.xml

<?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">


    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="5dp"
        android:background="#ffffff"
        android:padding="15dp">

        <ImageView
            android:id="@+id/imageView"
            android:layout_width="70dp"
            android:layout_height="86dp"
            android:layout_weight="1" />

        <TextView
            android:id="@+id/dname"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_marginLeft="26dp"
            android:layout_marginStart="26dp"
            android:layout_marginTop="16dp"
            android:layout_toEndOf="@id/imageView"
            android:layout_toRightOf="@id/imageView"
            android:textColor="#333"
            android:textSize="22dp"
            />

        <TextView
            android:id="@+id/dphone"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_marginLeft="26dp"
            android:layout_marginStart="26dp"
            android:layout_marginTop="50dp"
            android:layout_toEndOf="@id/imageView"
            android:layout_toRightOf="@id/imageView"
            android:textColor="#333"
            android:textSize="22dp"
            />

    </RelativeLayout>

</LinearLayout>

Answer 1

您正在使用空值初始化 AboutUsListView 类。 ArrayAdapter 的构造不允许 documentation 中所述的空值：

<块引用>

objects T：要在 ListView 中表示的对象。此值不能为空。

您应该检查名称数组是否为空并将其替换为空数组：

public AboutUsListView (Activity context, String[] name, String[] email, String[] imagepath) {
    super(context, R.layout.layout, name==null?new String[0]:name);
    this.context = context;
    this.name = name;
    this.email = email;
    this.imagepath = imagepath;
}