试图用数组文本填充列表视图

Question

我有一个列表视图，我试图通过将行 xml 的文本更改为文件夹中的文件路径来填充它（这样我以后可以对文件执行操作。我在启动活动时遇到了崩溃。

我的代码： FileView.java

package com.loopbreakr.firstpdf;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.io.File;
import java.util.List;

public class FileView extends AppCompatActivity {
    ListView listView;
    String filePath = "";

    List<String> stringList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file_view);
        listView = findViewById(R.id.listView);

        filePath = "PDF_files";
        File file = new File(getExternalFilesDir(filePath).toString());
        File[] fileList = file.listFiles();

        for (int i = 0; i < fileList.length; i++) {
            stringList.add(fileList[i].getName());
        }
        String[] modded = (String[]) stringList.toArray();

        MyAdapter adapter = new MyAdapter(this, modded);
        listView.setAdapter(adapter);
        
    }

    class MyAdapter extends ArrayAdapter<String> {

        Context context;
        String rTitle[];

        MyAdapter(Context c, String title[]) {
            super(c, R.layout.row, R.id.activityTitle, title);
            this.context = c;
            this.rTitle = title;
        }

        @NonNull
        @Override
        public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
            LayoutInflater layoutInflater = (LayoutInflater)getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            View row = layoutInflater.inflate(R.layout.row, parent, false);
            TextView myTitle = row.findViewById(R.id.activityTitle);
            
            myTitle.setText(rTitle[position]);
            return row;
        }
    }
}

activity_file_view.xml

<?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"
    android:orientation="vertical"
    tools:context=".FileView">
    
    <ListView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/listView"/>

</LinearLayout>

行.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="16dp">

    <ImageView
        android:layout_width="75dp"
        android:layout_height="75dp"
        android:src="@mipmap/ic_launcher"
        android:id="@+id/image"
        />
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">
            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:id="@+id/activityTitle"
                android:text="Main Title"
                android:textColor="#000"
                style="bold"
                android:layout_margin="5dp"
                android:textSize="20sp"/>
    </LinearLayout>
</LinearLayout>

我收到此错误：

2021-01-26 01:49:46.447 22787-22787/com.loopbreakr.firstpdf E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.loopbreakr.firstpdf, PID: 22787
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.loopbreakr.firstpdf/com.loopbreakr.firstpdf.FileView}: java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference
        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: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference
        at com.loopbreakr.firstpdf.FileView.onCreate(FileView.java:35)
        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)

似乎是一个空对象引用，但我不确定如何解决

Answer 1

问题的根源是stringList.add(fileList[i].getName());，其中stringList 未初始化。所以尝试以下，

listView = findViewById(R.id.listView);

// initialization code
stringList = new ArrayList();

String[] modded = (String[]) stringList.toArray(); // comment this line out

// pass the ArrayList instead of Array 
MyAdapter adapter = new MyAdapter(this, stringList);


//rest of the logic remains same

编辑：

虽然您的适配器似乎可以工作，但膨胀逻辑是错误的，因为您没有回收视图，而是仅为要显示的每个数据项创建 新视图，因此请尝试以下操作

// recommended adapter code
    class MyAdapter extends ArrayAdapter<String> {

        Context context;
        List<String> rTitle;

        MyAdapter(Context c, List<String> title) {
// since you will be inflating views manually, you don't need to pass any layoutId instead just pass 0
            super(c, 0 , title);
            this.context = c;
            this.rTitle = title;
        }

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

// convertView contains the old used view if any, else create a new view

          if(convertView == null){
            LayoutInflater layoutInflater = (LayoutInflater)getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = layoutInflater.inflate(R.layout.row, parent, false);
}

            TextView myTitle = convertView.findViewById(R.id.activityTitle);
            
            myTitle.setText(rTitle.get(position));

//return convertview
            return convertView;
        }
    }

Answer 2

你需要初始化你的数组列表

List<String> stringList = new ArrayList<>();