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dart:core›Classes›Object

Object ​

class Object

The base class for all Dart objects except null.

Because Object is a root of the non-nullable Dart class hierarchy, every other non-Null Dart class is a subclass of Object.

When you define a class, you should consider overriding toString to return a string describing an instance of that class. You might also need to define hashCode and operator ==, as described in the Implementing map keys section of the introduction to dart:core.

Implementers

Constructors ​

Object() const ​

const Object()

Creates a new Object instance.

Object instances have no meaningful state, and are only useful through their identity. An Object instance is equal to itself only.

Implementation
dart
@pragma("vm:recognized", "other")
const Object();

Properties ​

hashCode no setter ​

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Implementation
dart
external int get hashCode;

runtimeType no setter ​

Type get runtimeType

A representation of the runtime type of the object.

Implementation
dart
external Type get runtimeType;

Methods ​

noSuchMethod() ​

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dart
dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

dart
class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Implementation
dart
@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

toString() ​

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Implementation
dart
external String toString();

Operators ​

operator ==() ​

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

  • Total: It must return a boolean for all arguments. It should never throw.

  • Reflexive: For all objects o, o == o must be true.

  • Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

  • Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Implementation
dart
external bool operator ==(Object other);

Static Methods ​

hash() ​

int hash(
  Object? object1,
  Object? object2, [
  Object? object3 = sentinelValue,
  Object? object4 = sentinelValue,
  Object? object5 = sentinelValue,
  Object? object6 = sentinelValue,
  Object? object7 = sentinelValue,
  Object? object8 = sentinelValue,
  Object? object9 = sentinelValue,
  Object? object10 = sentinelValue,
  Object? object11 = sentinelValue,
  Object? object12 = sentinelValue,
  Object? object13 = sentinelValue,
  Object? object14 = sentinelValue,
  Object? object15 = sentinelValue,
  Object? object16 = sentinelValue,
  Object? object17 = sentinelValue,
  Object? object18 = sentinelValue,
  Object? object19 = sentinelValue,
  Object? object20 = sentinelValue,
])

Creates a combined hash code for a number of objects.

The hash code is computed for all arguments that are actually supplied, even if they are null, by numerically combining the Object.hashCode of each argument.

Example:

dart
class SomeObject {
  final Object a, b, c;
  SomeObject(this.a, this.b, this.c);
  bool operator ==(Object other) =>
      other is SomeObject && a == other.a && b == other.b && c == other.c;
  int get hashCode => Object.hash(a, b, c);
}

The computed value will be consistent when the function is called with the same arguments multiple times during the execution of a single program.

The hash value generated by this function is not guaranteed to be stable over different runs of the same program, or between code run in different isolates of the same program. The exact algorithm used may differ between different platforms, or between different versions of the platform libraries, and it may depend on values that change on each program execution.

The hashAll function gives the same result as this function when called with a collection containing the actual arguments to this function in the same order.

