大神们好,我是新手小白,今天在学习Java的IO操作中遇到一个百思不得其解的问题,下面的代码是今天做的关于DataInputStream类的练习,我很不解为什么DataInputStream的readUTF方法不需要任何参数,但是却在读取的时候可以知道自己读取多长,不明白他是怎么知道我当初在写入的时候写入的长度的,他是靠什么控制读取的范围的呀???比如下面的程序,readUTF方法就可以知道读取"昨天"这两个字,是怎么知道我就刚好需要读这两个字,而不会把下面的内容给读出。很是感谢大家的回答
package com.zhang.hello; import java.io.DataInputStream; import java.io.DataOutputStream; import java.io.File; import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; public class IODataStreamTest { public static void main(String[] args) throws IOException { File file=new File("f:/zhang.dat"); if(!file.exists()){ file.createNewFile(); } DataInputStream dis=new DataInputStream(new FileInputStream(file)); DataOutputStream dos=new DataOutputStream(new FileOutputStream(file)); int testInt=2016; double testDouble=10.12; long testLong=20161012; dos.writeInt(testInt); dos.writeDouble(testDouble); dos.writeLong(testLong); //utf-8 一个中文占3个字节 dos.writeUTF("昨天"); dos.writeUTF("今天是个好日子"); dos.close(); int testRInt=dis.readInt(); System.out.println(testRInt); double testRDouble=dis.readDouble(); System.out.println(testRDouble); long testRLong=dis.readLong(); System.out.println(testRLong); String testRStr=dis.readUTF();//为什么readUTF知道读取多长 System.out.println(testRStr); dis.close(); } }
执行结果:
2016
10.12
20161012
昨天
付费偷看金额在0.1-10元之间
实际上在调用
writeUTF
写入时jdk
内部有将字符串的字节数写入流中,读取时先读取到字节长度,按照指定的字节长度读取出相应的字符串。Reads two input bytes and returns an int value in the range 0 through
看源码,调用的第一句就获取了长度
int utflen = in.readUnsignedShort();
这个方法的Doc :
DataInputStream
(((a & 0xff) << 8) | (b & 0xff)) This method is suitable for reading
the bytes written by the writeShort method of interface DataOutput if
the argument to writeShort was intended to be a value in the range 0
through 65535. Returns: the unsigned 16-bit value read. Throws:
EOFException - if this stream reaches the end before reading all the
Reads from the stream in a representation of a Unicode character
string encoded in modified UTF-8 format; this string of characters is
then returned as a String. The details of the modified UTF-8
representation are exactly the same as for the readUTF method of
DataInput.
Let a be the first byte read and b be the second byte. The value returned is:
IOException - if an I/O error occurs.
readUTF的Doc:
DataOutputStream
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