Clean Code Chapter 10 Notes

Clean Code

Chapter 10 Class Organisation

Class structure:
- List of variables:
  - Public static constants
  - Private static variables
  - Private instance variables
- Public functions
- Private functions placed directly after the public function that utilises it

Classes Should Be Small!

The size of a class is dependent upon its number of responsibilities
The name of the class describes the responsibilities it should fulfill
- If a concise named can't be created, then it's an indicator the class is too large
- Words such as Processor, Manager and Super indicate an aggregation of responsibilities

Example of a "God class":

Consider reducing the class to the small number of highlighted methods, it still has multiple responsibilities
- Tracking version number
- Manages Java Swing components

public class SuperDashboard extends JFrame implements MetaDataUser {
    public String getCustomizerLanguagePath()
    public void setSystemConfigPath(String systemConfigPath)
    public String getSystemConfigDocument()

    ... // Many, many more methods

    public Component getLastFocusedComponent()
    public void setLastFocused(Component lastFocused)
    public int getMajorVersionNumber()
    public int getMinorVersionNumber()
    public int getBuildNumber()
}

The Single Responsibility Principle

SRP states that a class / module should only have one reason to change
The previous SuperDashboard example had two reasons to change

Example of methods on SuperDashboard refactored to a single responsibility class:

public class Version {
    public int getMajorVersionNumber()
    public int getMinorVersionNumber()
    public int getBuildNumber()
}

Codebases contain large amounts of logic and complexity
- Organisation, cleanliness and refactoring of code is just as important as getting the code to work
- Despite enforcing SRP and navigating more smaller class files, there is no extended functionality from the original uncoupled classes
  - The amount of knowledge to consume is the same
  - But the system has a well-defined structure containing smaller components, improving maintainability

Cohesion

Classes should have a small number of instance variables
Methods of the class should manipulate at least one of its instance variables
The cohesiveness of a method to its class is measured by the number of variables manipulated
Maximum cohesion is unrealistic, but high cohesion is ideal, indicating the methods and variables are closely related logically

Example of a very cohesive class:

public class Stack {
    private int topOfStack = 0;
    List<Integer> elements = new LinkedList<Integer>();

    public int size() {
        return topOfStack;
    }

    public void push(int element) {
        topOfStack++;
        elements.add(element):
    }

    public int pop() throws PoppedWhenEmpty {
        if (topOfStack === 0) {
            throw new PoppedWhenEmpty();
        }

        int element = elements.get(--topOfStack);
        elements.remove(topOfStack);

        return element;
    }
}

Maintaining Cohesion Results in Many Small Classes

Consider extracting a part of a function to a separate function
- The separate function uses variables declared in the original function, they need to access them
- Don't need to pass variables as arguments, just promote them to instance variables of the class
- But adding instance variables to enable few functions reduces cohesion, instead, split functions sharing certain variables into a new class

Listing 10-5 example:

Lots of responsibility taken on by the main function
Bad variable names, indentation and highly coupled
Should be split into 2 additional classes, one for calculating primes and one for printing primes

public class PrintPrimes {
    public static void main(String[] args) {
        final int M = 1000;
        final int RR = 50;
        ... // Lots of undescriptive variables

        while (K < M) {
            do {
                J = J + 2;
                if (J == SQUARE) {
                    ORD = ORD + 1;
                    SQUARE = P[ORD] * P[ORD];
                    MULT[ORD - 1] = J;
                }
                N = 2;
                JPRIME = true;
                while (N < ORD && JPRIME) {
                    while (MULT[N] < J) {
                        MULT[N] = MULT[N] + P[N] + P[N];
                    }
                    if (MULT[N] == J) {
                        JPRIME = false;
                    }
                    N = N + 1;
                }
            } while (!JPRIME);
        K = K + 1;
        P[K] = J;
        }
        {
            PAGENUMBER = 1;
            PAGEOFFSET = 1;
            while (PAGEOFFSET <= M) {
                System.out.println("The First " + M + " Prime Numbers --- Page " + PAGENUMBER);
                System.out.println("");
                for (ROWOFFSET = PAGEOFFSET; ROWOFFSET < PAGEOFFSET + RR; ROWOFFSET++) {
                    for (C = 0; C < CC;C++) {
                        if (ROWOFFSET + C * RR <= M) {
                            System.out.format("%10d", P[ROWOFFSET + C * RR]);
                        }
                    }
                    System.out.println("");
                }
                System.out.println("\f");
                PAGENUMBER = PAGENUMBER + 1;
                PAGEOFFSET = PAGEOFFSET + RR * CC;
            }
        }
    }
}

Organising for Change

Modifications to a class has the potential of breaking other code
Organise classes to reduce the risk of change when making modifications

Example of a work in-progress class subject to change (opened):

Violates SRP, class must change when adding a new statement or alter details of a single type of statement
Multiple private methods applying to different public methods can indicate potential refactoring

public class Sql {
    public Sql(String table, Column[] columns)
    public String create()
    public String insert(Object[] fields)
    public String selectAll()
    public String findByKey(String keyColumn, String keyValue)
    public String select(Column column, String pattern)
    public String select(Criteria criteria)
    public String preparedInsert()
    private String columnList(Column[] columns)
    private String valuesList(Object[] fields, final Column[] columns)
    private String selectWithCriteria(String criteria)
    private String placeholderList(Column[] columns)public Sql(String Table, Column[] columns)
}

Ideally, new features are implemented by extending the system, not by making modifications to existing code
Factor out public interface methods into its own subclass, also moving any necessary private methods along with it
- Classes become much simpler, comprehensible and testable
- Greatly reduced risk of breaking code since modifications are made in a subclass instead
- Supports SRP, small subclasses with one reason to change
- Supports Open-Closed Principle (OCP), the restructured subclass format enables new functionality but keeps other classes closed during this process

Example of some refactored Sql subclasses:

Sql is an abstract class which declares an abstract method to be implemented by its subclasses
The concrete subclasses each implement generate()
Each subclass has its own constructor to initialise the table name, columns and other specific parameters
Private methods specific to different queries are added to their subclass

abstract public class Sql {
    public Sql(String table, Column[] columns)
    abstract public String generate();
}

public class CreateSql extends Sql {
    public CreateSql(String table, Column[] columns)
    @Override public String generate()
}

public class InsertSql extends Sql {
    public InsertSql(String table, Column[] columns, Object[] fields)
    @Override public String generate()
    private String valuesList(Object[] fields, final Column[] columns)
}

public class SelectWithCriteriaSql extends Sql {
    public SelectWithCriteriaSql(String table, Column[] columns, Criteria criteria)
    @Override public String generate()
}

Isolating from Change

Dependencies upon concrete details make testing difficult because the expected result is volatile
Instead, the class should depend on an interface
- There will be two implementations of the interface, 1 used in real scenarios and 1 used in testing
- We have control over the test implementation, with the ability to fix any value and get an expected result
Support Dependency Inversion Principle (DIP) by minimising coupling this way
- Classes should depend upon abstractions, not on concrete details