Vlad's Roam Garden

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G22: Make Logical Dependencies Physical If one module depends upon another, that dependency should be physical, not just logical. The dependent module should not make assumptions (in other words, logical dependencies) about the module it depends upon. Rather it should explicitly ask that module for all the information it depends upon.

For example, imagine that you are writing a function that prints a plain text report of hours worked by employees. One class named HourlyReporter gathers all the data into a convenient form and then passes it to HourlyReportFormatter to print it. (See Listing 17-1 .)

Listing 17-1 HourlyReporter.java

   public class HourlyReporter {
     private HourlyReportFormatter formatter;
     private List<LineItem> page;
     private final int PAGE_SIZE = 55;
     public HourlyReporter(HourlyReportFormatter formatter) {
       this.formatter = formatter;
       page = new ArrayList<LineItem>();

     public void generateReport(List<HourlyEmployee> employees) {
       for (HourlyEmployee e : employees) {
         if (page.size() == PAGE_SIZE)
       if (page.size() > 0)

     private void printAndClearItemList() {
     private void addLineItemToPage(HourlyEmployee e) {
       LineItem item = new LineItem();
       item.name = e.getName();
       item.hours = e.getTenthsWorked() / 10;
       item.tenths = e.getTenthsWorked() % 10;
     public class LineItem {
       public String name;
       public int hours;
       public int tenths;

This code has a logical dependency that has not been physicalized. Can you spot it? It is the constant PAGE_SIZE. Why should the HourlyReporter know the size of the page? Page size should be the responsibility of the HourlyReportFormatter.

The fact that PAGE_SIZE is declared in HourlyReporter represents a misplaced responsibility [G17] that causes HourlyReporter to assume that it knows what the page size ought to be. Such an assumption is a logical dependency. HourlyReporter depends on the fact that HourlyReportFormatter can deal with page sizes of 55. If some implementation of HourlyReportFormatter could not deal with such sizes, then there would be an error.

We can physicalize this dependency by creating a new method in HourlyReport-Formatter named getMaxPageSize(). HourlyReporter will then call that function rather than using the PAGE_SIZE constant.