java - 自定义 ListView 中的空指针异常 View 持有者

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

package com.example.ahmadal_bargouthy.project1;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;

import static android.app.PendingIntent.getActivity;

public class MainActivity extends AppCompatActivity {


int[] IMAGES = {R.drawable.img1,R.drawable.img2,R.drawable.img3,R.drawable.img4,R.drawable.img5 ,R.drawable.img6,R.drawable.img7,R.drawable.img8,R.drawable.img9,R.drawable.img10};

    String[] NAMES = {"Lighter Watch","MINI TRACKER ANTI-LOST","FUNNY LION WIG","ROCKET LAUNCHER",
    "NEW SMART WATCH","LINE REWIND","LUMINOUS TEMPERED GLASS ","E58 DRONE WIFI","LED POWER MASK","PAINT SKULL 3D PRINTED HOODIES"};

    String[] DESC = {"Flameless windproof","SMART BLUETOOTH 4.0 TRACER GPS LOCATOR","FUNNY LION WIG HEAD CAP HAT WITH EAR",
            "ROCKET LAUNCHER DEFAULT GIRL SKIN DEFAULT SKIN FORTNITE","MEN WOMEN HEART RATE MONITOR BLOOD PRESSURE FITNESS TRACKER",
    "LINE REWIND 24K GOLD SERUM","CASE FOR IPHONE","FPV ANGLE HD CAMERA HIGH HOLD MODE"," "," "};

    String[] PRICE = {"20","15","12.95","25.98","27.98","23.99","14.95","24.95","38.18"};

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

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

        CustomAdapter customAdapter = new CustomAdapter(this,NAMES,DESC,PRICE,IMAGES);

        listView.setAdapter(customAdapter);

        Intent i = getIntent();
        String userName = i.getStringExtra("User_Name");
        Toast toast = Toast.makeText(this,"Hello " + userName,Toast.LENGTH_LONG);
        toast.show();
        setTitle("Welcome " + userName);
    }

    class CustomAdapter extends ArrayAdapter<String>{

        private String[] NAMES;
        private String[] DESC;
        private String[] PRICE;
        private int[] IMAGES;

        private Activity context;

        public CustomAdapter(Activity context, String[]NAMES, String[] DESC , String[] PRICE, int[] IMAGES) {
            super(context, R.layout.customlayout,NAMES);

            this.context = context;

            this.NAMES = NAMES;
            this.DESC  = DESC;
            this.PRICE = PRICE;
            this.IMAGES = IMAGES;
        }

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

                ViewHolder viewHolder = null;

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

            }

                ViewHolder vm = new ViewHolder();

                //vm.imageView.setImageResource(IMAGES[position]);

            int x = R.drawable.ic_launcher_background;

                viewHolder.imageView.setImageResource(x);
                viewHolder.nameTxt.setText(NAMES[position]);
                viewHolder.descTxt.setText(DESC[position]);
                viewHolder.priceTxt.setText(PRICE[position]);



            return r;
        }

        class ViewHolder{
            TextView nameTxt = (TextView)findViewById(R.id.Name);
            TextView descTxt = (TextView)findViewById(R.id.Desc);
            TextView priceTxt = (TextView)findViewById(R.id.Price);

            ImageView imageView =(ImageView)findViewById(R.id.imageView2);
        }
    }
}

这是我的代码，我想使用 ArrayAdapter 和 ViewHolder 创建自定义 ListView

我创建了 customAdapter 类，并在 GetView 方法中尝试从数组(IMAGES、NAMES、DESC、PRICE)获取数据

当我运行代码 java.lang.NullPointerException 错误时出现

  java.lang.NullPointerException: Attempt to read from field 'android.widget.ImageView com.example.ahmadal_bargouthy.project1.MainActivity$CustomAdapter$ViewHolder.imageView' on a null object reference
        at com.example.ahmadal_bargouthy.project1.MainActivity$CustomAdapter.getView(MainActivity.java:98)
        at android.widget.AbsListView.obtainView(AbsListView.java:2366)
        at android.widget.ListView.makeAndAddView(ListView.java:2052)
        at android.widget.ListView.fillDown(ListView.java:786)
        at android.widget.ListView.fillFromTop(ListView.java:847)
        at android.widget.ListView.layoutChildren(ListView.java:1826)
        at android.widget.AbsListView.onLayout(AbsListView.java:2165)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:444)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1812)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1656)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1565)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:753)
        at android.view.View.layout(View.java:20672)
        at android.view.ViewGroup.layout(ViewGroup.java:6194)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2792)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2319)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1460)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7183)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:949)
        at android.view.Choreographer.doCallbacks(Choreographer.java:761)
        at android.view.Choreographer.doFrame(Choreographer.java:696)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:935)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

请帮帮我

Answer 1

if (r==null){
    LayoutInflater layoutInflater = context.getLayoutInflater();

    r=layoutInflater.inflate(R.layout.customlayout,null,true);

            viewHolder.nameTxt = r.findViewById(R.id.Name);
            viewHolder.descTxt = r.findViewById(R.id.Desc);
            viewHolder.priceTxt = r.findViewById(R.id.Price);
            vewHolder.imageView = r.findViewById(R.id.imageView2);

                    r.setTag(viewHolder);
                }else{
                    viewHolder = (ViewHolder)r.getTag();
                }


       class ViewHolder{
            TextView nameTxt;
            TextView descTxt;
            TextView priceTxt;
            ImageView imageView;
        }