<?xml version="1.0" encoding="utf-8"?>

<LinearLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:orientation="vertical" >

    <Button

        android:id="@+id/save"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:text="保存数据(File)" />

    <Button

        android:id="@+id/read"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:text="读取数据(File)" />

</LinearLayout>  

package com.example.yanlei.wifi;

import android.os.Bundle;

import android.os.Environment;

import android.support.v7.app.AppCompatActivity;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.Toast;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.PrintStream;

import java.util.Scanner;

public class MainActivity extends AppCompatActivity {

    private Button btnSave=null;

    private Button btnRead=null;

    private File file=null;

    private static final String FILENAME="data.txt";

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        btnSave=(Button)super.findViewById(R.id.save);

        btnRead=(Button)super.findViewById(R.id.read);

        btnSave.setOnClickListener(new OnClickListener(){

            public void onClick(View v)

            {

                PrintStream ps=null;

                //判断外部存储卡是否存在

                if(!Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){

                    Toast.makeText(getApplicationContext(), "读取失败，SD存储卡不存在！", Toast.LENGTH_LONG).show();

                    return;

                }

                //初始化File

                String path=Environment.getExternalStorageDirectory().toString()

                        +File.separator

                        +"genwoxue"

                        +File.separator

                        +FILENAME;

                file=new File(path);

                //如果当前文件的父文件夹不存在，则创建genwoxue文件夹

                if(!file.getParentFile().exists())

                    file.getParentFile().mkdirs();

                //写文件

                try {

                    ps = new PrintStream(new FileOutputStream(file));

                    ps.println("跟我学网址：www.genwoxue.com");

                    ps.println("");

                    ps.println("电子邮件：hello@genwoxue.com");

                } catch (FileNotFoundException e) {

                    e.printStackTrace();

                }finally{

                    ps.close();

                }

                Toast.makeText(getApplicationContext(), "保存成功！", Toast.LENGTH_LONG).show();

            }

        });

        btnRead.setOnClickListener(new OnClickListener(){

            public void onClick(View v)

            {

                StringBuffer info=new StringBuffer();

                //判断外部存储卡是否存在

                if(!Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){

                    Toast.makeText(getApplicationContext(), "读取失败，SD存储卡不存在！", Toast.LENGTH_LONG).show();

                    return;

                }

                //初始化File

                String path=Environment.getExternalStorageDirectory().toString()

                        +File.separator

                        +"genwoxue"

                        +File.separator

                        +FILENAME;

                file=new File(path);

                if(!file.exists()){

                    Toast.makeText(getApplicationContext(), "文件不存在！", Toast.LENGTH_LONG).show();

                    return;

                }

                //读取文件内容

                Scanner scan=null;

                try {

                    scan=new Scanner(new FileInputStream(file));

                    while(scan.hasNext()){

                        info.append(scan.next()).append("☆☆☆\n");

                    }

                    Toast.makeText(getApplicationContext(), info.toString(), Toast.LENGTH_LONG).show();

                } catch (FileNotFoundException e) {

                    e.printStackTrace();

                }finally{

                    scan.close();

                }

            }

        });

    }

}

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

    package="com.example.yanlei.wifi" >

    <!-- 在SDCard中创建与删除文件权限 -->

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

    <!-- 往SDCard写入数据权限 -->

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

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

    <application

        android:allowBackup="true"

        android:icon="@mipmap/ic_launcher"

        android:label="@string/app_name"

        android:theme="@style/AppTheme" >

        <activity

            android:name=".MainActivity"

            android:label="@string/app_name" >

            <intent-filter>

                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />

            </intent-filter>

        </activity>

    </application>

</manifest>