达内课程-DataInputStream/DataOutputStream

DataInputStream/DataOutputStream

读写固定字节格式数据

DataOutputStream

创建对象
DataOutputStream out = new DataOutputStream(相接的流)

方法
writeInt(int i)//输出4个字节
writeDouble(double d)//输出8个字节
……
writeUTF()//先输出2个字节表示字符串的字节长度，再输出这些字节值

DataInputStream

方法
readInt()//读取4个字节
readDouble()//读取8个字节
……
readUTF()//先读取2个字节来确定字符串的字节长度，再读取这些字节值

读取结束，再读取会出现EOFException

栗子

DataOutputStream out = new DataOutputStream(
        new FileOutputStream("d:/abc/f2")
        );

        out.writeInt(20171213);
        out.writeUTF("阴，13度");
        out.writeInt(20171214);
        out.writeUTF("晴，14度");
        out.writeInt(20171215);
        out.writeUTF("雨，15度");
        out.close();

输出结果

读取栗子

DataInputStream in = new DataInputStream(
        new FileInputStream("d:/abc/f2")
        );
        try{
            while(true){
                int date = in.readInt();
                String s = in.readUTF();
                System.out.println(date);
                System.out.println(s);
            }
        }catch(EOFException e){
            //正确读取结束，不需要处理
        }
        in.close();

输出结果

20171213
阴，13度
20171214
晴，14度
20171215
雨，15度

练习：保存学生信息
用如下格式保存学生信息

学号 00 00 00 01
姓名 00 03 61 62 63
性别 00 61
年龄 00 00 00 16

xml

<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:background="@android:color/white"
              android:orientation="vertical"
              tools:context=".MainActivity">

    <EditText
        android:id="@+id/et1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="学号"
        />
    <EditText
        android:id="@+id/et2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="姓名"
        />
    <EditText
        android:id="@+id/et3"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="性别"
        />
    <EditText
        android:id="@+id/et4"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="年龄"
        />

    <Button
        android:id="@+id/btn1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="保存"/>
    <Button
        android:id="@+id/btn2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="读取"/>

    <TextView
        android:id="@+id/tv"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>

java

public class MainActivity extends Activity {

    private EditText et1;
    private EditText et2;
    private EditText et3;
    private EditText et4;
    private Button btn1;
    private Button btn2;
    private TextView tv;

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

    private void setViews() {
        et1 = findViewById(R.id.et1);
        et2 = findViewById(R.id.et2);
        et3 = findViewById(R.id.et3);
        et4 = findViewById(R.id.et4);
        btn1 = findViewById(R.id.btn1);
        btn2 = findViewById(R.id.btn2);
        tv = findViewById(R.id.tv);
    }

    private void setListeners() {
        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                baocun();
            }
        });
        btn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                duqu();
            }
        });
    }

    private void baocun() {
        //IO操作有IO异常，所以进行try...catch...
        /*
        *
        *           ┌DataOutputStream
        *    ┌FileOutputStream
        * sdcard
        */
        try{
            int id = Integer.parseInt(et1.getText().toString());
            String name = et2.getText().toString();
            String gender= et3.getText().toString();
            int age = Integer.parseInt(et4.getText().toString());

            DataOutputStream out = new DataOutputStream(
                    new FileOutputStream(Environment.getExternalStorageDirectory()+"/stu",true)
            );
            out.writeInt(id);
            out.writeUTF(name);
            out.writeChar(gender.charAt(0));
            out.writeInt(age);
            out.close();

            Toast.makeText(this,"保存成功",Toast.LENGTH_SHORT).show();
        }catch (Exception e){
            Toast.makeText(this,"保存失败",Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        }
    }

    private void duqu() {
        //IO操作有IO异常，所以进行try...catch...
        try{
            DataInputStream in = new DataInputStream(
              new FileInputStream(Environment.getExternalStorageDirectory()+"/stu")
            );

            try{
                tv.setText("");
                while (true){
                    int id = in.readInt();
                    String name = in.readUTF();
                    char gender = in.readChar();
                    int age = in.readInt();
                    tv.append("id:"+id+"\n"+"name:"+name+"\n"+"gender:"+gender+"\n"+"age:"+age+"\n");
                }
            }catch (EOFException e){

            }
            in.close();
            Toast.makeText(this,"读取成功",Toast.LENGTH_SHORT).show();
        }catch (Exception e){
            Toast.makeText(this,"读取失败",Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        }
    }
}

TIP
1、现在我们想在模拟机的中读写数据
相关文章查看模拟机中路径怎么写

2、保存失败，显示的错误信息为 Permission denied

所以在AndroidMenifest.xml中添加权限

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

并在模拟机中，打开应用的权限

再次运行程序，显示保存成功

从模拟机中把stu文件导出至桌面，查看内容
保存了3个学生信息

点击读取，读取信息