Computer Science 209

Introduction to Design Patterns:

Iterator

Composite

Decorator

Patterns and Design Problems

• A design pattern gives advice about solving a problem in software design

• Different problems often have features in common

• A design pattern captures a common way of solving problems in a set of rules

Example: Visiting a Set of Items in Sequence

StringTokenizer tokens = new StringTokenizer("The cat is on the mat");while (tokens.hasMoreTokens()) System.out.println(tokens.nextToken());

Iterator<String> iter = aSet.iterator();while (iter.hasNext()) System.out.println(iter.next ());

Scanner reader = new Scanner(blah blah blah);while (reader.hasNext()){ String data = reader.nextLine(); System.out.println(data);}

The Iterator Pattern

• The iterator pattern captures the common elements of this set of problems and gives advice, in the form of a set of rules, for solving them

• Each pattern has – a short name – a brief description of the context– a lengthy description of the problem– a prescription for a solution

Proven advice in a standard format

The Iterator Pattern: Context

• An aggregate contains elements

• Clients need to access the elements

• The aggregate should not expose its internal structure

• There may be multiple clients that need simultaneous access

The Iterator Pattern: Solution

• Define an iterator class that fetches one element at a time

• Each iterator object tracks the position of the next element

• Because there might be multiple aggregate and iterator classes, it is best to have a single iterator interface

The Context of the Composite Pattern

• I want to combine primitive objects into compound objects

• I want clients to treat these compounds as primitives

Solution of the Composite Pattern

• Define an interface as an abstraction for the primitive objects

• A composite object contains a collection of the primitives

• Both primitives and composites implement the interface

• Composite applies method to primitives and combines results

The Composite Pattern

Composite

<<interface>>Primitive

Leaf

method()

method()

*

The composite calls method() for each leaf and combines the results.

Example: GUI Components

• Some components, like buttons and fields, are primitives

• Other components, like frames and panels, are containers (composites)

• A container computes its preferred size by combining all the preferred sizes of its components

The Context of the Decorator Pattern

• I want to enhance the behavior of a class, called the component class

• A decorated component can be used in the same way as a plain component

• The component does not want the responsibility of the decoration

• There may be any number of decorations

Solution of the Decorator Pattern

• Define an interface as an abstraction for the component

• Component classes and decorator classes implement this interface

• A decorator object manages the component object that it decorates

• The decorator applies a method to a component and combines the result with the effect of the decoration

The Decorator Pattern

Decorator

<<interface>>Component

ConcreteComponent

method()

method()

1

The decorator calls method() for the component and augments the results.

Example: Scroll Panes

• A scroll pane can decorate several components, such as text areas and list boxes

• The scroll pane’s paint method paints both the decorated component and the scroll bars

Using JScrollPane

JTextArea outputArea = new JTextArea();

JScrollPane outputPane = new JScrollPane(outputArea);

container.add(outputPane);

// Reference outputArea later in the program

Example: File Streams

• java.io includes many classes for manipulating different kinds of I/O streams

• Start with a basic input or output stream and decorate it until you get the type of stream you want

Using Input StreamsFileInputStream stream = new FileInputStream("anyfile.txt");

InputStreamReader iStrReader = new InputStreamReader(stream); // Now read characters from iStrReader or

BufferedReader reader = new BufferedReader(iStrReader);

// Now read lines of text from reader or

StreamTokenizer nizer = new StreamTokenizer(reader);

// Now read tokens (words) from nizer

Each decoration adds a new layer of functionality

Example: A ScannerScanner reader = new Scanner(new File("anyfile.txt")); // Now read data from the file

The same pattern is applied in similar situations

Scanner reader = new Scanner(System.in); // Now read data from the keyboard

Scanner reader = new Scanner("any file many other words text"); // Now read data from the string

Where Do Patterns Come From?

• Software best practices

• Catalogued and discussed in the Gang of Four book, Design Patterns

• Web sites abound

How Do I Spot the Need for One?

• Remember a place where a pattern is put to use and see the similarity of the current situation to that place

• Look at the intent of a pattern to see whether it applies in the current situation

• Review the context and solution parts of each potential pattern in detail