Android SD卡读写

package com.jredu.zuiyou.activity;

import android.os.Bundle;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import com.jredu.zuiyou.R;
import com.jredu.zuiyou.entity.Student;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class SDActivity extends AppCompatActivity {
    ObjectOutputStream oos = null;
    ObjectInputStream ois = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sd);
        Button write = (Button) findViewById(R.id.write);
        write.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                WriteToSD();
            }
        });
        Button read = (Button) findViewById(R.id.read);
        read.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ReadFromSD();
            }
        });
    }
    public void WriteToSD(){
        String state = Environment.getExternalStorageState();
        if (state.equals(Environment.MEDIA_MOUNTED)){
            File root = Environment.getExternalStorageDirectory();
            File TargetDir = new File(root,super.getPackageName());
            if (!TargetDir.exists()){
                TargetDir.mkdir();
            }
            try {
                oos = new ObjectOutputStream(new FileOutputStream(new File(TargetDir,"yxl.txt")));
                Student student = new Student("小明",18);
                oos.writeObject(student);
                Toast.makeText(getApplication(),"写入成功",Toast.LENGTH_SHORT).show();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    oos.flush();
                    oos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }else {
            Toast.makeText(getApplication(),"没有找到SD卡",Toast.LENGTH_SHORT).show();
        }
    }
    public void ReadFromSD(){
        String state = Environment.getExternalStorageState();
        if (state.equals(Environment.MEDIA_MOUNTED)){
            File root = Environment.getExternalStorageDirectory();
            File TargetDir = new File(root,super.getPackageName());
            if (!TargetDir.exists()){
                TargetDir.mkdir();
            }
            try {
                ois = new ObjectInputStream(new FileInputStream(new File(TargetDir,"/yxl.txt")));
                Student student = (Student) ois.readObject();
                Toast.makeText(getApplication(),student.getName().toString()+"，"+student.getAge(),Toast.LENGTH_SHORT).show();

            } catch (IOException e) {
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }finally {
                try {
                    ois.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }else {
            Toast.makeText(getApplication(),"没有找到SD卡",Toast.LENGTH_SHORT).show();
        }
    }
}