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.

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 Rectangle#resize() method 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 the 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 makeSameSize(new Rectangle(12, 8), new Square(4, 4)). That is, the Square class is not substitutable for the Rectangle class.

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.