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I was asked this question recently during my job interview, and I couldn't answer it. So, what is the most used pattern in java.io and how is it used? What are other patterns used in common java libraries?

Ravindra babu
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jb.
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    See [this post](http://stackoverflow.com/questions/1673841/examples-of-gof-design-patterns/2707195#2707195) for a comprehensive list of GoF patterns used in the Java standard libraries. – Abhinav Sarkar Jun 18 '10 at 11:08
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    Check this post at Java Code Geeks about design patterns in JDK - http://www.javacodegeeks.com/2011/03/design-patterns-in-jdk.html – jb. Mar 16 '11 at 08:45

5 Answers5

33

BufferedReader etc implements decorator pattern. Any Reader, e.g. FileReader or StringReader, can be decorated with the buffering feature, which is really source-oblivious.

Other patterns

Anti-patterns

To add to what others have said, these are several anti-patterns in the Java libraries:

Antipattern: inheritance instead of composition

From Effective Java 2nd Edition, Item 16: Favor composition over inheritance:

There are a number of obvious violations of this principle in the Java platform libraries. For example, a stack is not a vector, so Stack should not extend Vector. Similarly, a property list is not a hash table, so Properties should not extend Hashtable. In both cases, composition would have been preferable.

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Antipattern: constant interfaces

From Effective Java 2nd Edition, Item 19: Use interfaces only to define types:

There are several constant interfaces in the Java platform libraries, such as java.io.ObjectStreamConstants. These interfaces should be regarded as anomalies and should not be emulated.

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Antipattern: telescoping constructor and JavaBeans patterns

From Effective Java 2nd Edition, Item 2: Consider a builder when faced with many constructor parameters (excerpt online):

Traditionally, programmers have used the telescoping constructor pattern, in which you provide a constructor with only the required parameters, another with a single optional parameters, a third with two optional parameters, and so on [...] The telescoping constructor pattern works, but it is hard to write client code when there are many parameters, and harder still to write it.

A second alternative when you are faced with many constructor parameters is the JavaBeans pattern, in which you call a parameterless constructor to create the object, and then call setter methods to set each required parameter, and each optional parameter of interest. [...] Unfortunately the JavaBeans pattern has serious disadvantages of its own [...] a JavaBean may be in an inconsistent state partway through its construction [and it] precludes the possibility of making a class immutable.

Bloch recommends using a builder pattern instead.

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Community
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polygenelubricants
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27

I guess they wanted to hear about the Decorator pattern which can be found in the various Streams, Readers and Writers.

Other patterns (small selection):

I'm pretty sure that one can find examples for almost all patterns listed on this wikipedia page in the Java SDK.

Andreas Dolk
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8

The decorator pattern is often used in java i/o.

Example

BufferedReader br = new BufferedReader(new FileReader("filename.txt"));
stacker
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4

Decorator pattern, I think. To create all flavors of Readers, Writers, input and output streams. See this, for example.

Marat Salikhov
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2

Patterns used in java.io package.

  1. Decorator_pattern.

    Examples:

    The abstract class java.io.FilterInputStream and its concrete subclasses : BufferedInputStream, CheckedInputStream etc

  2. Abstract_factory_pattern and Factory_method_pattern patterns:

    Examples:

    The abstract class InputStream and its concrete sub classes: ByteArrayInputStream, FileInputStream, FilterInputStream etc.

     InputStream input = new FileInputStream("some_file.txt");

    Below classes enable unification of input from a byte-array, a file, a network connection, a persistent storage, a pipe, a string, etc:

     class java.io.InputStream 
       class java.io.ByteArrayInputStream 
       class java.io.FileInputStream 
       class java.io.FilterInputStream 
       class java.io.ObjectInputStream
       class java.io.PipedInputStream 
       class java.io.SequenceInputStream 
       class java.io.StringBufferInputStream

    Examples:

    java.io.InputStreamReader translates a byte stream into a character stream, and a java.io.OutputStreamWriter translates a character stream into a byte stream.

  3. Template_method_pattern

    All non-abstract methods of java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer.

For all other patterns in java, refer to this post:

Examples of GoF Design Patterns in Java's core libraries

Ravindra babu
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