public void save(){
    String data = "Data to save";
    FileOutputStream out = null;
    BufferedWriter writer = null;
    try {
      out = openFileOutput("data", Context.MODE_PRIVATE);
      writer = new BufferedWriter(new OutputStreamWriter(out));
      writer.write(data);
    }catch (IOException e) {
      e.printStackTrace();
    } finally {
      try {
        if(writer!= null){
          writer.close();
        }
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/activity_main"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
 >
  <EditText
    android:id="@+id/edit"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />
</LinearLayout>

public class MainActivity extends AppCompatActivity {
  private EditText editText;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //获取editText实例
    editText = (EditText)findViewById(R.id.edit);
  }

  // 重写onDestroy(), 可以保证活动销毁之前一定会调用这个方法.
  @Override
  protected void onDestroy() {
    super.onDestroy();
    String inputText = editText.getText().toString();
    save(inputText);
  }

  public void save (String inputText){
    FileOutputStream out = null;
    BufferedWriter writer = null;

    try {
      out = openFileOutput("data", Context.MODE_PRIVATE);
      writer = new BufferedWriter(new OutputStreamWriter(out));
      writer.write(inputText);
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      try {
         if(writer!= null) {
           writer.close();
         }
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }

public String load (){
    FileInputStream in = null;
    BufferedReader reader = null;
    StringBuilder content = new StringBuilder();
    try {
      //获取FileInputStream对象
      in = openFileInput("data");
      //借助FileInputStream对象, 构建出一个BufferedReader对象
      reader = new BufferedReader(new InputStreamReader(in));
      String line = "";
      //通过BufferedReader对象进行一行行的读取, 把文件中的所有内容全部读取出来
      // 并存放在StringBuilder对象中
      while ((line = reader.readLine())!= null){
        content.append(line);
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if(reader!=null){
        try {
          reader.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
    //最后将读取到的内容返回
    return content.toString();
  }

public class MainActivity extends AppCompatActivity {
  private EditText editText;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //获取editText实例
    editText = (EditText)findViewById(R.id.edit);
    String inputText = load();
//TextUtils.isEmpty()可以一次性判断两种非空判断 传入null或者空, 都返回true
    if(!TextUtils.isEmpty((inputText))){
      editText.setText(inputText);
      //setSelection()表示将光标移动在文本框的末尾位置, 以便继续输入
      editText.setSelection(inputText.length());
      //弹出Toast, 给出一个提示, 表示读取数据成功
      Toast.makeText(this, "读取数据成功!", Toast.LENGTH_SHORT).show();
    }
  }

  // 重写onDestroy(), 可以保证活动销毁之前一定会调用这个方法.
  @Override
  protected void onDestroy() {
    super.onDestroy();
    String inputText = editText.getText().toString();
    save(inputText);
  }

  public void save (String inputText){
    FileOutputStream out = null;
    BufferedWriter writer = null;

    try {
      out = openFileOutput("data", Context.MODE_PRIVATE);
      writer = new BufferedWriter(new OutputStreamWriter(out));
      writer.write(inputText);
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      try {
         if(writer!= null) {
           writer.close();
         }
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }

  public String load (){
    FileInputStream in = null;
    BufferedReader reader = null;
    StringBuilder content = new StringBuilder();
    try {
      //获取FileInputStream对象
      in = openFileInput("data");
      //借助FileInputStream对象, 构建出一个BufferedReader对象
      reader = new BufferedReader(new InputStreamReader(in));
      String line = "";
      //通过BufferedReader对象进行一行行的读取, 把文件中的所有内容全部读取出来
      // 并存放在StringBuilder对象中
      while ((line = reader.readLine())!= null){
        content.append(line);
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if(reader!=null){
        try {
          reader.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
    //最后将读取到的内容返回
    return content.toString();
  }
}