Dart: Dart Operators

Q: Q: What's the difference between / and ~/ ?

A: / always returns a double (e.g., 17 / 5 = 3.4), while ~/ returns the integer division result (e.g., 17 ~/ 5 = 3). Use ~/ when you need an integer result.

Q: Q: What is the return value of the cascade operator .. ?

A: .. returns the object itself (not the return value of the last method), allowing continued chaining. This is a key difference from a regular method call.

Q: Q: What's the difference between ??= and ?? ?

A: ?? is the null coalescing operator, which returns the non-null value. ??= is the null-aware assignment operator, which only assigns if the variable is null and returns the result. x ??= y is equivalent to x = x ?? y.

Q: Q: When should I use as instead of is ?

A: Prefer is for type checking, as the compiler performs automatic type promotion. Only use as when you are sure of the type or need an explicit cast, as an incorrect cast will throw an exception.

Q: Q: When to use ?.. vs .. ?

A: Use ?.. when the object might be null, and .. when you are sure it is non-null. ?.. skips the entire cascade if the object is null.

Q: Q: Does == compare references or values?

A: Dart's == defaults to reference comparison (same as Java), but many built-in types (String, int, double) override == to compare values. Custom classes need to override both == and hashCode.

Last updated: 2026-08-26

Operators are the verbs of code — master them to make data "move."

1. What You Will Learn

Arithmetic / Relational / Logical / Bitwise Operators
Assignment and Compound Assignment Operators
Cascade operator .. and ?.. (a powerful tool for method chaining)
Null Safety Operators: ?. / ?? / ??=
Type Test Operators: is / is! / as

2. A Developer's Real Story

(1) Pain Point: NullPointerException Crashing Batch Tasks

Bob's DataPipeline frequently crashed while processing millions of orders because some orders lacked optional fields. The code was littered with if (value != null) checks, but one was missed, causing the processing of 300,000 orders to halt and requiring a complete re-run.

(2) The Null Safety Operator Solution

Dart provides three operators — ?. (safe access), ?? (null coalescing), and ??= (null-aware assignment) — that make null handling concise and safe.

DART

// Before: verbose null checking
String getCity(Order order) {
  if (order.customer != null) {
    if (order.customer!.address != null) {
      return order.customer!.address!.city;
    }
  }
  return 'Unknown';
}

// After: null-safe operators
String getCity(Order order) =>
    order.customer?.address?.city ?? 'Unknown';

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

(3) Benefits

60% reduction in lines of code, making null-handling logic clear at a glance.
Cascade operator .. enables smoother method chaining.
Type test operators make type checking safer.

3. Arithmetic and Relational Operators

(1) Arithmetic Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Basic Arithmetic

DART

void main() {
  int a = 17;
  int b = 5;

  print(a + b);   // 22 - Addition
  print(a - b);   // 12 - Subtraction
  print(a * b);   // 85 - Multiplication
  print(a / b);   // 3.4 - Division (returns double)
  print(a ~/ b);  // 3 - Integer division
  print(a % b);   // 2 - Modulo (remainder)

  // Prefix and postfix increment
  int count = 10;
  print(count++); // 10 (returns then increments)
  print(count);   // 11
  print(++count); // 12 (increments then returns)
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Operator	Meaning	Example	Result
`+`	Addition	`17 + 5`	`22`
`-`	Subtraction	`17 - 5`	`12`
`*`	Multiplication	`17 * 5`	`85`
`/`	Division (returns double)	`17 / 5`	`3.4`
`~/`	Integer Division	`17 ~/ 5`	`3`
`%`	Modulo (Remainder)	`17 % 5`	`2`

(2) Relational Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Comparison Operations

DART

void main() {
  int orders = 1500;
  int threshold = 1000;

  print(orders > threshold);     // true
  print(orders < threshold);     // false
  print(orders >= 1500);         // true
  print(orders <= 1500);         // true
  print(orders == 1500);         // true
  print(orders != 1000);         // true

