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

  <EditText
    android:id="@+id/edit"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:hint="Type something here"/>
</LinearLayout>

public class MainActivity extends AppCompatActivity {
  private EditText edit;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    edit=(EditText) findViewById(R.id.edit);
    String inputText=load();
    if(!TextUtils.isEmpty(inputText)){                 //对字符串进行非空判断
      edit.setText(inputText);
      edit.setSelection(inputText.length());
      Toast.makeText(this,"Restoring succeeded",Toast.LENGTH_SHORT).show();
    }

  }
  @Override
  protected void onDestroy(){                      //重写onDestroy()保证在活动销毁之前一定调用这个方法
    super.onDestroy();
    String inputText=edit.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{
      in=openFileInput("data");
      reader=new BufferedReader(new InputStreamReader(in));
      String line="";
      while((line=reader.readLine())!=null){
        content.append(line);
      }
    }catch(IOException e){
      e.printStackTrace();
    }finally {
      if(reader!=null){
        try{
          reader.close();
        }catch (IOException e){
          e.printStackTrace();
        }
      }
    }
    return content.toString();
  }
}