Dart: Dart Collections

Q: Q: Should I use [] or <Type>[] for an empty List?

A: Use [] when type inference context is available. Use [] when declaring independently to avoid inferring as List .

Q: Q: What's the difference between Map's putIfAbsent and update ?

A: putIfAbsent only sets the value if the key doesn't exist. update only updates the value if the key exists (use the ifAbsent parameter to handle non-existent cases).

Q: Q: How can I sort a large List efficiently?

A: List.sort() sorts in-place, which is more efficient than [...list].sort() (which creates a copy). If you don't want to modify the original list, create a copy first with [...list] and then sort.

Last updated: 2026-08-26

Collections are the army of data — ordered lists, deduplicated sets, indexed maps. Combine them and you become invincible.

1. What You'll Learn

List: Create / Traverse / Sort / Spread (...) and Collection-if/for Elements
Set: Deduplication and Set Operations (Intersection / Union / Difference)
Map: Key-Value Operations and Iteration
Collection Type Inference and Const Collections
Bob's Scenario: Grouping and Aggregating Millions of Orders

2. A Developer's Real Story

(1) The Pain Point: Manual Grouping and Aggregation is Time-Consuming and Error-Prone

Bob needs to group 1,200,000 orders by category, calculating total revenue and order count for each. He manually implemented the grouping logic with a for loop, writing 50 lines of code, but encountered issues like duplicate key overwriting and unstable sorting. Processing 1,200,000 records took 8 seconds.

(2) The Solution: Collection Operations

Dart's collection operations combined with higher-order functions reduce the grouping and aggregation to just 10 lines of code, with a 40% speed improvement.

DART

// Group orders by category and aggregate
final byCategory = <String, List<Order>>{};
for (final order in orders) {
  byCategory.putIfAbsent(order.category, () => []).add(order);
}

// Or use fold for aggregation
final revenueByCategory = orders.fold<Map<String, double>>(
  {}, (acc, o) => acc..update(o.category, (v) => v + o.amount, ifAbsent: () => o.amount));

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

(3) The Benefits

80% reduction in code volume, with clearer logic
Set deduplication avoids duplicate data, Map grouping replaces manual indexing
Spread and collection-if make collection building more declarative

3. List

(1) Creation and Basic Operations

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: List Creation

DART

void main() {
  // Literal creation
  List<int> counts = [1, 2, 3, 4, 5];
  var amounts = <double>[1500.0, 3200.0, 890.0];

  // Growable list
  var orders = <String>['ORD-001', 'ORD-002'];
  orders.add('ORD-003');

  // Fixed-length list
  var buffer = List<double>.filled(5, 0.0);
  buffer[0] = 100.0;

  // Generate list
  var batch = List.generate(10, (i) => 'Batch-${i + 1}');

  print(counts);      // [1, 2, 3, 4, 5]
  print(orders);      // [ORD-001, ORD-002, ORD-003]
  print(buffer);      // [100.0, 0.0, 0.0, 0.0, 0.0]
  print(batch);       // [Batch-1, ..., Batch-10]
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: List Traversal and Sorting

DART

void main() {
  var amounts = [1500.0, 890.0, 3200.0, 50.0];

  // Sort in-place
  amounts.sort();
  print(amounts);  // [50.0, 890.0, 1500.0, 3200.0]

  // Sort descending (create new list)
  var descending = [...amounts]..sort((a, b) => b.compareTo(a));
  print(descending);  // [3200.0, 1500.0, 890.0, 50.0]

  // Access elements
  print(amounts.first);  // 50.0
  print(amounts.last);   // 3200.0
  print(amounts[2]);     // 1500.0

  // Sublist
  print(amounts.sublist(1, 3));  // [890.0, 1500.0]
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Spread and Collection-if/for

DART

void main() {
  var base = ['ORD-001', 'ORD-002'];
  var extra = ['ORD-003', 'ORD-004'];

  // Spread operator
  var combined = [...base, ...extra];
  print(combined);  // [ORD-001, ORD-002, ORD-003, ORD-004]

  // Null-aware spread
  List<String>? maybeList;
  var safe = ['header', ...?maybeList, 'footer'];
  print(safe);  // [header, footer]

  // Collection-if
  bool includePremium = true;
  var tiers = [
    'Standard',
    if (includePremium) 'Premium',
    'Enterprise',
  ];
  print(tiers);  // [Standard, Premium, Enterprise]

  // Collection-for
  var batch = [
    for (int i = 1; i <= 3; i++) 'Batch-$i',
  ];
  print(batch);  // [Batch-1, Batch-2, Batch-3]
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

Operation	Syntax	Description
spread	`...list`	Spreads elements
null-aware spread	`...?list`	Spreads a nullable list
collection-if	`if (cond) expr`	Conditional inclusion
collection-for	`for (var x in list) expr`	Loop generation

