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.

📦 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 {
    //...    
    private void adjustSalaries(AdminStaff adminStaff){ //violates ISP
        //...
    }

}

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

📦 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 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 Rectangle#resize() can take any integers for height and width. This contract is violated by the subclass Square#resize() because it does not accept a height that is different from the width.

class Rectangle {
    ...
    /** sets the size to given height and width*/
    void resize(int height, int width){
        ...
    }
}


class Square extends Rectangle {
    
    @Override
    void resize(int height, int width){
        if (height != width) {
            //error
       }
    }
}

Now consider the following method that is written to work with the Rectangle class.

void makeSameSize(Rectangle original, Rectangle toResize){
    toResize.resize(original.getHeight(), original.getWidth());
}

This code will fail if it is called as maekSameSize(new Rectangle(12,8), new Square(4, 4)) That is, Square class is not substitutable for the Rectangle class.

📦 In design (a), the higher level class Payroll depends on the lower level class Employee, a violation of DIP. In design (b), both Payroll and Employee depends 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. Here, Payee is the abstraction and is owned by the Payroll class.

📦 Examples for concerns:

• A specific feature, such as the code related to 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 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();
}

📦 In the example above, foo is already coupled to Bar. Upholding LoD avoids foo being coupled to Goo as well.

📦 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.

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

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