<?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"
android:padding="16dp">

<EditText
  android:id="@+id/ed_file_save"
  android:layout_width="match_parent"
  android:layout_height="wrap_content" />

<Button
  android:id="@+id/btn_file_save"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:text="保存内容" />

<Button
  android:id="@+id/btn_file_read"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:text="读取内容" />

<TextView
  android:id="@+id/tv_read_file"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:textColor="#000"
  android:textSize="14sp" />

</LinearLayout>

package com.example.datasave;

import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * Created by Devin on 2016/7/19.
 */
public class FileDataActivity extends AppCompatActivity {
private EditText ed_file_save;
private Button btn_file_save;
private Button btn_file_read;
private TextView tv_read_file;
private String fileName = " hello.txt";

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_file);
  ed_file_save = (EditText) findViewById(R.id.ed_file_save);
  btn_file_save = (Button) findViewById(R.id.btn_file_save);
  btn_file_read = (Button) findViewById(R.id.btn_file_read);
  tv_read_file = (TextView) findViewById(R.id.tv_read_file);

  btn_file_save.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
      String fileContent = ed_file_save.getText().toString();
      try {
        save(fileContent);
        ToastUtils.showToast(FileDataActivity.this, "文件写入成功");
      } catch (Exception e) {
        e.printStackTrace();
        ToastUtils.showToast(FileDataActivity.this, "文件写入失败");
      }
    }
  });
  btn_file_read.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
      try {
        String content = read();
        tv_read_file.setText("文件的内容是：" + content);
      } catch (IOException e) {
        e.printStackTrace();
        ToastUtils.showToast(FileDataActivity.this, "读取文件失败!");
      }
    }
  });
}

public void save(String fileContent) throws Exception {
  FileOutputStream output = this.openFileOutput(fileName, Context.MODE_PRIVATE);
  output.write(fileContent.getBytes());
  output.close();
}

public String read() throws IOException {
  //打开文件输入流
  FileInputStream input = this.openFileInput(fileName);
  byte[] temp = new byte[1024];
  StringBuffer stringBuffer = new StringBuffer("");
  int len = 0;
  while ((len = input.read(temp)) > 0) {
    stringBuffer.append(new String(temp, 0, len));
  }
  //关闭输入流
  input.close();
  return stringBuffer.toString();
}
}

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

<EditText
  android:id="@+id/ed_file_save_sd"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="20dp" />

<Button
  android:id="@+id/btn_file_save_sd"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:text="写入到SDcard" />

<Button
  android:id="@+id/btn_file_read_sd"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:text="从SDcard读取" />

<TextView
  android:id="@+id/tv_read_file_sd"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:textColor="#000"
  android:textSize="14sp" />

  ed_file_save_sd = (EditText) findViewById(R.id.ed_file_save_sd);
  tv_read_file_sd = (TextView) findViewById(R.id.tv_read_file_sd);
  btn_file_read_sd = (Button) findViewById(R.id.btn_file_read_sd);
  btn_file_read_sd.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
      try {
        String content = readFromSD();
        tv_read_file_sd.setText("从SDCard读取到的内容是：" + content);
      } catch (Exception e) {
        e.printStackTrace();
        ToastUtils.showToast(FileDataActivity.this, "读取文件失败!");
      }
    }
  });
  btn_file_save_sd = (Button) findViewById(R.id.btn_file_save_sd);
  btn_file_save_sd.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
      String content = ed_file_save_sd.getText().toString();
      try {
        save2SDCard(content);
        ToastUtils.showToast(FileDataActivity.this, "文件写入SDCard成功");
      } catch (Exception e) {
        e.printStackTrace();
        ToastUtils.showToast(FileDataActivity.this, "文件写入SDCard失败");
      }
    }
  });

public void save2SDCard(String content) throws Exception {
  if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) { // 如果sdcard存在
    String fileName3 = Environment.getExternalStorageDirectory().getCanonicalPath() + File.separator + "test" + File.separator + fileName2;
    File file = new File(fileName3);
    if (!file.getParentFile().exists()) {
      file.getParentFile().mkdir();
    }
    FileOutputStream fileOutputStream = new FileOutputStream(file);
    fileOutputStream.write(content.getBytes());
    fileOutputStream.close();
  } else {
    ToastUtils.showToast(this, "SDCard不存在");
  }

