Chapter 10: Classes The Single Responsibility Principle To Co

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Learning objectives

• Explain the main ideas and vocabulary in Classes The Single Responsibility Principle To Co.
• Work through the source examples for Classes The Single Responsibility Principle To Co without depending on raw chunk order.
• Use Classes The Single Responsibility Principle To Co as selective reference when learner modules point back to Clean Code.

Prerequisites

• Earlier prerequisite concepts leading into Chapter 10: Classes The Single Responsibility Principle To Co.

Module targets

• module-01-ood-foundations-and-smells
• module-03-clean-code

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Source-of-truth note

This unit is anchored to Clean Code and the source chapter "Chapter 10: Classes The Single Responsibility Principle To Co". Use external resources only to clarify, extend, or modernize details without replacing the chapter's conceptual spine.

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Source provenance

• Primary source: Clean Code
• Source chapter 10: Chapter 10: Classes The Single Responsibility Principle To Co
• Raw source file: 041-chapter-10-classes-the-single-responsibility-principle-to-co.md
• Raw source file: 042-maintaining-cohesion-results-in-many-small-classes.md

Merged source

Chapter 10 Classes The Single Responsibility Principle To Co

Chapter 10: Classes: The Single Responsibility Principle to Cohesion

The Single Responsibility Principle

The Single Responsibility Principle (SRP)2 states that a class or module should have one, and only one, reason to change. This principle gives us both a definition of responsibility, and a guidelines for class size. Classes should have one responsibility-one reason to change.

2. You can read much more about this principle in [PPP].

The seemingly small SuperDashboard class in Listing 10-2 has two reasons to change. First, it tracks version information that would seemingly need to be updated every time the software gets shipped. Second, it manages Java Swing components (it is a derivative of JFrame, the Swing representation of a top-level GUI window). No doubt we'll want to update the version number if we change any of the Swing code, but the converse isn't necessarily true: We might change the version information based on changes to other code in the system. Trying to identify responsibilities (reasons to change) often helps us recognize and create better abstractions in our code. We can easily extract all three SuperDashboard methods that deal with version information into a separate class named Version. (See

Listing 10-3.) The Version class is a construct that has a high potential for reuse in other

applications!

Listing 10-3

A single-responsibility class
public class Version {
    public int getMajorVersionNumber()
    public int getMinorVersionNumber()
    public int getBuildNumber()
}

SRP is one of the more important concept in OO design. It's also one of the simpler concepts to understand and adhere to. Yet oddly, SRP is often the most abused class design principle. We regularly encounter classes that do far too many things. Why? Getting software to work and making software clean are two very different activities. Most of us have limited room in our heads, so we focus on getting our code to work more than organization and cleanliness. This is wholly appropriate. Maintaining a separation of concerns is just as important in our programming activities as it is in our programs. The problem is that too many of us think that we are done once the program works. We fail to switch to the other concern of organization and cleanliness. We move on to the next problem rather than going back and breaking the overstuffed classes into decoupled units with single responsibilities. At the same time, many developers fear that a large number of small, single-purpose classes makes it more difficult to understand the bigger picture. They are concerned that they must navigate from class to class in order to figure out how a larger piece of work gets accomplished. However, a system with many small classes has no more moving parts than a system with a few large classes. There is just as much to learn in the system with a few large classes. So the question is: Do you want your tools organized into toolboxes with many small drawers each containing well-defined and well-labeled components? Or do you want a few drawers that you just toss everything into? Every sizable system will contain a large amount of logic and complexity. The primary goal in managing such complexity is to organize it so that a developer knows where to look to find things and need only understand the directly affected complexity at any given time. In contrast, a system with larger, multipurpose classes always hampers us by insisting we wade through lots of things we don't need to know right now. To restate the former points for emphasis: We want our systems to be composed of many small classes, not a few large ones. Each small class encapsulates a single responsibility, has a single reason to change, and collaborates with a few others to achieve the desired system behaviors.

Cohesion

Classes should have a small number of instance variables. Each of the methods of a class should manipulate one or more of those variables. In general the more variables a method manipulates the more cohesive that method is to its class. A class in which each variable is used by each method is maximally cohesive. In general it is neither advisable nor possible to create such maximally cohesive classes; on the other hand, we would like cohesion to be high. When cohesion is high, it means that the methods and variables of the class are co-dependent and hang together as a logical whole. Consider the implementation of a Stack in Listing 10-4. This is a very cohesive class. Of the three methods only size() fails to use both the variables.

Listing 10-4

Stack.java A 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;
  }
}

The strategy of keeping functions small and keeping parameter lists short can sometimes lead to a proliferation of instance variables that are used by a subset of methods. When this happens, it almost always means that there is at least one other class trying to get out of the larger class. You should try to separate the variables and methods into two or more classes such that the new classes are more cohesive.

