Apex Collections Patterns

Apex Collections Patterns

Use this skill when reviewing or writing Apex code that uses List, Set, or Map to aggregate, de-duplicate, or look up SObject data in triggers, batch classes, or service layers. The skill covers safe null handling, heap-efficient accumulation patterns, and idiomatic bulkification using Map<Id, List>.

Before Starting

Gather this context before working on anything in this domain:

• Is the code running in a trigger (200-record scope), a batch execute() chunk (configurable scope), or synchronous Apex? The heap pressure and loop patterns differ.
• Is the class implementing Database.Stateful? If so, any instance-level Map or List grows across every execute() chunk and can exhaust the 6 MB heap before the job finishes.
• Are Set intersection or subtraction operations needed? If so, confirm whether mutating the receiver is acceptable — retainAll() and removeAll() modify the Set in place.
• What is the maximum expected volume of records? A Map keyed on Id with one value per key is O(n); a Map<Id, List> with unbounded inner lists can be O(n²) if records share the same key frequently.

Core Concepts

Collections Are Heap-Allocated Against the 6 MB Limit

Every List, Set, and Map in an Apex transaction contributes to the heap. A Map<Id, List> is the standard bulkification container in trigger handlers: one SOQL returns all related records, and the Map groups them by parent Id. However, each inner List object also consumes heap. In a Database.Stateful batch job, instance-level Map or List fields persist across every execute() call — growing unboundedly until the job finishes or the 6 MB limit kills the transaction. The Apex Developer Guide states the heap limit as 6 MB for synchronous transactions and async transactions alike.

Map.get() Returns null — Not an Exception

Map.get(key) returns null when the key is absent. This is different from Java's behavior and different from what many LLMs assume. Calling .size(), iterating, or performing any operation on a null return causes a NullPointerException. The correct guard is Map.containsKey(key) before Map.get(key), or assigning to a variable and null-checking before use. This applies equally to Trigger.oldMap.get(Id) inside after-update triggers.

Set Mutation — retainAll() and removeAll() Are In-Place

Set.retainAll(otherCollection) modifies the receiver Set to keep only elements present in both collections (intersection). Set.removeAll(otherCollection) removes all elements present in the argument (subtraction). Both are destructive to the original Set. If the original Set is needed after the operation, copy it first with new Set<Id>(originalSet) before calling retainAll() or removeAll(). Building a new Set manually in a loop instead of calling retainAll() is verbose, slower, and a frequent LLM anti-pattern.

putAll(List) Keys on the SObject Id Field

Map<Id, SObject>.putAll(List<SObject>) inserts all records into the Map using each record's Id field as the key. Records with a null Id (unsaved records) will cause a NullPointerException. Records with duplicate Ids — possible in Trigger.new on update when the same record appears — will silently overwrite the prior entry. This is intentional behavior for trigger maps (most recent value wins) but can be surprising in other contexts.

Common Patterns

Map<Id, List> for Bulkified Trigger Lookups

When to use: An after-insert or after-update trigger on a child object needs to group child records by their parent Id before performing a single DML or SOQL operation at the parent level.

How it works:

1. Query all relevant parent records using the set of parent Ids extracted from Trigger.new.
2. Build a Map<Id, List<Child__c>> by iterating the query results once, using Map.containsKey() guard before Map.get().
3. Iterate Trigger.new, look up each record's parent group from the Map, and accumulate changes.
4. Perform a single bulkified DML call outside all loops.

Reference the templates/apex/TriggerHandler.cls scaffold for the handler structure. The collection building belongs in the handler's afterInsert() / afterUpdate() methods, not in a trigger body directly.

Why not the alternative: Querying inside a for loop over Trigger.new runs one SOQL per record, burning the 100-query limit on any bulk load of 100+ records.

Safe Set Intersection With retainAll()

When to use: A service method needs to find the overlap between two Sets — for example, the set of record Ids that are both in a new batch and in an existing do-not-process exclusion list.

