Single responsibility principle (SRP): A class should have one, and only one, reason to change. -- Robert C. Martin
If a class has only one responsibility, it needs to change only when there is a change to that responsibility.
Consider a TextUi
class that does parsing of the user commands as well as interacting with the user. That class needs to change when the formatting of the UI changes as well as when the syntax of the user command changes. Hence, such a class does not follow the SRP.
Gather together the things that change for the same reasons. Separate those things that change for different reasons. ―- Agile Software Development, Principles, Patterns, and Practices by Robert C. Martin
The Open-Closed Principle aims to make a code entity easy to adapt and reuse without needing to modify the code entity itself.
Open-closed principle (OCP): A module should be open for extension but closed for modification. That is, modules should be written so that they can be extended, without requiring them to be modified. -- proposed by Bertrand Meyer
In object-oriented programming, OCP can be achieved in various ways. This often requires separating the specification (i.e. interface) of a module from its implementation.
In the design given below, the behavior of the CommandQueue
class can be altered by adding more concrete Command
subclasses. For example, by including a Delete
class alongside List
, Sort
, and Reset
, the CommandQueue
can now perform delete commands without modifying its code at all. That is, its behavior was extended without having to modify its code. Hence, it is open to extensions, but closed to modification.
The behavior of a Java generic class can be altered by passing it a different class as a parameter. In the code below, the ArrayList
class behaves as a container of Students
in one instance and as a container of Admin
objects in the other instance, without having to change its code. That is, the behavior of the ArrayList
class is extended without modifying its code.
ArrayList students = new ArrayList<Student>();
ArrayList admins = new ArrayList<Admin>();
Liskov substitution principle (LSP): Derived classes must be substitutable for their base classes. -- proposed by Barbara Liskov
LSP sounds the same as substitutability but it goes beyond substitutability; LSP implies that a subclass should not be more restrictive than the behavior specified by the superclass. As you know, Java has language support for substitutability. However, if LSP is not followed, substituting a subclass object for a superclass object can break the functionality of the code.
Suppose the Payroll
class depends on the adjustMySalary(int percent)
method of the Staff
class. Furthermore, the Staff
class states that the adjustMySalary
method will work for all positive percent values. Both the Admin
and Academic
classes override the adjustMySalary
method.
Now consider the following:
Admin#adjustMySalary
method works for both negative and positive percent values.Academic#adjustMySalary
method works for percent values 1..100
only.In the above scenario,
Admin
class follows LSP because it fulfills Payroll
’s expectation of Staff
objects (i.e. it works for all positive values). Substituting Admin
objects for Staff
objects will not break the Payroll
class functionality.Academic
class violates LSP because it will not work for percent values over 100
as expected by the Payroll
class. Substituting Academic
objects for Staff
objects can potentially break the Payroll
class functionality.The five OOP principles given below are known as SOLID Principles (an acronym made up of the first letter of each principle):
Separation of concerns principle (SoC): To achieve better modularity, separate the code into distinct sections, such that each section addresses a separate concern. -- Proposed by Edsger W. Dijkstra
A concern in this context is a set of information that affects the code of a computer program.
Examples for concerns:
add employee
featurepersistence
or security
Employee
entityApplying reduces functional overlaps among code sections and also limits the ripple effect when changes are introduced to a specific part of the system.
If the code related to persistence is separated from the code related to security, a change to how the data are persisted will not need changes to how the security is implemented.
This principle can be applied at the class level, as well as at higher levels.
The n-tier architecture utilizes this principle. Each layer in the architecture has a well-defined functionality that has no functional overlap with each other.
This principle should lead to higher cohesion and lower coupling.
Law of Demeter (LoD):
Also known as
More concretely, a method m
of an object O
should invoke only the methods of the following kinds of objects:
O
itselfm
m
(directly or indirectly)O
The following code fragment violates LoD due to the following reason: while b
is a ‘friend’ of foo
(because it receives it as a parameter), g
is a ‘friend of a friend’ (which should be considered a ‘stranger’), and g.doSomething()
is analogous to ‘talking to a stranger’.
void foo(Bar b) {
Goo g = b.getGoo();
g.doSomething();
}
LoD aims to prevent objects from navigating the internal structures of other objects.
An analogy for LoD can be drawn from Facebook. If Facebook followed LoD, you would not be allowed to see posts of friends of friends, unless they are your friends as well. If Jake is your friend and Adam is Jake’s friend, you should not be allowed to see Adam’s posts unless Adam is a friend of yours as well.