Question

I have been studying design patterns lately. I am struggling with finding a scenario that I can implement builder pattern for that scenario. Has anyone got any example?

Any answer will be much appreciated.

Answer 1

SQLOrm serves as a nice example of combining the builder pattern with a proper set of methodnames making the construction of SQL feel like working with a DSL.

eg

private String getPersonAndmaybePet_JavaStr(boolean personName, boolean isAdult, int[] petTypes) {
  StringBuilder sb = new StringBuilder();
  // do SELECT
  sb.append("SELECT ");
  if(personName) 
    sb.append("person.name,");
  if(petTypes != null) 
    sb.append("pet.*,");

  // do FROM
  sb.delete(sb.length() - 1, sb.length()); // remove last comma
  sb.append(" FROM person,");
  if(petTypes != null) 
    sb.append(" Pet,");

  // do WHERE
  sb.delete(sb.length() - 1, sb.length()); // remove last comma
  sb.append(" WHERE ");
  if(isAdult) 
    sb.append(" person.age >= 18");
  else 
    sb.append(" person.age < 18");

  if(petTypes != null) {
    StringBuilder commaList = new StringBuilder();
    for(int id : petTypes) {
        commaList.append(id);
        commaList.append(",");
    }
    commaList.delete(commaList.length() - 1, commaList.length());
    sb.append(" AND person.pet_id = pet.id AND pet.pettype IN (" + commaList.toString() + ")");
  }
  return sb.toString();
}

becomes

private String getPersonAndmaybePet(boolean personName, boolean isAdult, int[] petTypes) {
  SelectBuilder qry = new SelectBuilder();

  if(personName) 
    qry.select("person.name").from("person");    

  if(isAdult) 
    qry.from("person").where().and("person.age >= 18");
  else 
    qry.from("person").where().and("person.age < 18");

  if(petTypes != null) {
    qry.select("pet.*").from("Pet") //
            .where() //
            .and("person.pet_id = pet.id") //
            .and("pet.pettype IN (#)", QueryBuilderHelper.numbers(petTypes));
  }
  return qry.toSql();
}