How it works:

1. Construct the first Set from the incoming Ids: Set<Id> incoming = new Set<Id>(triggerIds);
2. Construct or load the exclusion Set from a SOQL or Custom Metadata query.
3. Create a working copy if the original Set must be preserved: Set<Id> overlap = new Set<Id>(incoming);
4. Call overlap.retainAll(exclusionIds); — the result is the intersection in one platform operation.
5. Subtract from the working set to get records that are NOT excluded: incoming.removeAll(exclusionIds);

Why not the alternative: Building a new Set by iterating and adding manually is O(n) extra code, allocates additional intermediate objects, and is more likely to introduce off-by-one bugs.

Decision Guidance

Recommended Workflow

Step-by-step instructions for an AI agent or practitioner working on this task:

1. Identify the Apex context — trigger, batch, or service — and note the scope size and whether Database.Stateful is involved.
2. Locate every collection declaration and flag any that are instance-level fields in a Database.Stateful class; these are candidates for heap exhaustion.
3. Find every Map.get() call and verify a Map.containsKey() guard or null-check precedes it; unguarded calls will throw NullPointerException when the key is absent.
4. Find every Set.retainAll() or Set.removeAll() call and confirm the caller does not expect the original Set to be unmodified; if it does, the Set must be copied first.
5. Verify that SOQL queries and DML statements are outside all for loops; collections should accumulate changes across the loop, with a single bulk operation after.
6. Run python3 scripts/check_apex_collections_patterns.py --manifest-dir <path> against the target metadata directory.
7. Review the output artifacts against the Review Checklist below before closing the task.

Review Checklist

• All Map.get() calls are preceded by Map.containsKey() or the return value is null-checked before use.
• No Database.Stateful batch class accumulates unbounded Map or List values across execute() chunks.
• Set.retainAll() and Set.removeAll() are called on copies, not the original collection, when the original is needed afterward.
• No SOQL query or DML statement appears inside a for loop.
• Map<Id, SObject> construction from List<SObject> uses the Map constructor (new Map<Id, SObject>(list)) rather than a manual loop where possible.
• Inner Lists in Map<Id, List<SObject>> are initialized with containsKey guard (not overwriting an existing list).
• Any putAll(List<SObject>) call is used only on records with guaranteed non-null Ids.

Salesforce-Specific Gotchas

1. Map.get() returns null silently — unlike Java's optional approach, Apex Map.get() returns null for a missing key with no exception. Code that chains .size() or iterates the result without a null guard will throw NullPointerException at runtime — not at compile time — and only on data paths where the key is absent.

2. Database.Stateful instance collections grow across every execute() chunk — if a Database.Stateful batch class declares a Map<Id, List<SObject>> or List<SObject> as an instance field, the collection grows with every chunk processed. For a 200-scope batch over 100,000 records, the collection accumulates data from 500 chunks before finish() runs. Heap exhaustion causes the entire job to fail with a LimitException and no partial rollback. The fix is to flush accumulated data to the database at the end of each execute() chunk and keep only lightweight counters in instance fields.

3. putAll(List) silently overwrites duplicate keys — Map.putAll(list) uses each SObject's Id as the key. If two records in the list share an Id (possible in upsert scenarios or test data with re-used Ids), the later record silently replaces the earlier one. This produces data loss bugs that are difficult to reproduce in unit tests where each test record has a unique fake Id.

4. retainAll() and removeAll() mutate the receiver — calling mySet.retainAll(otherSet) changes mySet in place. Code that passes a Set to a helper method and then continues to use the Set after the helper called retainAll() internally will observe a mutated Set with no indication that mutation occurred. This is a silent logic bug, not an exception.

5. Set construction from a List does not preserve order — new Set<String>(myList) de-duplicates but does not guarantee insertion order. Code that converts a List to a Set for de-duplication and then iterates the Set expecting the original order will produce non-deterministic behavior across Salesforce releases.

Output Artifacts

Related Skills

• apex/trigger-framework — use when the handler structure around the collection patterns is the primary concern.
• apex/batch-apex-patterns — use when the broader batch design (scope, start/execute/finish, error handling) is the focus.
• apex/governor-limits — use when heap or CPU limits are being hit and broader limit strategy is needed.
• apex/soql-fundamentals — use when the underlying SOQL driving collection population needs optimization.
• apex/exception-handling — use when NullPointerException from unguarded Map.get() is part of a broader error handling review.