Principles

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.

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>();

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:

• The Admin#adjustMySalary method works for both negative and positive percent values.
• The Academic#adjustMySalary method works for percent values 1..100 only.

In the above scenario,

• The 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.
• The 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 Payroll class should not depend on the AdminStaff class because it does not use the arrangeMeeting() method. Instead, it should depend on the SalariedStaff interface.

public class Payroll {
    // violates ISP
    private void adjustSalaries(AdminStaff adminStaff) {
        // ...
    }

}

public class Payroll {
    // does not violate ISP
    private void adjustSalaries(SalariedStaff staff) {
        // ...
    }
}

Example:

In design (a), the higher level class Payroll depends on the lower level class Employee, which is a violation of DIP. In design (b), both Payroll and Employee depend on the Payee interface (note that inheritance is a dependency).

Design (b) is more flexible (and less coupled) because now the Payroll class need not change when the Employee class changes.

Examples for concerns:

• A specific feature, such as the code related to the add employee feature
• A specific aspect, such as the code related to persistence or security
• A specific entity, such as the code related to the Employee entity

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.

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.

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();
}

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.

A functionality being implemented twice is a violation of the DRY principle even if the two implementations are different.

The value of a system-wide timeout being defined in multiple places is a violation of DRY.