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