| Lesson 3 | The different filter streams |
| Objective | Examine the Purposes of the Filter Stream Classes. |
Purpose of Java Filter Stream Classes
There are several stream classes in the java.io package that perform different kinds of filtering. You can also write your own classes to do custom filtering. Furthermore, you can chain multiple filters together to get the effect of a combined filter.
Buffered reads and writes
The BufferedInputStream and BufferedOutputStream classes buffer reads and writes by reading or writing data first into a buffer.
Thus an application can read or write bytes to the stream without necessarily calling the underlying native method. The data is read from or written into the buffer in blocks; subsequent accesses go straight to the buffer.
This improves performance in many situations. BufferedInputStream also allows the reader to back up and reread data that's already been read.
5.3 Filter Stream Classes
java.io.FilterInputStream and java.io.FilterOutputStream are concrete superclasses
for input and output stream subclasses that somehow modify or manipulate data of an
underlying stream:
public class FilterInputStream extends InputStream
public class FilterOutputStream extends OutputStream
Each of these classes has a single protected constructor that specifies the underlying stream
from which the filter stream reads or writes data:
protected FilterInputStream(InputStream in)
protected FilterOutputStream(OutputStream out)
These constructors set protected InputStream and OutputStream fields, called in and out,
inside the FilterInputStream and FilterOutputStream classes, respectively.
protected InputStream in
protected OutputStream out
Since the constructors are protected, filter streams may only be created by subclasses. Each subclass implements a particular filtering operation. Normally, such a pattern suggests that polymorphism is going to be used heavily, with subclasses standing in for the common superclass; however, it is uncommon to use filter streams polymorphically as instances of FilterInputStream or FilterOutputStream. Most of the time, references to a filter stream are either references to a more specific subclass like BufferedInputStream or they are polymorphic references to InputStream or OutputStream with no hint of the filter left.
Reading and writing data types
The
DataInputStream and
DataOutputStream classes read and write primitive Java data types and strings in a
machine independent way.
PrintStream
You have already encountered the java.io.PrintStream class. It allows very simple printing of primitive values, objects, and string literals.
It uses the platform's default character encoding to convert characters into bytes. This class traps all IOExceptions and is primarily intended for debugging. System.out and System.err are instances of the
PrintStream class, but you can connect a PrintStream filter to other output streams as well. For example, you can chain a PrintStream to a FileOutputStream to easily write text into the file.
Pushback
The PushbackInputStream class has a one-byte pushback buffer so a program can unread the last character read.
The next time data is read from the stream, the unread character is reread.
Java7 NIO 2