---

Maintaining Cohesion Results In Many Small Classes

Maintaining Cohesion Results in Many Small Classes

Maintaining Cohesion Results in Many Small Classes

Just the act of breaking large functions into smaller functions causes a proliferation of classes. Consider a large function with many variables declared within it. Let's say you want to extract one small part of that function into a separate function. However, the code you want to extract uses four of the variables declared in the function. Must you pass all four of those variables into the new function as arguments? Not at all! If we promoted those four variables to instance variables of the class, then we could extract the code without passing any variables at all. It would be easy to break the function up into small pieces. Unfortunately, this also means that our classes lose cohesion because they accumulate more and more instance variables that exist solely to allow a few functions to share them. But wait! If there are a few functions that want to share certain variables, doesn't that make them a class in their own right? Of course it does. When classes lose cohesion, split them! So breaking a large function into many smaller functions often gives us the opportunity to split several smaller classes out as well. This gives our program a much better organization and a more transparent structure. As a demonstration of what I mean, let's use a time-honored example taken from Knuth's wonderful book Literate Programming.3 Listing 10-5 shows a translation into Java of Knuth's PrintPrimes program. To be fair to Knuth, this is not the program as he wrote it but rather as it was output by his WEB tool. I'm using it because it makes a great starting place for breaking up a big function into many smaller functions and classes.

Listing 10-5

PrintPrimes.java
package literatePrimes;
public class PrintPrimes {
  public static void main(String[] args) {
    final int M = 1000;
    final int RR = 50;
    final int CC = 4;
    final int WW = 10;
    final int ORDMAX = 30;
    int P[] = new int[M + 1];
    int PAGENUMBER;
    int PAGEOFFSET;
    int ROWOFFSET;
    int C;

3. [Knuth92].

Listing 10-5 (continued)

PrintPrimes.java
    int J;
    int K;
    boolean JPRIME;
    int ORD;
    int SQUARE;
    int N;
    int MULT[] = new int[ORDMAX + 1];
    J = 1;
    K = 1;
    P[1] = 2;
    ORD = 2;
    SQUARE = 9;
    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;
      }
    }
  }
}

This program, written as a single function, is a mess. It has a deeply indented structure, a plethora of odd variables, and a tightly coupled structure. At the very least, the one big function should be split up into a few smaller functions.

Listing 10-6 through Listing 10-8 show the result of splitting the code in Listing 10-5

into smaller classes and functions, and choosing meaningful names for those classes, functions, and variables.

Listing 10-6

PrimePrinter.java (refactored)
package literatePrimes;
public class PrimePrinter {
  public static void main(String[] args) {
    final int NUMBER_OF_PRIMES = 1000;
    int[] primes = PrimeGenerator.generate(NUMBER_OF_PRIMES);
    final int ROWS_PER_PAGE = 50;
    final int COLUMNS_PER_PAGE = 4;
    RowColumnPagePrinter tablePrinter =
      new RowColumnPagePrinter(ROWS_PER_PAGE,
                               COLUMNS_PER_PAGE,
"The First " + NUMBER_OF_PRIMES +
                                 " Prime Numbers");
    tablePrinter.print(primes);
  }
}

Listing 10-7

RowColumnPagePrinter.java
package literatePrimes;
import java.io.PrintStream;
public class RowColumnPagePrinter {
  private int rowsPerPage;
  private int columnsPerPage;
  private int numbersPerPage;
  private String pageHeader;
  private PrintStream printStream;
  public RowColumnPagePrinter(int rowsPerPage,
                              int columnsPerPage,
                              String pageHeader) {
    this.rowsPerPage = rowsPerPage;
    this.columnsPerPage = columnsPerPage;
    this.pageHeader = pageHeader;
    numbersPerPage = rowsPerPage * columnsPerPage;
    printStream = System.out;
  }

Listing 10-7 (continued)