4. Set

(1) Creation and Set Operations

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Set Deduplication and Operations

DART

void main() {
  // Create from list (removes duplicates)
  var allOrders = ['ORD-001', 'ORD-002', 'ORD-001', 'ORD-003'];
  var uniqueOrders = <String>{...allOrders};
  print(uniqueOrders);  // {ORD-001, ORD-002, ORD-003}

  // Set operations
  var setA = {1, 2, 3, 4, 5};
  var setB = {4, 5, 6, 7, 8};

  print(setA.intersection(setB));  // {4, 5} - Intersection
  print(setA.union(setB));         // {1, 2, 3, 4, 5, 6, 7, 8} - Union
  print(setA.difference(setB));    // {1, 2, 3} - Difference

  // Membership test
  print(setA.contains(3));    // true
  print(setA.containsAll({1, 2}));  // true
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Application of Set in Deduplication Scenarios

DART

void main() {
  // Deduplicate categories from order list
  var orders = [
    {'id': 'ORD-001', 'category': 'Electronics'},
    {'id': 'ORD-002', 'category': 'Books'},
    {'id': 'ORD-003', 'category': 'Electronics'},
    {'id': 'ORD-004', 'category': 'Clothing'},
    {'id': 'ORD-005', 'category': 'Books'},
  ];

  var categories = orders.map((o) => o['category'] as String).toSet();
  print(categories);  // {Electronics, Books, Clothing}
  print('Unique categories: ${categories.length}');  // 3
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

Operation	Method	Result
Intersection	`a.intersection(b)`	Common elements
Union	`a.union(b)`	All elements
Difference	`a.difference(b)`	Elements in `a` not in `b`
Contains	`a.contains(e)`	bool
Contains All	`a.containsAll(b)`	bool

5. Map

(1) Creation and Operations

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Map Creation and Basic Operations

DART

void main() {
  // Literal creation
  var config = <String, dynamic>{
    'appName': 'DataPipeline',
    'maxRecords': 1000000,
    'taxRate': 0.08,
    'verbose': true,
  };

  // Access values
  print(config['appName']);       // DataPipeline
  print(config['missing']);       // null
  print(config['missing'] ?? 'N/A');  // N/A

  // Add/update entries
  config['outputPath'] = '/tmp/reports';  // Add
  config['taxRate'] = 0.10;               // Update

  // Safe update
  config.update('maxRecords', (v) => v * 2, ifAbsent: () => 500000);

  // Iterate
  config.forEach((key, value) => print('  $key: $value'));
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Map Iteration and Aggregation

DART

void main() {
  var orders = [
    {'id': 'ORD-001', 'category': 'Electronics', 'amount': 1500.0},
    {'id': 'ORD-002', 'category': 'Books', 'amount': 50.0},
    {'id': 'ORD-003', 'category': 'Electronics', 'amount': 3200.0},
    {'id': 'ORD-004', 'category': 'Clothing', 'amount': 890.0},
    {'id': 'ORD-005', 'category': 'Books', 'amount': 120.0},
  ];

  // Group by category
  final byCategory = <String, List<Map<String, dynamic>>>{};
  for (final order in orders) {
    final cat = order['category'] as String;
    byCategory.putIfAbsent(cat, () => []).add(order);
  }

  // Aggregate revenue by category
  final revenue = <String, double>{};
  for (final entry in byCategory.entries) {
    final total = entry.value.fold<double>(
      0, (sum, o) => sum + (o['amount'] as double));
    revenue[entry.key] = total;
  }

  // Print results
  revenue.forEach((cat, total) =>
      print('$cat: \$${total.toStringAsFixed(2)} USD'));
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

6. Collection Type Inference and Const Collections

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Type Inference and Const Collections

DART

void main() {
  // Type inference
  var list1 = [1, 2, 3];           // List<int>
  var list2 = [1, 2, 3.0];         // List<num> (mixed int and double)
  var list3 = <String>[];          // Explicit generic for empty list

  // Const collections - deeply immutable
  const categories = ['Electronics', 'Books', 'Clothing'];
  const config = <String, int>{'batchSize': 10000, 'timeout': 30};
  const flags = {true, false};

  // categories.add('Sports');  // Error! Cannot modify const

  // Const in class
  const defaultFormats = ['json', 'csv', 'html'];
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

Feature	var	final	const
Mutable Content	Yes	Yes (immutable reference)	No
Type Inference	Automatic	Automatic	Automatic
Compile-time Constant	No	No	Yes

7. Bob's Scenario: Grouping and Aggregating Millions of Orders

flowchart LR
  A[List: 1.2M Orders] --> B[where: Filter]
  B --> C[groupBy: Category]
  C --> D[Map: Category → Orders]
  D --> E[fold: Aggregate]
  E --> F[Output: Statistics]

