Choosing the Right Collection

Use Case	Best Choice
Ordered data	List
Remove duplicates	Set
Fast lookup	Map
Avoid nested loops	Map
Bulk processing	List + Map

Correct choice improves performance and readability.

Pattern 1 – Convert List to Set for Uniqueness

Example:

List<String> emails = new List<String>{
    'a@test.com', 'b@test.com', 'a@test.com'
};

Set<String> uniqueEmails = new Set<String>(emails);

Use this to:
● Remove duplicates
● Prepare IN clause filters
● Improve performance

Pattern 2 – Map for Fast Lookup (Avoid Nested Loops)

Bad Pattern

for(Account acc : accounts) {
    for(Contact con : contacts) {
        if(con.AccountId == acc.Id) {
            // heavy processing
        }
    }
}

Optimized Pattern

Map<Id, List<Contact>> contactMap = new Map<Id, List<Contact>>();

for(Contact con : contacts) {
    if(!contactMap.containsKey(con.AccountId)) {
        contactMap.put(con.AccountId, new List<Contact>());
    }
    contactMap.get(con.AccountId).add(con);
}

Now processing is efficient and scalable.

Pattern 3 – Bulk Processing with Maps

Example:

Set<Id> accIds = new Set<Id>();

for(Contact con : Trigger.new) {
    accIds.add(con.AccountId);
}

Map<Id, Account> accMap = new Map<Id, Account>(
    [SELECT Id, Name FROM Account WHERE Id IN :accIds]
);

This pattern:
● Prevents SOQL inside loops
● Scales automatically
● Is trigger-safe

Pattern 4 – Group Records Using Map

Example:

Map<String, List<Account>> industryMap = new Map<String, List<Account>>();

for(Account acc : accounts) {
    if(!industryMap.containsKey(acc.Industry)) {
        industryMap.put(acc.Industry, new List<Account>());
    }
    industryMap.get(acc.Industry).add(acc);
}

Used for:
● Reporting logic
● Batch grouping
● Aggregation

Pattern 5 – Defensive Collection Handling

Always protect against nulls and missing keys.

if(accountMap.containsKey(accId)) {
    Account acc = accountMap.get(accId);
}

Common Pitfalls

● Nested loops on large data

● Using List instead of Map for lookup
● Forgetting containsKey checks
● SOQL inside loops

Productivity Tips

● Build maps once and reuse

● Use Set for IN filters
● Log collection sizes during debugging
● Keep collections small and focused

Conclusion

You now understand:
● Real-world collection patterns

● Performance optimization
● Bulk-safe design
● Efficient data processing

Keep Building! 💪

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Apex Collections Deep Dive