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

package net.bwie.localdata.activity;

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
 
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import net.bwie.localdata.R;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;

 
public class FileActivity extends AppCompatActivity implements View.OnClickListener {

    protected Button mReadFileBtn;
    protected Button mWriteFileBtn;
    protected TextView mResultTv;

    public static void startActivity(Context context) {
        context.startActivity(new Intent(context, FileActivity.class
 
));
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        super.setContentView(R.layout.activity_file);
        initView();
    }

    @Override
    public void onClick(View view) {
        if (view.getId() == R.id.read_file_btn) {
            String result = readFile();
            mResultTv.setText(result);

        } else if (view.getId() == R.id.write_file_btn) {
            writeFile();
        }
    }

    // 讀取文件
    private String readFile() {
        String filePath = Environment.getExternalStorageDirectory().getPath() + "/abc/";
        String fileName = "xyz.txt";

        File file = new File(filePath, fileName);

        BufferedReader br = null;
        try {
            br = new BufferedReader(new InputStreamReader(new FileInputStream(file)));

            String result = "";
            String line = "";

            while ((line = br.readLine()) != null) {
                result += line;
            }
            return result;


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

    // 寫入文件
    private void writeFile() {

        // 外部存儲私有路徑：Android文件夾
//        String privatePath = getExternalFilesDir(null).getPath();// 私有路徑不分類為null
//        String filePath = privatePath + "/abc/";

        // 外部存儲公共路徑：DICM，DOWNLOAD，MUSIC等系統提供的文件夾
//        String publicPath = Environment
//                .getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS)
//                .getPath();
//        String filePath = publicPath + "/abc/";

        // 自定義文件路徑
        String rootPath = Environment.getExternalStorageDirectory().getPath();// 外部存儲路徑（根目錄）
        String filePath = rootPath + "/abc/";

        String fileName = "xyz.txt";

        File file = new File(filePath, fileName);

        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream(file);
            fos.write("asdasdas".getBytes());
            Toast.makeText(this, "成功", Toast.LENGTH_SHORT).show();
        } catch (Exception e) {
            e.printStackTrace();
            Log.d("1507", "error: " + e.getMessage());
        } finally {
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

    }

    private void initView() {
        mReadFileBtn = (Button) findViewById(R.id.read_file_btn);
        mReadFileBtn.setOnClickListener(FileActivity.this);
        mWriteFileBtn = (Button) findViewById(R.id.write_file_btn);
        mWriteFileBtn.setOnClickListener(FileActivity.this);
        mResultTv = (TextView) findViewById(R.id.result_tv);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    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"
    android:orientation="vertical"
    tools:context="net.bwie.localdata.activity.FileActivity">

    <Button
        android:id="@+id/read_file_btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="讀取文件"/>

    <Button
        android:id="@+id/write_file_btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="寫入文件"/>

    <TextView
        android:id="@+id/result_tv"
        android:text="結果"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>

android 文件存儲