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

▶ Example

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

: Practical Grouping and Aggregation

DART

class Order {
  final String id;
  final String category;
  final double amount;
  final String status;

  Order({required this.id, required this.category, required this.amount, required this.status});
}

Map<String, Map<String, dynamic>> analyzeOrders(List<Order> orders) {
  // Filter completed orders
  final completed = orders.where((o) => o.status == 'completed').toList();

  // Group by category
  final grouped = <String, List<Order>>{};
  for (final order in completed) {
    grouped.putIfAbsent(order.category, () => []).add(order);
  }

  // Aggregate per category
  final result = <String, Map<String, dynamic>>{};
  for (final entry in grouped.entries) {
    final total = entry.value.fold<double>(0, (s, o) => s + o.amount);
    final avg = total / entry.value.length;
    result[entry.key] = {
      'count': entry.value.length,
      'total': total,
      'average': avg,
    };
  }
  return result;
}

void main() {
  final orders = [
    Order(id: 'ORD-001', category: 'Electronics', amount: 1500.0, status: 'completed'),
    Order(id: 'ORD-002', category: 'Books', amount: 50.0, status: 'completed'),
    Order(id: 'ORD-003', category: 'Electronics', amount: 3200.0, status: 'pending'),
    Order(id: 'ORD-004', category: 'Clothing', amount: 890.0, status: 'completed'),
    Order(id: 'ORD-005', category: 'Books', amount: 120.0, status: 'completed'),
  ];

  final analysis = analyzeOrders(orders);
  print('=== Category Analysis ===');
  for (final entry in analysis.entries) {
    final stats = entry.value;
    print('${entry.key}:');
    print('  Orders:  ${stats['count']}');
    print('  Total:   \$${(stats['total'] as double).toStringAsFixed(2)} USD');
    print('  Average: \$${(stats['average'] as double).toStringAsFixed(2)} USD');
  }
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

8. Full Example: DataPipeline Collection Analyzer

DART

// ============================================
// DataPipeline Collection Analyzer
// Full demonstration of List, Set, Map operations
// ============================================

class Order {
  final String id;
  final String category;
  final double amount;
  final String status;
  final String region;

  const Order({
    required this.id,
    required this.category,
    required this.amount,
    required this.status,
    this.region = 'US',
  });
}

class OrderAnalyzer {
  final List<Order> orders;

  OrderAnalyzer(this.orders);

  // Unique categories (Set)
  Set<String> get categories => orders.map((o) => o.category).toSet();

  // Unique regions (Set)
  Set<String> get regions => orders.map((o) => o.region).toSet();

  // Completed orders (List filter)
  List<Order> get completed =>
      orders.where((o) => o.status == 'completed').toList();

  // Total revenue (fold)
  double get totalRevenue =>
      completed.fold(0.0, (sum, o) => sum + o.amount);

  // Revenue by category (Map grouping)
  Map<String, double> get revenueByCategory {
    return completed.fold<Map<String, double>>({}, (acc, o) {
      acc.update(o.category, (v) => v + o.amount, ifAbsent: () => o.amount);
      return acc;
    });
  }

  // Order count by region and status
  Map<String, Map<String, int>> get countByRegionStatus {
    final result = <String, Map<String, int>>{};
    for (final order in orders) {
      result.putIfAbsent(order.region, () => {});
      result[order.region]!.update(
        order.status, (v) => v + 1, ifAbsent: () => 1);
    }
    return result;
  }

  // Top N categories by revenue
  List<MapEntry<String, double>> topCategories(int n) {
    final sorted = revenueByCategory.entries.toList()
      ..sort((a, b) => b.value.compareTo(a.value));
    return sorted.take(n).toList();
  }

  void printReport() {
    print('=== DataPipeline Analytics Report ===');
    print('Total orders:    ${orders.length}');
    print('Completed:       ${completed.length}');
    print('Unique categories: ${categories.length}');
    print('Regions:         ${regions}');
    print('Total revenue:   \$${totalRevenue.toStringAsFixed(2)} USD');

    print('\n--- Revenue by Category ---');
    for (final entry in revenueByCategory.entries) {
      print('  ${entry.key}: \$${entry.value.toStringAsFixed(2)} USD');
    }

    print('\n--- Top Categories ---');
    for (final entry in topCategories(3)) {
      print('  ${entry.key}: \$${entry.value.toStringAsFixed(2)} USD');
    }

    print('\n--- Orders by Region & Status ---');
    for (final region in countByRegionStatus.entries) {
      print('  ${region.key}: ${region.value}');
    }
  }
}

void main() {
  final orders = [
    Order(id: 'ORD-001', category: 'Electronics', amount: 1500.0, status: 'completed', region: 'US'),
    Order(id: 'ORD-002', category: 'Books', amount: 50.0, status: 'completed', region: 'EU'),
    Order(id: 'ORD-003', category: 'Electronics', amount: 3200.0, status: 'pending', region: 'US'),
    Order(id: 'ORD-004', category: 'Clothing', amount: 890.0, status: 'completed', region: 'EU'),
    Order(id: 'ORD-005', category: 'Books', amount: 120.0, status: 'completed', region: 'US'),
    Order(id: 'ORD-006', category: 'Electronics', amount: 2100.0, status: 'completed', region: 'AP'),
  ];

