Before this, you need to obtain read-write access to the memory. Add the following in AndroidManifest.xml:

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

1.Android creates local folders

1.1. Declaration path:
The path I use here is:

 path = getExternalFilesDir("exter_test").getPath();

Where getExternalFilesDir("extern? Test") is the path to get the folder "extern? Test", and getPath() is the relative path;
1.2. Set folder name:
Here I set it to:

fileName = "test.txt";

1.3. Editing interface

Place one in the interface Button And one EditText，Also for Button Binding event.
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.a14553.localdocument.MainActivity">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <Button
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="31dp"
            android:onClick="onClick"
            android:text="Button" />

        <EditText
            android:id="@+id/editText"
            android:layout_width="200dp"
            android:layout_height="200dp"
            android:layout_centerHorizontal="true"
            android:layout_centerVertical="true"
            android:ems="10"
            android:inputType="textMultiLine" />
    </RelativeLayout>
</android.support.constraint.ConstraintLayout>

1.4. Content in mainactivity

public void onClick(View view){
        newDirectory(path,"Test2");
        //check();
        //newFile(path,fileName);
        //edt.setText(readFile(path,fileName));
        // save(edt.getText().toString());
    }

public void newDirectory(String _path,String dirName){
        File file = new File(_path+"/"+dirName);
        try {
            if (!file.exists()) {
                file.mkdir();
            }
        }catch (Exception e){
            e.printStackTrace();
        }
    }

2.Android creates local files

2.1-2.3 is the same as 1.1-1.3
2.4. Main activity

public void onClick(View view){
        //newDirectory(path,"Test2");
        //check();
        newFile(path,fileName);
        //edt.setText(readFile(path,fileName));
        // save(edt.getText().toString());
    }

public void newFile(String _path,String _fileName){
        File file=new File(_path+"/"+_fileName);
        try {
            if(!file.exists()) {
                file.createNewFile();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

3.Android access to local files

3.1-3.3 same as 1.1-1.3
3.4. Main activity

public void onClick(View view){
        newDirectory(path,"Test2");
        //check();
        //newFile(path,fileName);
        //edt.setText(readFile(path,fileName));
        // save(edt.getText().toString());
    }

public String readFile(String _path,String _fileName){
        String res = "";
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(_path+"/"+_fileName)));
            String line = "";
            while ((line = reader.readLine())!=null){
                res+=line;
            }
        }catch (IOException e){
            e.printStackTrace();
        }
        return res;
    }

Source engineering code
-----—–2018.03.08-----—–