  // String comparison is lexicographic
  print('apple' < 'banana');     // true
  print('USD' == 'USD');         // true
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

4. Logical and Bitwise Operators

(1) Logical Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Logical Operations

DART

void main() {
  bool isProduction = true;
  bool hasErrors = false;

  print(isProduction && !hasErrors);  // true - AND + NOT
  print(isProduction || hasErrors);   // true - OR
  print(!isProduction);               // false - NOT

  // Short-circuit evaluation
  String? name = null;
  // name.length > 0 would crash, but:
  print(name != null && name.isNotEmpty);  // false (safe)
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Operator	Meaning	Short-Circuit
`&&`	Logical AND	Left false -> right not evaluated
`		`
`!`	Logical NOT	—

(2) Bitwise Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Bitwise Operations

DART

void main() {
  int flags = 0b1010;  // 10 in binary
  int mask = 0b1100;   // 12 in binary

  print(flags & mask);   // 8  (0b1000) - AND
  print(flags | mask);   // 14 (0b1110) - OR
  print(flags ^ mask);   // 6  (0b0110) - XOR
  print(~flags);         // -11 (inverted bits) - NOT
  print(flags << 2);     // 40 (0b101000) - Left shift
  print(flags >> 1);     // 5  (0b0101) - Right shift
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

5. Assignment and Compound Assignment Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Compound Assignment

DART

void main() {
  int total = 0;
  double revenue = 0.0;

  total += 100;          // total = total + 100 = 100
  total -= 30;           // total = 70
  total *= 2;            // total = 140
  total ~/= 3;           // total = 46 (integer division)
  revenue += 52500.75;   // revenue = 52500.75

  // Null-aware assignment
  String? outputPath;
  outputPath ??= '/tmp/output';  // Assign if null
  print(outputPath);             // /tmp/output
  outputPath ??= '/other/path';  // Not assigned (already non-null)
  print(outputPath);             // /tmp/output
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Operator	Equivalent To	Example
`+=`	`a = a + b`	`total += 100`
`-=`	`a = a - b`	`total -= 30`
`*=`	`a = a * b`	`total *= 2`
`~/=`	`a = a ~/ b`	`total ~/= 3`
`??=`	`a = a ?? b`	`path ??= '/default'`

6. Cascade Operator `..` and `?..`

(1) Cascade Operator

The cascade operator .. allows performing multiple operations on the same object without repeatedly referencing the variable name.

graph TD
  A[Operator System] --> B[Arithmetic + Relational + Logical]
  A --> C[Cascade ..]
  A --> D[Null Safety ?. ?? ??=]
  A --> E[Type Test is/as]
  C --> C1["object..method1()..method2()"]

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Cascade Operator

DART

class DataPipeline {
  String name = '';
  int batchSize = 0;
  bool verbose = false;
  final List<String> sources = [];

  void start() => print('$name started');
}

void main() {
  // Without cascade - repetitive
  final pipeline1 = DataPipeline();
  pipeline1.name = 'Analytics';
  pipeline1.batchSize = 50000;
  pipeline1.verbose = true;
  pipeline1.sources.add('orders.csv');
  pipeline1.start();

  // With cascade - fluent and clean
  final pipeline2 = DataPipeline()
    ..name = 'Analytics'
    ..batchSize = 50000
    ..verbose = true
    ..sources.add('orders.csv')
    ..start();
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

▶ Example

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> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Null-Safe Cascade ?..

DART

class Customer {
  String? name;
  String? email;
}

void main() {
  Customer? customer;

  // Null-safe cascade - skipped if object is null
  customer?..name = 'Bob'..email = 'bob@example.com';
  print(customer);  // null (cascade was skipped)

  customer = Customer();
  customer?..name = 'Bob'..email = 'bob@example.com';
  print(customer?.name);  // Bob
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Operator	Meaning	When Object is null
`..`	Cascade operation	Error
`?..`	Null-safe cascade	Skips entire cascade

7. Null Safety Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: Three Null Safety Operators

DART

class Address {
  final String city;
  Address(this.city);
}

class Customer {
  final String name;
  final Address? address;
  Customer(this.name, {this.address});
}

class Order {
  final String id;
  final Customer? customer;
  Order(this.id, {this.customer});
}

void main() {
  // ?. - null-safe access
  Order order = Order('ORD-001');
  print(order.customer?.name);          // null (safe)
  // print(order.customer.name);        // Runtime error!