  OrderAnalyzer(orders).printReport();
}

TEXT 📖 Display only

> **Output:** Run locally in DartPad or via `dart run`. All examples in the Dart course are based on Dart 3.x / Flutter 3.x. Results may vary slightly depending on the SDK version.

Output:

TEXT 📖 Display only

=== DataPipeline Analytics Report ===
Total orders:    6
Completed:       5
Unique categories: 3
Regions:         {US, EU, AP}
Total revenue:   $4660.00 USD

--- Revenue by Category ---
  Electronics: $3600.00 USD
  Clothing: $890.00 USD
  Books: $170.00 USD

--- Top Categories ---
  Electronics: $3600.00 USD
  Clothing: $890.00 USD
  Books: $170.00 USD

--- Orders by Region & Status ---
  US: {completed: 2, pending: 1}
  EU: {completed: 2}
  AP: {completed: 1}

❓ FAQ

Q: What's the performance difference between List and Set? A: List stores elements in insertion order, with O(1) index access and O(n) lookup. Set is hash-based, offering O(1) lookup/contains checks but no index access. Use Set for deduplication scenarios and List when order matters.

Q: Can the keys of a Map be any type? A: Yes, but when using custom classes as keys, you must override == and hashCode; otherwise, the Map cannot find entries correctly. It's recommended to use String or int as keys.

Q: Should I use [] or <Type>[] for an empty List? A: Use [] when type inference context is available. Use <Type>[] when declaring independently to avoid inferring as List<dynamic>.

Q: Can collection-if and collection-for only be used in List literals? A: No, List, Set, and Map literals all support collection-if and collection-for. For example: {for (var i in items) i.name: i.value}.

Q: What's the difference between Map's putIfAbsent and update? A: putIfAbsent only sets the value if the key doesn't exist. update only updates the value if the key exists (use the ifAbsent parameter to handle non-existent cases).

Q: What's the difference between const collections and final collections? A: A const collection is a compile-time constant, with entirely immutable contents (deep immutability). A final collection has an immutable reference, but the contents can be mutable (e.g., a final List can have elements added).

Q: How can I sort a large List efficiently? A: List.sort() sorts in-place, which is more efficient than [...list].sort() (which creates a copy). If you don't want to modify the original list, create a copy first with [...list] and then sort.

📖 Summary

List is ordered and allows duplicates, supporting spread (...) and collection-if/for for declarative construction.
Set is unordered and prevents duplicates, with O(1) lookup, supporting intersection/union/difference operations.
Map stores key-value pairs; putIfAbsent and update are powerful tools for grouping and aggregation.
const collections are deeply immutable; final collections have immutable references but mutable contents.
Higher-order functions combined with collection operations can reduce data processing from 50 lines to just 10.

📝 Exercises

Basic (Difficulty ⭐): Create a List containing 10 order amounts. Use where to filter amounts greater than 500, map to format them as USD, and reduce to calculate the total sum.
Intermediate (Difficulty ⭐⭐): Given a set of order data, use Map's putIfAbsent and fold to group orders by category, calculating the order count and total revenue for each category, and output a formatted report.
Challenge (Difficulty ⭐⭐⭐): Implement a CollectionPipeline class that supports chaining operations like where, map, sortBy, groupBy, and aggregate, ultimately outputting the analysis results. Hint: Each step returns a new collection or Map.

← Previous Lesson | Next Lesson →

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Dart: Dart Collections

1. What You'll Learn

2. A Developer's Real Story

(1) The Pain Point: Manual Grouping and Aggregation is Time-Consuming and Error-Prone

(2) The Solution: Collection Operations

(3) The Benefits

3. List

(1) Creation and Basic Operations

▶ Example

▶ Example

▶ Example

4. Set

(1) Creation and Set Operations

▶ Example

▶ Example

5. Map

(1) Creation and Operations

▶ Example

▶ Example

6. Collection Type Inference and Const Collections

▶ Example

7. Bob's Scenario: Grouping and Aggregating Millions of Orders

▶ Example

8. Full Example: DataPipeline Collection Analyzer

❓ FAQ

📖 Summary

📝 Exercises

🙏 帮我们做得更好