Dart functions, lists, maps and null safety

Flutter apps are written in Dart. Before building screens, get comfortable with Dart's functions, collections and its null safety, which prevents the most common app crash ("null is not a subtype of...").

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Functions

Dart

double withVat(double amount, {double rate = 16}) {   // named parameter with a default
  return amount * (1 + rate / 100);
}

String greet(String name, [String? title]) {          // [ ] = optional positional
  return title == null ? 'Habari $name' : 'Habari $title $name';
}

int square(int n) => n * n;                           // arrow function for one expression

void main() {
  print(withVat(1000));              // 1160.0
  print(withVat(1000, rate: 0));     // named argument
  print(greet('Wanjiru'));
  print(greet('Otieno', 'Dr.'));
  print(square(9));
}

Parameter style	Declared as	Called as
Required positional	`f(int a)`	`f(3)`
Optional positional	`f([int? a])`	`f()` or `f(3)`
Named (optional)	`f({int a = 0})`	`f(a: 3)`
Named required	`f({required int a})`	`f(a: 3)`

Flutter widgets use named parameters everywhere: Text('Hi', style: ...), Padding(padding: ..., child: ...).

Lists

Dart

void main() {
  final towns = <String>['Nyeri', 'Meru'];
  towns.add('Embu');
  towns.insert(0, 'Nanyuki');
  print('${towns.length} towns, first: ${towns.first}');

  final prices = [180, 350, 60, 1200];
  final withVat = prices.map((p) => (p * 1.16).round()).toList();
  final cheap = prices.where((p) => p < 300).toList();
  final total = prices.reduce((a, b) => a + b);
  print('$withVat $cheap $total');

  prices.sort();
  print(prices);
  for (final p in prices) {
    print('KSh $p');
  }

  final menu = ['Home', 'Shop', if (true) 'Offers', for (var i = 1; i <= 2; i++) 'Page $i'];
  print(menu);   // collection if/for: used a lot for building widget lists
}

Maps and sets

Dart

void main() {
  final stock = <String, int>{'Unga': 12, 'Sugar': 0, 'Oil': 7};
  stock['Rice'] = 20;
  stock.update('Unga', (v) => v - 2);
  print(stock['Oil']);            // 7
  print(stock['Beans']);          // null: missing key
  print(stock.containsKey('Sugar'));
  stock.forEach((item, qty) => print('$item: $qty'));

  final unique = {'Otieno', 'Wanjiru', 'Otieno'};   // a Set
  print(unique.length);           // 2
}

Null safety

In Dart, a variable **cannot be null unless its type ends with ?**. The compiler forces you to handle the null case.

Dart

void main() {
  String name = 'Amina';        // can never be null
  String? nickname;             // may be null (starts as null)

  // print(nickname.length);    // compile error: nickname might be null
  print(nickname?.length);      // ?. : null if nickname is null
  print(nickname ?? 'No nickname');   // ?? : a default value

  nickname = 'Mina';
  if (nickname != null) {
    print(nickname.length);     // safe: Dart knows it's not null here
  }

  int? maybeQty = int.tryParse('abc');   // null if not a number
  final qty = maybeQty ?? 1;
  print(qty);

  late String token;            // late: "I promise to set it before use"
  token = 'abc123';
  print(token);
}

Syntax	Meaning
`String?`	May be null
`x?.y`	Access only if x isn't null
`x ?? y`	Use y if x is null
`x ??= y`	Assign y only if x is null
`x!`	"I'm sure it's not null" (crashes if you're wrong: avoid)
`late`	Initialised later, before first use

String interpolation

Dart

final item = 'Cake';
final qty = 2;
final price = 1800.0;
print('$qty x $item = KSh ${(qty * price).toStringAsFixed(2)}');

$name inserts a variable; ${expression} inserts any expression.

Why Dart functions and collections matter for Flutter

Every Flutter screen is built from Dart: lists of products become ListViews, maps hold JSON from APIs, and functions handle button taps, validation and calculations. Fluency with Dart's collection methods (map, where, fold), named parameters and null safety makes Flutter code shorter, safer and easier to read. Flutter developers are in demand for cross-platform apps used by fintechs, retailers, logistics companies and startups.

Named, required and default parameters

Dart

String formatPrice(double amount, {String currency = 'KSh', int decimals = 0}) {
  return '$currency ${amount.toStringAsFixed(decimals)}';
}

String orderSummary({required String customer, required int items, double delivery = 0}) {
  final fee = delivery == 0 ? 'free delivery' : 'delivery ${formatPrice(delivery)}';
  return '$customer: $items items, $fee';
}

void main() {
  print(formatPrice(2500));
  print(formatPrice(19.5, currency: 'USD', decimals: 2));
  print(orderSummary(customer: 'Wanjiru', items: 3));
  print(orderSummary(customer: 'Otieno', items: 1, delivery: 250));
}

This is the same style Flutter widgets use: Text('Hi', style: ...), Padding(padding: ..., child: ...). required makes a named parameter compulsory.

Functions as values and closures

Dart

typedef PriceRule = double Function(double price);

PriceRule percentOff(double percent) => (price) => price * (1 - percent / 100);
PriceRule flatOff(double amount) => (price) => (price - amount).clamp(0, double.infinity).toDouble();

double applyRules(double price, List<PriceRule> rules) =>
    rules.fold(price, (current, rule) => rule(current));

void main() {
  final rules = [percentOff(10), flatOff(100)];
  print(applyRules(2000, rules));        // 2000 -> 1800 -> 1700

  var counter = 0;
  void increment() => counter++;         // closure captures counter
  increment();
  increment();
  print('Counter: $counter');
}

Callbacks like onPressed: () => setState(() => count++) in Flutter are exactly these closures.