  // ?? - null coalescing
  String city = order.customer?.address?.city ?? 'Unknown';
  print(city);                          // Unknown

  // ??= - null-aware assignment
  String? outputPath;
  outputPath ??= '/tmp/reports';
  print(outputPath);                    // /tmp/reports
  outputPath ??= '/other/path';
  print(outputPath);                    // /tmp/reports (unchanged)
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Operator	Meaning	Example	Result
`?.`	Safe access	`obj?.method()`	null or call result
`??`	Null coalescing	`value ?? default`	value if non-null, else default
`??=`	Null-aware assignment	`var ??= value`	Assigns if null

8. Type Test Operators

▶ Example

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

: is / is! / as

DART

void main() {
  Object value = '1,500.00 USD';

  // is - type check (returns bool)
  if (value is String) {
    print('String length: ${value.length}');  // Type promoted!
  }

  // is! - negative type check
  if (value is! int) {
    print('Not an integer');
  }

  // as - type cast (throws if wrong)
  String text = value as String;       // OK
  // int number = value as int;        // Runtime error!

  // Safe cast pattern
  if (value is String) {
    String safe = value;               // No cast needed (promoted)
    print(safe.toUpperCase());
  }
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Operator	Meaning	On Failure
`is`	Type check	Returns false
`is!`	Negated type check	Returns true
`as`	Type cast	Throws TypeError

9. Full Example: DataPipeline Order Processing with Operators

DART

// ============================================
// DataPipeline Order Processing with Operators
// Demonstrates all operator categories
// ============================================

class Order {
  final String id;
  final double amount;
  String? discountCode;
  double? discountPercent;

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

  // Null-safe operators for discount calculation
  double get discountedAmount =>
      amount - (amount * (discountPercent ?? 0));

  double get tax => discountedAmount * 0.08;

  double get total => discountedAmount + tax;

  String formatUSD() => '\$${total.toStringAsFixed(2)} USD';
}

class Report {
  final List<Order> orders = [];
  var totalRevenue = 0.0;
  var orderCount = 0;

  void addOrder(Order order) {
    orders.add(order);
    totalRevenue += order.total;       // Compound assignment
    orderCount++;
  }

  double get averageOrderValue =>
      orderCount > 0 ? totalRevenue / orderCount : 0;

  bool get isHighVolume => orderCount >= 1000;

  String get statusLabel => isHighVolume ? 'High Volume' : 'Normal';
}

void main() {
  final report = Report()
    ..addOrder(Order(id: 'ORD-001', amount: 1500.0)
      ..discountPercent = 0.1)
    ..addOrder(Order(id: 'ORD-002', amount: 3250.50)
      ..discountCode = 'SAVE20'
      ..discountPercent = 0.2)
    ..addOrder(Order(id: 'ORD-003', amount: 890.25));

  print('=== DataPipeline Order Report ===');
  for (final order in report.orders) {
    final discount = order.discountPercent != null
        ? '${(order.discountPercent! * 100).toInt()}%'
        : 'None';
    print('  ${order.id}: ${order.formatUSD()} (Discount: $discount)');
  }

  print('\n--- Summary ---');
  print('Orders:     ${report.orderCount}');
  print('Revenue:    \$${report.totalRevenue.toStringAsFixed(2)} USD');
  print('Average:    \$${report.averageOrderValue.toStringAsFixed(2)} USD');
  print('Status:     ${report.statusLabel}');

  // Type test
  print('\nType check: report is ${report.runtimeType}');
  print('Is Report: ${report is Report}');
}

TEXT 📖 Display only

> **Output:** Run in local DartPad or `dart run`. All Dart course examples are based on Dart 3.x / Flutter 3.x; results may vary slightly with SDK version.

