Java 学习笔记- I/O - DataStream

Data Streams支持 primitive 数据类型以及字符串类型的字节流I/O，所有的Data Stream 类都实现了DataInput 与DataqOutput 这两个接口，在实现这两个接口的类中最为常用是：DataInputStream，DataOutputStream。官方文档的例子：

package io.datastream;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.EOFException;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class DataStreams {

    static final String dataFile = "D:\\360云盘\\javase\\src\\io\\invoicedata";

    static final double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 };

    static final int[] units = { 12, 8, 13, 29, 50 };

    static final String[] descs = { "Java T-shirt", "Java Mug", "Duke Juggling Dolls", "Java Pin", "Java Key Chain" };

    public static void main(String[] args) throws IOException {
        DataOutputStream out = null;
        try {
            out = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(dataFile)));
            for (int i = 0; i < prices.length; i++) {
                out.writeDouble(prices[i]);
                out.writeInt(units[i]);
                out.writeUTF(descs[i]);
            }

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (out != null) {
                out.close();
            }
        }

        // ========================= Read the file ============

        DataInputStream in = null;
        double price;
        int unit;
        String desc;
        double total = 0.0;
        try {
            in = new DataInputStream(new BufferedInputStream(new FileInputStream(dataFile)));
            try {
                while (true) {
                    price = in.readDouble();
                    unit = in.readInt();
                    desc = in.readUTF();
                    System.out.format("You ordered %d" + " units of %s at $%.2f%n", unit, desc, price);
                    total += price * unit;
                }

            } catch (EOFException e) {
                e.printStackTrace();
            }
            System.out.format("Total amount %.2f", total);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            in.close();
        }
    }
}

 这段代码是先将3个分别存放着相同个数，不同primitive类型或字符串类型的数组的数据写到invoicedata文件中，然后再将这些数据从文件中读出。上面这段代码的运行结果为：

 

运行结果1： 

You ordered 12 units of Java T-shirt at $19.99
You ordered 8 units of Java Mug at $9.99
You ordered 13 units of Duke Juggling Dolls at $15.99
You ordered 29 units of Java Pin at $3.99
You ordered 50 units of Java Key Chain at $4.99
java.io.EOFException
at java.io.DataInputStream.readFully(DataInputStream.java:197)
at java.io.DataInputStream.readLong(DataInputStream.java:416)
at java.io.DataInputStream.readDouble(DataInputStream.java:468)
at io.datastream.DataStreams.main(DataStreams.java:51)
Total amount 892.88

 上面得到的结果都是正常的，但是如果将上面的代码稍作改动：

 

 

package io.datastream;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.EOFException;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class DataStreams {

    static final String dataFile = "D:\\360云盘\\javase\\src\\io\\invoicedata";

    static final double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 };

    static final int[] units = { 12, 8, 13, 29, 50 };

    static final String[] descs = { "Java T-shirt", "Java Mug", "Duke Juggling Dolls", "Java Pin", "Java Key Chain" };

    public static void main(String[] args) throws IOException {
        DataOutputStream out = null;
        DataInputStream in = null;
        double price;
        int unit;
        String desc;
        double total = 0.0;

        try {
            out = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(dataFile)));
            for (int i = 0; i < prices.length; i++) {
                out.writeDouble(prices[i]);
                out.writeInt(units[i]);
                out.writeUTF(descs[i]);
            }
            
            // ========================= Read the file =====================
            in = new DataInputStream(new BufferedInputStream(new FileInputStream(dataFile)));
            try {
                while (true) {
                    price = in.readDouble();
                    unit = in.readInt();
                    desc = in.readUTF();
                    System.out.format("You ordered %d" + " units of %s at $%.2f%n", unit, desc, price);
                    total += price * unit;
                }

            } catch (EOFException e) {
                e.printStackTrace();
            }
            System.out.format("Total amount %.2f", total);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (in != null) {
                in.close();
            }
            if (out != null) {
                out.close();
            }
        }

    }
}

 

 将读取文件的代码移动写入文件代码片段后面，而且是在同一个try里面，也就是在outputstream关闭之前读取，其结果：

 

运行结果2 

java.io.EOFException
at java.io.DataInputStream.readFully(DataInputStream.java:197)
at java.io.DataInputStream.readLong(DataInputStream.java:416)
at java.io.DataInputStream.readDouble(DataInputStream.java:468)
at io.datastream.DataStreams.main(DataStreams.java:42)
Total amount 0.00

 也就是读取文件的代码并没有读到希望读到的数据。

所以我猜测再输出流关闭之前对文件的修改，输入流是读取不到了？

不知道是不是这样？请懂的多多赐教。

后来发现，在输出流向文件写入完以后调用一下flush方法，这样DataInputStream 就可以读取到文件的内容了，其中，flush方法就是把数据流从内存刷新到文件中。