Android SDcard 文件读写，RandomAccessFile操作

SD Card 相关的权限：

    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /> 

<?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"
    tools:context="shortcut.song.com.myapplication.SDCardTestActivity">

    <Button
        android:id="@+id/btn_read_sdcard"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="read"/>

    <Button
        android:id="@+id/btn_write_sdcard"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="write"/>

    <EditText
        android:id="@+id/edit_1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="read"/>
    <EditText
        android:id="@+id/edit_2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="write"/>
</LinearLayout>

package shortcut.song.com.myapplication;

import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;

public class SDCardTestActivity extends AppCompatActivity {
    private String FILE_NAME = "/sdandroid.bin";

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

        final Button read = (Button)findViewById(R.id.btn_read_sdcard);
        final Button write = (Button)findViewById(R.id.btn_write_sdcard);

        final EditText edit1 = (EditText)findViewById(R.id.edit_1);
        final EditText edit2 = (EditText)findViewById(R.id.edit_2);

        read.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //将edit1输入的内容写到文件中
                edit2.setText(read());
            }
        });

        write.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                write(edit1.getText().toString());
                edit1.setText("");
            }
        });

    }

    private String read() {
        try {
            //如果设置插入了SD卡，并且应用程序具有访问SD卡的权限
            if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                //获取SD卡对应的存储目录
                File sdCardDir = Environment.getExternalStorageDirectory();
                //获取指定文件对应的输入流
                FileInputStream fis = new FileInputStream(sdCardDir.getCanonicalPath() + FILE_NAME);
                //将指定的输入流包装成BufferedReader
                BufferedReader br = new BufferedReader(new InputStreamReader(fis));
                StringBuilder sb = new StringBuilder("");
                String line = null;
                //循环读取文件内容
                while ( (line = br.readLine()) != null)
                {
                    sb.append(line);
                }
                //关闭资源
                br.close();
                return sb.toString();
            }else{
                Toast.makeText(SDCardTestActivity.this, "Not Find SD card.", Toast.LENGTH_LONG).show();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }


    private void write(String content) {
        try {
            //如果设置插入了SD卡，并且应用程序具有访问SD卡的权限
            if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                //获取SD卡对应的存储目录
                File sdCardDir = Environment.getExternalStorageDirectory();
                    File targetFile = new File(sdCardDir.getCanonicalPath() + FILE_NAME);
                //以指定文件创建RandomAccessFile 对象
                RandomAccessFile raf = new RandomAccessFile(targetFile, "rw");
                //将文件记录移动到文件最后
                raf.seek(targetFile.length());
                //输出文件内容
                raf.write(content.getBytes());
                //关闭RandomAccessFile
                raf.close();
            }else{
                Toast.makeText(SDCardTestActivity.this, "Not Find SD card.", Toast.LENGTH_LONG).show();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

运行效果：