SD卡读写 android

package com.jredu.helloworld.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.helloworld.R;
import com.jredu.helloworld.entity.SDcard;
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 {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sd); //XML文件没有给出，就是两个按钮
        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                writetoSD();
            }
        });
        Button button1 = (Button) findViewById(R.id.button1);
        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                readtoSD();
            }
        });
    }
    
    private void writetoSD() {
        ObjectOutputStream oos = null;
        String state = Environment.getExternalStorageState();//获取SD卡状态
        if (state.equals(Environment.MEDIA_MOUNTED)) {  //外部存储正常
            File root = Environment.getExternalStorageDirectory();//获取SD根路径
            File targetdir = new File(root, super.getPackageName());//在根路径下创建子路径
            if (!targetdir.exists()) {
                targetdir.mkdir();
            }
            try {
                oos = new ObjectOutputStream(new FileOutputStream(new File(targetdir,"www.txt")));
                SDcard sDcard = new SDcard("hello word", "by me");
                oos.writeObject(sDcard);
                Toast.makeText(SDActivity.this, "读入成功", Toast.LENGTH_SHORT).show();
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                try {
                    oos.flush();
                    oos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    private void readtoSD() {　　　　//读和写大致一样
        ObjectInputStream ois=null;
        String state = Environment.getExternalStorageState();
        if (state.equals(Environment.MEDIA_MOUNTED)) {
            File root = Environment.getExternalStorageDirectory();
            File targetdir = new File(root, super.getPackageName());
            if (targetdir.exists()) {
                try {
                    ois = new ObjectInputStream(new FileInputStream(new File(targetdir,"www.txt")));
                    SDcard sDcard = (SDcard) ois.readObject();
                    Toast.makeText(SDActivity.this,sDcard.getTitle()+sDcard.getContent(), Toast.LENGTH_SHORT).show();
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (ClassNotFoundException e) {
                    e.printStackTrace();
                }finally {
                    try {
                        ois.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }
}

注意：

new File(targetdir,"www.txt")读和写,或+ 要用就要一致。
还有要关闭输入输出流