Collection methods in depth

Dart

class Product {
  final String name;
  final String category;
  final double price;
  final int stock;
  const Product(this.name, this.category, this.price, this.stock);
}

void main() {
  const products = [
    Product('Unga 2kg', 'Food', 180, 12),
    Product('Sugar 1kg', 'Food', 210, 0),
    Product('Soap', 'Home', 60, 30),
    Product('Cooking oil 1L', 'Food', 350, 7),
  ];

  final inStock = products.where((p) => p.stock > 0).toList();
  final names = inStock.map((p) => p.name).join(', ');
  final stockValue = products.fold<double>(0, (sum, p) => sum + p.price * p.stock);
  final cheapest = products.reduce((a, b) => a.price < b.price ? a : b);
  final anyOut = products.any((p) => p.stock == 0);
  final sorted = [...products]..sort((a, b) => b.price.compareTo(a.price));

  print('In stock: $names');
  print('Stock value: KSh ${stockValue.toStringAsFixed(0)}');
  print('Cheapest: ${cheapest.name}');
  print('Anything out of stock? $anyOut');
  print('Most expensive first: ${sorted.map((p) => p.name).toList()}');

  final byCategory = <String, List<Product>>{};
  for (final p in products) {
    byCategory.putIfAbsent(p.category, () => []).add(p);
  }
  byCategory.forEach((cat, list) => print('$cat: ${list.length} products'));
}

[...products]..sort(...) copies the list with the spread operator and sorts the copy using the cascade operator .., leaving the original unchanged.

Cascades: configuring objects fluently

Dart

class Receipt {
  final lines = <String>[];
  double total = 0;
  void add(String item, double amount) {
    lines.add('$item: ${amount.toStringAsFixed(0)}');
    total += amount;
  }
}

void main() {
  final r = Receipt()
    ..add('Unga', 180)
    ..add('Milk', 120)
    ..add('Bread', 65);
  print(r.lines.join('\n'));
  print('Total: ${r.total.toStringAsFixed(0)}');
}

Records and pattern matching (Dart 3)

Dart

(double min, double max) range(List<double> values) {
  final sorted = [...values]..sort();
  return (sorted.first, sorted.last);
}

String describePayment(Map<String, Object?> json) {
  return switch (json) {
    {'method': 'mpesa', 'phone': String phone} => 'M-Pesa from $phone',
    {'method': 'card', 'last4': String last4} => 'Card ending $last4',
    _ => 'Unknown payment',
  };
}

void main() {
  final (low, high) = range([67, 82, 45, 90]);
  print('Lowest $low, highest $high');

  print(describePayment({'method': 'mpesa', 'phone': '0712345678'}));
  print(describePayment({'method': 'card', 'last4': '4242'}));
  print(describePayment({'method': 'cash'}));
}

Records return several values without creating a class; patterns safely pull values out of JSON-like maps, with the compiler checking types.

Null safety with collections

Dart

void main() {
  final stock = <String, int>{'unga': 40, 'sugar': 25};
  final salt = stock['salt'];               // int? because the key may be missing
  print(salt ?? 0);
  print(stock['unga']! + 10);               // ! only when you're sure

  final List<String?> phones = ['0712000001', null, '0722000002'];
  final valid = phones.whereType<String>().toList();   // removes nulls
  print(valid);

  final first = phones.firstWhere((p) => p != null && p.startsWith('07'), orElse: () => null);
  print(first);
}

whereType<String>() turns a List<String?> into a list of non-null strings, a common clean-up step for API data.

Common mistakes

Mistake	Fix
Forgetting `.toList()` after `map`/`where`	They return lazy iterables; call `.toList()` when you need a list
`sort()` returns void, so `final s = list.sort()` fails	Sort in place, or `[...list]..sort()`
Using `!` everywhere	Use `??`, `?.` and proper checks
Mutating a `const` list	Copy it first with the spread operator
`reduce` on an empty list	Throws; use `fold` with a starting value

Practice

Write double cartTotal(List<Map<String, num>> items, {double vat = 0.16}).
Group a list of students by form into a Map<int, List<String>>.
Return the lowest and highest mark as a record and destructure it.
Use a switch with patterns to describe different notification maps (sms, email, push).
Clean a list of nullable phone numbers with whereType and keep only valid 10-digit numbers.

Think about it: Why does final total = items.map((i) => i.price).reduce((a, b) => a + b); crash for an empty cart, and what's safer?Show answer

reduce needs at least one element and throws a StateError on an empty list. fold<double>(0, (sum, i) => sum + i.price) starts from 0 and works for empty lists, returning 0.

Check yourself

Which free website runs Dart and Flutter code in the browser?
Show answer
DartPad
Which symbol after a type means the variable may be null?
Show answer
?
Which operator gives a default value when something is null?
Show answer
??
Which keyword makes a named parameter compulsory?
Show answer
required
Which list method keeps only matching items?
Show answer
where
Which operator calls several methods on the same object in a row?
Show answer
..
Which collection method combines items with a starting value and works on empty lists?
Show answer
fold
Which keyword makes a named parameter compulsory in Dart?
Show answer
required
Which method removes nulls from a List<String?>? (whereType<...>)
Show answer
whereType

Lesson 2 of 10 in Dart & Flutter (mobile apps) · Printable course notes