Output:

TEXT 📖 Display only

=== DataPipeline Order Report ===
  ORD-001: $1458.00 USD (Discount: 10%)
  ORD-002: $2807.43 USD (Discount: 20%)
  ORD-003: $961.47 USD (Discount: None)

--- Summary ---
Orders:     3
Revenue:    $5226.90 USD
Average:    $1742.30 USD
Status:     Normal

Type check: report is Report
Is Report: true

❓ FAQ

Q: What's the difference between / and ~/? A: / always returns a double (e.g., 17 / 5 = 3.4), while ~/ returns the integer division result (e.g., 17 ~/ 5 = 3). Use ~/ when you need an integer result.

Q: What is the return value of the cascade operator ..? A: .. returns the object itself (not the return value of the last method), allowing continued chaining. This is a key difference from a regular method call.

Q: What's the difference between ??= and ??? A: ?? is the null coalescing operator, which returns the non-null value. ??= is the null-aware assignment operator, which only assigns if the variable is null and returns the result. x ??= y is equivalent to x = x ?? y.

Q: When should I use as instead of is? A: Prefer is for type checking, as the compiler performs automatic type promotion. Only use as when you are sure of the type or need an explicit cast, as an incorrect cast will throw an exception.

Q: When to use ?.. vs ..? A: Use ?.. when the object might be null, and .. when you are sure it is non-null. ?.. skips the entire cascade if the object is null.

Q: Does == compare references or values? A: Dart's == defaults to reference comparison (same as Java), but many built-in types (String, int, double) override == to compare values. Custom classes need to override both == and hashCode.

Q: Are bitwise operators commonly used in practice? A: Not frequently, but they are essential in scenarios like permission systems (flags), network protocol parsing, and encryption algorithms. Daily development mainly uses arithmetic and logical operators.

📖 Summary

Note the difference between arithmetic / (returns double) and ~/ (integer division).
Logical operators && and || have short-circuit properties, useful for safe access.
Cascade operator .. simplifies method chaining; ?.. is its null-safe version.
The Null Safety trio: ?. (safe access), ?? (null coalescing), ??= (null-aware assignment).
Type tests: is checks and promotes types; as forces a cast (throws exception if unsafe).

📝 Exercises

Basic (Difficulty ⭐): Write code using arithmetic operators to calculate the average amount per order for 1,500,000 orders with a total of 75,000,000 USD. Use the correct division operator.
Intermediate (Difficulty ⭐⭐): Use the cascade operator .. to create and configure an object, setting at least 4 properties and calling 2 methods. Then compare it to the version without using cascade.
Challenge (Difficulty ⭐⭐⭐): Implement a SafeValue<T> class using the ?., ??, and ??= operators to provide safe value access, default values, and lazy assignment, ensuring no null pointer exceptions are thrown during any operation.

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Dart: Dart Operators

1. What You Will Learn

2. A Developer's Real Story

(1) Pain Point: NullPointerException Crashing Batch Tasks

(2) The Null Safety Operator Solution

(3) Benefits

3. Arithmetic and Relational Operators

(1) Arithmetic Operators

▶ Example

(2) Relational Operators

▶ Example

4. Logical and Bitwise Operators

(1) Logical Operators

▶ Example

(2) Bitwise Operators

▶ Example

5. Assignment and Compound Assignment Operators

▶ Example

6. Cascade Operator .. and ?..

(1) Cascade Operator

▶ Example

▶ Example

7. Null Safety Operators

▶ Example

8. Type Test Operators

▶ Example

9. Full Example: DataPipeline Order Processing with Operators

❓ FAQ

📖 Summary

📝 Exercises

🙏 帮我们做得更好

6. Cascade Operator `..` and `?..`