package com.itheima.writesd;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Toast;

/**
 * 写sd卡一定记得添加权限
 */
public class MainActivity extends Activity {

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

    /**
     * 模拟向sd卡写一个视频文件.
     */
    public void click(View view) {
        // 检查的sd卡的状态是否被拔出或者是不可用
        if (Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED)) {
            File sdFile = Environment.getExternalStorageDirectory();// 外部存储空间
            long sdsize = sdFile.getFreeSpace();
            if (sdsize >5* 1024 * 1024) {
                File file = new File(Environment.getExternalStorageDirectory(),//根目录，mnt/sdcard/hlw.3gp
                        System.currentTimeMillis()+"hlw.3gp");//空文件
                try {
                    FileOutputStream fos = new FileOutputStream(file);//文件输出流来写文件
                    byte[] buffer = new byte[1024];//1K,空数据
                    for (int i = 0; i < 5*1024; i++) {//hlw.3gp为5M的空数据文件
                        fos.write(buffer);
                    }
                    fos.close();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }else{
                Toast.makeText(this, "sd卡空间不足", 0).show();
            }
        } else {
            Toast.makeText(this, "sd卡被拔出或者是不可用", 0).show();
        }
    }
}

清单文件：
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

package com.itheima.haker;

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

import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends Activity {

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

    public void click(View view){
        try {
            File file = new File("/data/data/com.itheima.createfile/files/haha.txt");//内部存储目录
            FileInputStream fis = new FileInputStream(file);//文件输入流
            BufferedReader br = new BufferedReader(new InputStreamReader(fis));//输入流进行包装
            String line = br.readLine();//读取第一行
            Toast.makeText(this, line, 0).show();
        } catch (Exception e) {
            Toast.makeText(this, "读取失败", 0).show();//文件在手机内部存储空间默认是私有的，别的应用程序是不能访问的，也可以改为公有。
            e.printStackTrace();
        }
    }

}