Java IO note 1

Return value of read()

For instance, according to the Java class library documentation, the read( ) method of java.io.InputStream returns "the next byte of data, or -1 if the end of the stream is reached." Upon reflection, this sounds suspicious. How is a -1 that appears as part of the stream data to be distinguished from a -1 indicating end of stream? In point of fact, the read( ) method does not return a byte; its signature shows that it returns an int:

1 public   abstract   int  read( )  throws  IOException

This int is not a Java byte with a value between -128 and 127 but a more general unsigned byte with a value between 0 and 255. Hence, -1 can easily be distinguished from valid data values read from the stream.

JVM and byte, byte array

In fact, a single byte still takes up four bytes of space inside the Java virtual machine, but a byte array occupies only the amount of space it actually needs. The virtual machine includes special instructions for operating on byte arrays but does not include any instructions for operating on single bytes. They're just promoted to ints.

Input, Output, Reader and Writer

For the most part, these classes have methods that are extremely similar to the equivalent stream classes. Often the only difference is that a byte in the signature of a stream method becomes a char in the signature of the matching reader or writer method. For example, the java.io.OutputStream class declares these three write( ) methods:

The java.io.Writer class, therefore, declares these three write( ) methods:

As you can see, the signatures match except that in the latter two methods the byte array data has changed to a char array. There's also a less obvious difference not reflected in the signature. While the int passed to the OutputStream write( ) method is reduced modulo 256 before being output, the int passed to the Writer write( ) method is reduced modulo 65,536. This reflects the different ranges of chars and bytes.

java.io.Writer also has two more write( ) methods that take their data from a string:

        public void write(String s) 
throws IOException
public void write(String s, int offset, int length) throws IOException

    

Because streams don't know how to deal with character-based data, there are no corresponding methods in the java.io.OutputStream class.

Hello World!

 

1   byte [] hello  =   { 72 ,  101 ,  108 ,  108 ,  111 ,  32 ,  87 ,  111 ,  114 ,  108 ,  100 ,  33 ,  10 , 13 } ;
2 System.out.write(hello);

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