Then the code wraps that computer in a wrapper that adds a hard disk in Java

Encoder Code 128C in Java Then the code wraps that computer in a wrapper that adds a hard disk

public class Test { public static void main(String args[]) { Computer computer = new Computer(); computer = new Disk(computer); } }

public class Test { public static void main(String args[]) { Computer computer = new Computer(); computer = new Disk(computer); computer = new Monitor(computer); } }

Then, you might as well add not just one CD drive, but two cost is no consideration here Finally, you can display the resulting wrapped computer s full configuration by calling the computer s final description method

public class Test { public static void main(String args[]) { Computer computer = new Computer(); computer computer computer computer = = = = new new new new Disk(computer); Monitor(computer); CD(computer); CD(computer);

Systemoutprintln( You re getting a + computerdescription() + ); } }

You already know about decorator classes if you ve ever worked with the file system in Java That s how Java structures its file system classes as decorators Do you want to read data in a buffered way You might take a look at a basic file-reading object, an InputStream object but there s no accessible buffering there So you might wrap an InputStream object inside a FilterInputStream object, and then wrap that in a BufferedInputStream object The final wrapper, BufferedInputStream, will give you the buffering you want Here s the class hierarchy: javalangObject |_javaioInputStream |_javaioFilterInput Stream |_javaioBuffered InputStream And there you go; a BufferedInputStream object buffers what it gets from the objects it s wrapped, which in this case is a FilterInputStream object, which in turn wraps an InputStream object That s the Decorator pattern at work, pure and simple Here s what the Java 15 documents on FilterInputStream have to say note how this description says Decorator in just about every line: A FilterInputStream contains some other input stream, which it uses as its basic source of data, possibly transforming the data along the way or providing additional functionality The class FilterInputStream itself simply overrides all methods of InputStream with versions that pass all requests to the contained input stream Subclasses of FilterInputStream may further override some of these methods and may also provide additional methods and fields

Not bad You were able to extend the core object simply by wrapping it in various decorator wrappers, avoiding modification of the core code Each successive wrapper called the description method of the object it wrapped in this case and added something to it That s how you use the Decorator design pattern

Here, in your capacity of highly paid, hotshot, design pattern pro for MegaGigaCo, you re creating a new database connection object Behold the new operator at work, creating an object in a Java application:

Connection connection = new OracleConnection();

Not bad, you think, after finishing the coding for your OracleConnection class Now you can connect to Oracle databases But, wails the MegaGigaCo CEO, what about connecting to Microsoft s SQL Server Alright, you say, calm down Let me think about this You go off to lunch and then return to find the CEO and board of directors waiting anxiously in your office and asking, Is it done yet You get to work and create a new database connection class, SqlServerConnection And you re able to create objects of this new class like this:

Connection connection = new SqlServerConnection();

Great! cries the CEO Um, what about connecting to MySQL We want to make that the default connection Jeez, you think But you set to work, and presently, you get the useful MySqlConnection put together and presto, now you can connect to MySQL databases

Connection connection = new MySqlConnection();

But now you ve got three different kinds of connections to make: Oracle, SQL Server, and MySQL So you might start to adapt your code to make a connection based on the value in a variable named type: Oracle , SQL Server , or anything else (which results in the default connection to MySQL)

Connection connection; if (typeequals( Oracle )){ connection= new OracleConnection(); } else if (typeequals( SQL Server )){ connection = new SqlServerConnection(); } else { connection = new MySqlConnection(); }

That s all fine, you think, but there are about 200 places in your code where you need to create a database connection So maybe it s time to put this code into a method, createConnection, and pass the type of connection you want to that method as follows:

public Connection createConnection(String type) { }

You can return the correct connection object, depending on what type of connection is required:

public Connection createConnection(String type) { if (typeequals( Oracle )){ return new OracleConnection(); } else if (typeequals( SQL Server )){ return new SqlServerConnection(); } else { return new MySqlConnection(); } }

Bingo, you think What could go wrong with that Bad news, cries the CEO, running into your office suddenly We need to rework your code to handle secure connections to all database servers as well! The board of our Western division is demanding it