Download I/O Streams

I/O
Ji Zhenyan
[email protected]
Tel: 063 165713
Ji Zhenyan, ITM, Mitthögskolan
Contents
• What’s I/O?
• I/O Streams
¾ Character streams
¾ Byte Streams
•
•
•
•
•
•
File streams
Pipe Streams
How to Concatenate Files
DataInputStream & DataOutputStream
Writing Customized Filtered Streams
Object Serialization
¾ Write to an ObjectOutputStream
¾ Read from an ObjectOutputStream
¾ Providing Object Serialization
Ji Zhenyan, ITM, Mitthögskolan
What’s I/O?
• “I” means “input”: a program opens a stream on an
information source (a file, memory, a socket) and reads the
information serially.
• “O” means “output”: a program can send information to an
external destination by opening a stream to a destination
and writing the information out serially.
Ji Zhenyan, ITM, Mitthögskolan
I/O Streams
• java.io contains character streams and byte
streams that perform the same processing but for
the different data type:
¾ java.io. Reader --streams that read 16-bit characters
¾ java.io.Writer --streams that write 16-bit characters.
¾ java.io.InputStream --streams that read 8-bit bytes.
¾ java.io.OutputStream --streams that write 8-bit bytes.
Ji Zhenyan, ITM, Mitthögskolan
Character streams
• Hierarchy of input character streams:
Ji Zhenyan, ITM, Mitthögskolan
• Hierarchy of output character streams:
Ji Zhenyan, ITM, Mitthögskolan
Byte Streams
• Hierarchy of input byte streams:
Ji Zhenyan, ITM, Mitthögskolan
• Hierarchy of output byte streams:
Ji Zhenyan, ITM, Mitthögskolan
File Streams
• The file streams -- FileReader, FileWriter,
FileInputStream, and FileOutputStream --each
read or write from a file on the native file system.
• A file stream can be created from a file name in
the form of a string, a File object, or a
FileDescriptor object.
• Examples:
¾ Copy.java
¾ CopyBytes.java
Ji Zhenyan, ITM, Mitthögskolan
Use Pipe Streams
• Pipes are used to channel the output from one program (or
thread) into the input of another.
¾ Without pipe streams, the program would have to store the results
somewhere (such as in a file or in memory) between each step.
Ji Zhenyan, ITM, Mitthögskolan
¾ With pipe streams, the output from one method could be piped into
the next
Ji Zhenyan, ITM, Mitthögskolan
• PipedReader and PipedWriter (and their input and
output stream counterparts PipedInputStream and
PipedOutputStream ) implement the input and output
components of a pipe.
• Example:
¾ RhymindWords, ReverseThread & SortThread
Ji Zhenyan, ITM, Mitthögskolan
How to Concatenate Files
• The class java.io.SequenceInputStream
can create a single input stream from
multiple input sources.
• Example:
¾Concatenate.java & ListOfFiles.java
Ji Zhenyan, ITM, Mitthögskolan
DataInputStream &
DataOutputStream
• DataOutputStream, like other filtered
output streams, must be attached to some
other OutputStream.
• DataInputStream also must be attached to
some other InputStream.
• Example:
¾DataIOTest.java
Ji Zhenyan, ITM, Mitthögskolan
Writing Customized Filtered Streams
• Steps:
¾ Create a subclass of FilterInputStream and
FilterOutputStream. Input and output streams often
come in pairs, so it's likely that you will need to create
both input and output versions of your filter stream.
¾ Override the read and write methods.
¾ Override any other methods that you might need.
¾ Make sure the input and output streams work together.
Ji Zhenyan, ITM, Mitthögskolan
Object Serialization
• The key to writing an object is to represent its
state in a serialized form sufficient to
reconstruct the object as it is read. Thus
reading and writing objects is a process called
object serialization.
• Two streams, java.io.ObjectInputStream
and java.io.ObjectOutputStream, can read
and write objects.
Ji Zhenyan, ITM, Mitthögskolan
Write to an ObjectOutputStream
•
FileOutputStream out = new FileOutputStream("theTime");
ObjectOutputStream s = new ObjectOutputStream(out);
s.writeObject("Today");
s.writeObject(new Date());
s.flush();
™ ObjectOutputStream must be constructed on another
stream. This code constructs an ObjectOutputStream on a
FileOutputStream, thereby serializing the object to a file
named theTime.
Ji Zhenyan, ITM, Mitthögskolan
Read from an ObjectOutputStream
• FileInputStream in = new FileInputStream("theTime");
ObjectInputStream s = new ObjectInputStream(in);
String today = (String)s.readObject();
Date date = (Date)s.readObject();
™ ObjectInputStream must be constructed on another stream.
In this example, the objects were archived in a file, so the
code constructs an ObjectInputStream on a FileInputStream.
• Example: DataIOTest2.java
Ji Zhenyan, ITM, Mitthögskolan
Providing Object Serialization
• An object is serializable only if its class
implements the Serializable interface
• Serializable is an empty interface
public class MySerializableClass implements Serializable { ... }
Ji Zhenyan, ITM, Mitthögskolan
Ji Zhenyan, ITM, Mitthögskolan