}
public String readFromSD() throws Exception {
  StringBuffer stringBuffer = new StringBuffer("");
  if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
    String fileName3 = Environment.getExternalStorageDirectory().getCanonicalPath() + File.separator + "test" + File.separator + fileName2;
    File file = new File(fileName3);
    if (!file.getParentFile().exists()) {
      file.getParentFile().mkdir();
    }
    FileInputStream fileInputStream = new FileInputStream(file);
    byte[] temp = new byte[1024];
    int len = 0;
    while ((len = fileInputStream.read(temp)) > 0) {
      stringBuffer.append(new String(temp, 0, len));
    }
    fileInputStream.close();
  } else {
    ToastUtils.showToast(this, "SDCard不存在");
  }
  return stringBuffer.toString();
}

AssetManager assetsManager = getAssets(); 
InputStream inputStream = assetsManager.open("fileName");

package com.example.datasave.io;

import android.content.Context;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

/**
 * IO流 工具类<br>
 * 很简单,仅支持文本操作
 */
public class IOUtils {

/**
 * 文本的写入操作
 *
 * @param filePath 文件路径。一定要加上文件名字 <br>
 *         例如：../a/a.txt
 * @param content 写入内容
 * @param append  是否追加
 */
public static void write(String filePath, String content, boolean append) {
  BufferedWriter bufw = null;
  try {
    bufw = new BufferedWriter(new OutputStreamWriter(
        new FileOutputStream(filePath, append)));
    bufw.write(content);

  } catch (Exception e1) {
    e1.printStackTrace();
  } finally {
    if (bufw != null) {
      try {
        bufw.close();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
}

/**
 * 文本的读取操作
 *
 * @param path 文件路径,一定要加上文件名字<br>
 *       例如：../a/a.txt
 * @return
 */
public static String read(String path) {
  BufferedReader bufr = null;
  try {
    bufr = new BufferedReader(new InputStreamReader(
        new FileInputStream(path)));
    StringBuffer sb = new StringBuffer();
    String str = null;
    while ((str = bufr.readLine()) != null) {
      sb.append(str);
    }
    return sb.toString();
  } catch (Exception e) {
    e.printStackTrace();
  } finally {
    if (bufr != null) {
      try {
        bufr.close();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
  return null;
}

/**
 * 文本的读取操作
 *
 * @param is 输入流
 * @return
 */
public static String read(InputStream is) {
  BufferedReader bufr = null;
  try {
    bufr = new BufferedReader(new InputStreamReader(is));
    StringBuffer sb = new StringBuffer();
    String str = null;
    while ((str = bufr.readLine()) != null) {
      sb.append(str);
    }
    return sb.toString();
  } catch (Exception e) {
    e.printStackTrace();
  } finally {
    if (bufr != null) {
      try {
        bufr.close();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
  return null;
}


/**
 * @param context 上下文
 * @param fileName 文件名
 * @return 字节数组
 */
public static byte[] readBytes(Context context, String fileName) {
  FileInputStream fin = null;
  byte[] buffer = null;
  try {
    fin = context.openFileInput(fileName);
    int length = fin.available();
    buffer = new byte[length];
    fin.read(buffer);
  } catch (FileNotFoundException e) {
    e.printStackTrace();
  } catch (IOException e) {
    e.printStackTrace();
  } finally {
    try {
      if (fin != null) {
        fin.close();
        fin = null;
      }
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
  return buffer;
}

/**
 * 快速读取程序应用包下的文件内容
 *
 * @param context 上下文
 * @param filename 文件名称
 * @return 文件内容
 * @throws IOException
 */
public static String read(Context context, String filename)
    throws IOException {
  FileInputStream inStream = context.openFileInput(filename);
  ByteArrayOutputStream outStream = new ByteArrayOutputStream();
  byte[] buffer = new byte[1024];
  int len = 0;
  while ((len = inStream.read(buffer)) != -1) {
    outStream.write(buffer, 0, len);
  }
  byte[] data = outStream.toByteArray();
  return new String(data);
}


}