A simple lazy collections framework in Apex

The operations are run only when toList() or toMapByField() is called on the collection. The records are looped over only once

• Usage

Account[] result = acctCollection
  .mapValues(new AccountMap())
  .filterValues(new AccountRevenueFilter())
  .mapValues(new AccountMap())
  .toList();

• Aternatively, you can retrieve a map of record by field value

Map<String,Account> acctIndustryMap = new Map<String,Account>();
acctCollection
  .mapValues(new AccountMap())
  .filterValues(new AccountRevenueFilter())
  .mapValues(new AccountMap())
  .toMapByField(Account.Industry,acctIndustryMap);
//acctIndustryMap will contain the mapped values. If there are multiple records with the same 'Industry' value, the last record in the list with the value will be returned in the map

• or, you can retrieve a map of list of records by field value

Map<String,List<Account>> acctsIndustryMap = new Map<String,List<Account>>();
acctCollection
  .mapValues(new AccountMap())
  .filterValues(new AccountRevenueFilter())
  .mapValues(new AccountMap())
  .toMapByField(Account.Industry,acctsIndustryMap);
//acctIndustryMap will contain the mapped values

• Creating a Collection

Account[] accounts = [Select Name,AnnualRevenue from Accounts];
Collection acctCollection = Collection.of(accounts);

• How to implement a mapper class

class AccountMap implements MapOperation {
  public SObject doMap(SObject a) {
    Account acc = (Account) a;
    acc.AnnualRevenue = acc.Name.startsWith(('Ac)
      ? acc.AnnualRevenue == null ? 10000 : acc.AnnualRevenue * 2
      : 5000;
    return acc;
  }
}

• How to implement a filter class

class AccountRevenueFilter implements FilterOperation {
  public Boolean doFilter(SObject a) {
    return ((Account) a).AnnualRevenue > 5001;
  }
}