RowColumnPagePrinter.java
  public void print(int data[]) {
    int pageNumber = 1;
    for (int firstIndexOnPage = 0;
         firstIndexOnPage < data.length;
         firstIndexOnPage += numbersPerPage) {
      int lastIndexOnPage =
        Math.min(firstIndexOnPage + numbersPerPage - 1,
                 data.length - 1);
      printPageHeader(pageHeader, pageNumber);
      printPage(firstIndexOnPage, lastIndexOnPage, data);
      printStream.println("\f");
      pageNumber++;
    }
  }
  private void printPage(int firstIndexOnPage,
                         int lastIndexOnPage,
                         int[] data) {
    int firstIndexOfLastRowOnPage =
      firstIndexOnPage + rowsPerPage - 1;
    for (int firstIndexInRow = firstIndexOnPage;
         firstIndexInRow <= firstIndexOfLastRowOnPage;
         firstIndexInRow++) {
      printRow(firstIndexInRow, lastIndexOnPage, data);
      printStream.println("");
    }
  }
  private void printRow(int firstIndexInRow,
                        int lastIndexOnPage,
                        int[] data) {
    for (int column = 0; column < columnsPerPage; column++) {
      int index = firstIndexInRow + column * rowsPerPage;
      if (index <= lastIndexOnPage)
        printStream.format("%10d", data[index]);
    }
  }
  private void printPageHeader(String pageHeader,
                               int pageNumber) {
    printStream.println(pageHeader + " --- Page " + pageNumber);
    printStream.println("");
  }
  public void setOutput(PrintStream printStream) {
    this.printStream = printStream;
  }
}

Listing 10-8

PrimeGenerator.java
package literatePrimes;
import java.util.ArrayList;
public class PrimeGenerator {
  private static int[] primes;
  private static ArrayList<Integer> multiplesOfPrimeFactors;
  protected static int[] generate(int n) {
    primes = new int[n];
    multiplesOfPrimeFactors = new ArrayList<Integer>();
    set2AsFirstPrime();
    checkOddNumbersForSubsequentPrimes();
    return primes;
  }
  private static void set2AsFirstPrime() {
    primes[0] = 2;
    multiplesOfPrimeFactors.add(2);
  }
  private static void checkOddNumbersForSubsequentPrimes() {
    int primeIndex = 1;
    for (int candidate = 3;
         primeIndex < primes.length;
         candidate += 2) {
      if (isPrime(candidate))
        primes[primeIndex++] = candidate;
    }
  }
  private static boolean isPrime(int candidate) {
    if (isLeastRelevantMultipleOfNextLargerPrimeFactor(candidate)) {
      multiplesOfPrimeFactors.add(candidate);
      return false;
    }
    return isNotMultipleOfAnyPreviousPrimeFactor(candidate);
  }
  private static boolean
  isLeastRelevantMultipleOfNextLargerPrimeFactor(int candidate) {
    int nextLargerPrimeFactor = primes[multiplesOfPrimeFactors.size()];
    int leastRelevantMultiple = nextLargerPrimeFactor * nextLargerPrimeFactor;
    return candidate == leastRelevantMultiple;
  }
  private static boolean
  isNotMultipleOfAnyPreviousPrimeFactor(int candidate) {
    for (int n = 1; n < multiplesOfPrimeFactors.size(); n++) {
      if (isMultipleOfNthPrimeFactor(candidate, n))
        return false;
    }

Listing 10-8 (continued)

PrimeGenerator.java
    return true;
  }
  private static boolean
  isMultipleOfNthPrimeFactor(int candidate, int n) {
    return
      candidate == smallestOddNthMultipleNotLessThanCandidate(candidate, n);
  }
  private static int
  smallestOddNthMultipleNotLessThanCandidate(int candidate, int n) {
    int multiple = multiplesOfPrimeFactors.get(n);
    while (multiple < candidate)
      multiple += 2 * primes[n];
    multiplesOfPrimeFactors.set(n, multiple);
    return multiple;
  }
}

The first thing you might notice is that the program got a lot longer. It went from a little over one page to nearly three pages in length. There are several reasons for this growth. First, the refactored program uses longer, more descriptive variable names. Second, the refactored program uses function and class declarations as a way to add commentary to the code. Third, we used whitespace and formatting techniques to keep the program readable. Notice how the program has been split into three main responsibilities. The main program is contained in the PrimePrinter class all by itself. Its responsibility is to handle the execution environment. It will change if the method of invocation changes. For example, if this program were converted to a SOAP service, this is the class that would be affected. The RowColumnPagePrinter knows all about how to format a list of numbers into pages with a certain number of rows and columns. If the formatting of the output needed changing, then this is the class that would be affected. The PrimeGenerator class knows how to generate a list prime numbers. Notice that it is not meant to be instantiated as an object. The class is just a useful scope in which its variables can be declared and kept hidden. This class will change if the algorithm for computing prime numbers changes. This was not a rewrite! We did not start over from scratch and write the program over again. Indeed, if you look closely at the two different programs, you'll see that they use the same algorithm and mechanics to get their work done. The change was made by writing a test suite that verified the precise behavior of the first program. Then a myriad of tiny little changes were made, one at a time. After each change the program was executed to ensure that the behavior had not changed. One tiny step after another, the first program was cleaned up and transformed into the second.