Java 试图用数组文本填充listview

我有一个listview，我正试图通过将行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）。启动活动时，我会遇到崩溃

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;
        }
    }
}

---

似乎是空对象引用，但我不确定如何解决

您需要初始化数组列表

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

List-stringList=new-ArrayList（）；

问题的根源是

stringList.add（fileList[i].getName（））；

其中

stringList

未初始化。请尝试以下操作

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，而只是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;
        }
    }

//推荐的适配器代码
类MyAdapter扩展了ArrayAdapter{
语境；
列表标题；
MyAdapter（上下文c，列表标题）{
//由于您将手动膨胀视图，因此不需要传递任何layoutId，只需传递0即可
超级（c，0，标题）；
this.context=c；
this.rTitle=标题；
}
@非空
@凌驾
公共视图getView（int位置，@Nullable视图convertView，@NonNull视图组父级）{
//convertView包含旧使用的视图（如果有），否则创建新视图
if（convertView==null）{
LayoutInflater LayoutInflater=（LayoutInflater）getApplicationContext（）.getSystemService（Context.LAYOUT\u INFLATER\u SERVICE）；
convertView=LayoutFlater.充气（R.layout.row，父级，false）；
}
TextView myTitle=convertView.findViewById（R.id.activityTitle）；
myTitle.setText（rTitle.get（position））；
//返回转换视图
返回视图；
}
}

感谢您在逻辑上的更正。尽管我使用您的代码时仍然遇到了崩溃：2021-01-26 13:42:38.939 24325-24325/com.loopbreakr.firstpdf E/AndroidRuntime:FATAL EXCEPTION:main Process:com.loopbreakr.firstpdf，PID:24325 java.lang.RuntimeException:无法启动活动组件信息{com.loopbreakr.firstpdf/com.loopbreakr.firstpdf.FileView}:java.lang.ClassCastException:java.lang.Object[]不能强制转换为java.lang.String[]现在检查答案，更改的要点-直接将列表与适配器一起使用，而不是字符串[]

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

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

// 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;
        }
    }