Java中InputStream和InputStreamReader之间的区别

Question

I read that InputStream is used for byte based reading it reads 1 byte at a time. And InputStreamReader is used for character based reading so it reads one character at a time so no need to convert this first to int then read it.

Here is reading using InputStream.

 InputStream input=new FileInputStream("D:/input.txt");

 int c;

 while((c=input.read())!=-1)
 {
    System.out.print((char)c);
 }

here is reading using InputStreamReader

InputStream input=new FileInputStream("D:/input.txt");

reader=new InputStreamReader(input,"UTF-8");

int c;

while((c=reader.read())!=-1)
{
    System.out.print((char)c);
}

What is difference between InputStream and InputStreamReader ? In both case I have to use a int and then read it and at the end if I want to print that data I have to cast that with "(char)c".

So what is advantage of using InputStreamReader ?

Answer 1

There's a big difference between what is an InputStream and an InputStreamReader . One reads bytes whereas the other one reads character. Depending on the encoding used, one character may be more than 1 byte.

From InputStream.read() :

Reads the next byte of data from the input stream

From InputStreamReader.read() :

Reads a single character.

InputStreamReader serves as a bridge between those two ways of reading data in Java: a way of bridging byte streams to character streams. From its Javadoc :

An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted.

Each invocation of one of an InputStreamReader 's read() methods may cause one or more bytes to be read from the underlying byte-input stream. To enable the efficient conversion of bytes to characters, more bytes may be read ahead from the underlying stream than are necessary to satisfy the current read operation.

Answer 2

Some encodings have characters that span multiple bytes. By reading with InputStream, you're stupidly reading the next byte as opposed to InputStreamReader that may read ahead as necessary in order to give you the next character as opposed to byte. In other words, suppose you have 0x00 0xA7 as the next two bytes on the stream. InputStream will give you 0x0 on first read and then 0xA7 on the next. An InputStreamReader with unicode encoding will return you 0x00A7 on first read which is character §.

Please note that it is probably best if you wrap InputStreamReader with BufferedReader in most cases.

Answer 3

An InputStream is typically always connected to some data source, like a file, network connection, pipe etc. This is also explained in more detail in the Java IO Overview text.

InputStreamReader takes an inputstream and converts the bytes Strem into characters when you are reading it. For example, some UTF characters are 2 bytes, a call to an input stream reader will read the two bytes and convert it into a character automatically. It is used to read text streams.

Answer 4

In the case of InputStreamReader you can use 'char c' in place of 'int c' and instead of '-1' value, you can use 'null'. Like this :

InputStream input=new FileInputStream("D:/input.txt");

reader=new InputStreamReader(input,"UTF-8");

char c;

while((c=reader.read())!=null)
{
System.out.print(c);
}

Because you are using InputStreamReader which do character based reading. So, there is no need to set datatype 'int' and then to cast it using 'char', it is unnecessary. We need to do this in the case of InputStream as it does byte based reading.