写出Mainactivity文件和布局文件实现：a)文件写入，在输入框中输入内容，在文件名输入框中输入文件名，点击SAVE按钮，弹出文件保存成功信息提示。 b)文件读出，点击READ按钮，弹出app私有目录下的文件列表，选择列表项，读取文件内容和文件名在内容显示框文件名显示框中显示。a)在上述的布局下，增加PICTURE按钮和图片显示控件，点击按钮，显示照片选择页面，选择照片后，将照片现在图片显示控件上。

MainActivity.java：

import android.content.Context;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    EditText editTextContent, editTextFileName;
    Button buttonSave, buttonRead, buttonPicture;
    TextView textViewContent, textViewFileName;

    private static final int REQUEST_CODE_SELECT_PICTURE = 1;

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

        editTextContent = findViewById(R.id.editTextContent);
        editTextFileName = findViewById(R.id.editTextFileName);
        buttonSave = findViewById(R.id.buttonSave);
        buttonRead = findViewById(R.id.buttonRead);
        buttonPicture = findViewById(R.id.buttonPicture);
        textViewContent = findViewById(R.id.textViewContent);
        textViewFileName = findViewById(R.id.textViewFileName);

        buttonSave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String content = editTextContent.getText().toString();
                String fileName = editTextFileName.getText().toString();
                if (content.isEmpty() || fileName.isEmpty()) {
                    Toast.makeText(getApplicationContext(), "请输入内容和文件名", Toast.LENGTH_SHORT).show();
                } else {
                    saveFile(fileName, content);
                }
            }
        });

        buttonRead.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showFileList();
            }
        });

        buttonPicture.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                selectPicture();
            }
        });
    }

    private void saveFile(String fileName, String content) {
        File file = new File(getExternalFilesDir(null), fileName);
        try {
            OutputStream outputStream = new FileOutputStream(file);
            outputStream.write(content.getBytes());
            outputStream.close();
            Toast.makeText(getApplicationContext(), "文件保存成功", Toast.LENGTH_SHORT).show();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void showFileList() {
        File dir = getExternalFilesDir(null);
        File[] files = dir.listFiles();
        List<String> fileNames = new ArrayList<>();
        for (File file : files) {
            if (file.isFile()) {
                fileNames.add(file.getName());
            }
        }
        String[] fileNameArray = new String[fileNames.size()];
        fileNameArray = fileNames.toArray(fileNameArray);
        FileListAdapter fileListAdapter = new FileListAdapter(this, fileNameArray);
        fileListAdapter.setOnItemClickListener(new FileListAdapter.OnItemClickListener() {
            @Override
            public void onItemClick(String fileName) {
                showFileContent(fileName);
            }
        });
        FileListDialogFragment fileListDialogFragment = new FileListDialogFragment(fileListAdapter);
        fileListDialogFragment.show(getSupportFragmentManager(), "fileListDialog");
    }

    private void showFileContent(String fileName) {
        File file = new File(getExternalFilesDir(null), fileName);
        try {
            InputStream inputStream = getResources().getAssets().open(file.getName());
            byte[] buffer = new byte[inputStream.available()];
            inputStream.read(buffer);
            inputStream.close();
            String content = new String(buffer, "UTF-8");
            textViewFileName.setText(fileName);
            textViewContent.setText(content);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void selectPicture() {
        Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
        intent.addCategory(Intent.CATEGORY_OPENABLE);
        intent.setType("image/*");
        startActivityForResult(intent, REQUEST_CODE_SELECT_PICTURE);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == REQUEST_CODE_SELECT_PICTURE && resultCode == RESULT_OK && data != null) {
            Uri uri = data.getData();
            ImageView imageView = findViewById(R.id.imageView);
            imageView.setImageURI(uri);
        }
    }
}

activity_main.xml：

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/editTextContent"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="输入文件内容" />

    <EditText
        android:id="@+id/editTextFileName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="输入文件名" />

    <Button
        android:id="@+id/buttonSave"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="SAVE" />

    <Button
        android:id="@+id/buttonRead"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="READ" />

    <TextView
        android:id="@+id/textViewFileName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/textViewContent"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <Button
        android:id="@+id/buttonPicture"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="PICTURE" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:adjustViewBounds="true"
        android:scaleType="centerCrop"
        android:visibility="gone" />

</LinearLayout>