Classes

• A class should have as few responsibilities as possible as opposed to number of lines of code, e.g. it should do one thing ONLY (single responsibility principle)
• You should be able to describe the class in around 25 words without using "if", "and", "or", "but" - e.g.
  • The SuperDashboard provides access to the component that last held the focus, and it also allows us to track the version and build numbers
    • the and indicates the existence of multiple responsibilities
• Goal should be to have many small classes, all with a single responsibility, instead of one large class
• Private method behavior that applies only to a small subset of the class can be an indicator that something needs to change/ violates the single responsibility principle. e.g. change

      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)
      }
  

  to

      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 SelectSql extends Sql {
          public SelectSql(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()
      }
      public class SelectWithMatchSql extends Sql {
          public SelectWithMatchSql(
          String table, Column[] columns, Column column, String pattern)
          @Override public String generate()
      }
      ...
  

  etc.

  The code in each class becomes "excruciatingly simple." This means

  • It takes practically 0 effort to understand it
  • adding to the class won't break anything else (just add a new inherited class)

• Dependency Inversion Principle - classes should depend on abstractions, NOT implementation details - it means we should be able to change our concrete implementation details without breaking the other classes that use it.