Builder Pattern says that "construct a complex object
from simple objects using step-by-step approach"
Builder pattern was introduced to solve some of the problems with
Factory and Abstract Factory design patterns when the Object contains a lot of
attributes.
There are three major issues with Factory and Abstract Factory
design patterns when the Object contains a lot of attributes.
- Too
Many arguments to pass from client program to the Factory class that can
be error prone because most of the time, the type of arguments are same
and from client side it’s hard to maintain the order of the argument.
- Some of
the parameters might be optional but in Factory pattern, we are forced to
send all the parameters and optional parameters need to send as NULL.
- If the
object is heavy and its creation is complex, then all that complexity will
be part of Factory classes that is confusing.
We can solve the issues with large number of parameters by
providing a constructor with required parameters and then different setter
methods to set the optional parameters. The problem with this approach is that
the Object state will be inconsistent until unless all the
attributes are set explicitly.
Builder pattern solves the issue with large number of optional
parameters and inconsistent state by providing a way to build the object
step-by-step and provide a method that will actually return the final Object.
For example, you can consider construction of a home. Home
is the final end product (object) that is to be returned as the output of the
construction process. It will have many steps, like basement construction, wall
construction and so on roof construction. Finally, the whole home object is
returned. Here using the same process, you can build houses with different
properties.
GOF says,
GOF says,
Advantage of Builder Design Pattern
The main advantages of
Builder Pattern are as follows:
- It provides clear separation between the construction
and representation of an object.
- It provides better control over construction process.
- It supports to change the internal representation of
objects.
Builder
Design Pattern Example in JDK
DocumentBuilderFactory, StringBuffer, StringBuilder
are some examples of builder pattern usage in java API.
Some
of the builder pattern example in Java classes are;
- java.lang.StringBuilder#append()
(unsynchronized)
- java.lang.StringBuffer#append()
(synchronized)
Usage examples:
- Builder
pattern may be very useful while writing Unit Tests. When in order to construct
the object under the test, need to pass a lot of parameters to the constructor
and some of these parameters are completely irrelevant for the specific test.
Builder class creation with separate methods for each parameter that should be
tested, which returns by the end complete object under the test will help to
write many UTs effectively, without duplicating the code.
- Building
an XML document with HTML elements (<html>,<h1>,<h2>,
<body>,<p> and etc)
- Building
a smartphone object with attributes like RAM, size, resolution, OS, waterproof
and so on.
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