Implementation
dart
@Since("2.14")
static int hash(
  Object? object1,
  Object? object2, [
  Object? object3 = sentinelValue,
  Object? object4 = sentinelValue,
  Object? object5 = sentinelValue,
  Object? object6 = sentinelValue,
  Object? object7 = sentinelValue,
  Object? object8 = sentinelValue,
  Object? object9 = sentinelValue,
  Object? object10 = sentinelValue,
  Object? object11 = sentinelValue,
  Object? object12 = sentinelValue,
  Object? object13 = sentinelValue,
  Object? object14 = sentinelValue,
  Object? object15 = sentinelValue,
  Object? object16 = sentinelValue,
  Object? object17 = sentinelValue,
  Object? object18 = sentinelValue,
  Object? object19 = sentinelValue,
  Object? object20 = sentinelValue,
]) {
  if (sentinelValue == object3) {
    return SystemHash.hash2(object1.hashCode, object2.hashCode, _hashSeed);
  }
  if (sentinelValue == object4) {
    return SystemHash.hash3(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object5) {
    return SystemHash.hash4(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object6) {
    return SystemHash.hash5(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object7) {
    return SystemHash.hash6(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object8) {
    return SystemHash.hash7(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object9) {
    return SystemHash.hash8(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object10) {
    return SystemHash.hash9(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object11) {
    return SystemHash.hash10(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object12) {
    return SystemHash.hash11(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object13) {
    return SystemHash.hash12(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object14) {
    return SystemHash.hash13(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object15) {
    return SystemHash.hash14(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      object14.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object16) {
    return SystemHash.hash15(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      object14.hashCode,
      object15.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object17) {
    return SystemHash.hash16(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      object14.hashCode,
      object15.hashCode,
      object16.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object18) {
    return SystemHash.hash17(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      object14.hashCode,
      object15.hashCode,
      object16.hashCode,
      object17.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object19) {
    return SystemHash.hash18(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      object14.hashCode,
      object15.hashCode,
      object16.hashCode,
      object17.hashCode,
      object18.hashCode,
      _hashSeed,
    );
  }
  if (sentinelValue == object20) {
    return SystemHash.hash19(
      object1.hashCode,
      object2.hashCode,
      object3.hashCode,
      object4.hashCode,
      object5.hashCode,
      object6.hashCode,
      object7.hashCode,
      object8.hashCode,
      object9.hashCode,
      object10.hashCode,
      object11.hashCode,
      object12.hashCode,
      object13.hashCode,
      object14.hashCode,
      object15.hashCode,
      object16.hashCode,
      object17.hashCode,
      object18.hashCode,
      object19.hashCode,
      _hashSeed,
    );
  }
  return SystemHash.hash20(
    object1.hashCode,
    object2.hashCode,
    object3.hashCode,
    object4.hashCode,
    object5.hashCode,
    object6.hashCode,
    object7.hashCode,
    object8.hashCode,
    object9.hashCode,
    object10.hashCode,
    object11.hashCode,
    object12.hashCode,
    object13.hashCode,
    object14.hashCode,
    object15.hashCode,
    object16.hashCode,
    object17.hashCode,
    object18.hashCode,
    object19.hashCode,
    object20.hashCode,
    _hashSeed,
  );
}

hashAll() ​

int hashAll(Iterable<Object?> objects)

Creates a combined hash code for a sequence of objects.

The hash code is computed for elements in objects, even if they are null, by numerically combining the Object.hashCode of each element in iteration order.

The result of hashAll([o]) is not o.hashCode.

Example:

dart
class SomeObject {
  final List<String> path;
  SomeObject(this.path);
  bool operator ==(Object other) {
    if (other is SomeObject) {
      if (path.length != other.path.length) return false;
      for (int i = 0; i < path.length; i++) {
        if (path[i] != other.path[i]) return false;
      }
      return true;
    }
    return false;
  }

  int get hashCode => Object.hashAll(path);
}

The computed value will be consistent when the function is called again with objects that have the same hash codes in the same order during an execution of a single program.

The hash value generated by this function is not guaranteed to be stable over different runs of the same program, or between code run in different isolates of the same program. The exact algorithm used may differ between different platforms, or between different versions of the platform libraries, and it may depend on values that change on each program execution.

Implementation
dart
@Since("2.14")
static int hashAll(Iterable<Object?> objects) {
  int hash = _hashSeed;
  for (var object in objects) {
    hash = SystemHash.combine(hash, object.hashCode);
  }
  return SystemHash.finish(hash);
}

hashAllUnordered() ​

int hashAllUnordered(Iterable<Object?> objects)

Creates a combined hash code for a collection of objects.

The hash code is computed for elements in objects, even if they are null, by numerically combining the Object.hashCode of each element in an order independent way.

The result of hashAllUnordered({o}) is not o.hashCode.

Example:

dart
bool setEquals<T>(Set<T> set1, Set<T> set2) {
  var hashCode1 = Object.hashAllUnordered(set1);
  var hashCode2 = Object.hashAllUnordered(set2);
  if (hashCode1 != hashCode2) return false;
  // Compare elements ...
}

The computed value will be consistent when the function is called again with objects that have the same hash codes during an execution of a single program, even if the objects are not necessarily in the same order,

The hash value generated by this function is not guaranteed to be stable over different runs of the same program. The exact algorithm used may differ between different platforms, or between different versions of the platform libraries, and it may depend on values that change on each program execution.

Implementation
dart
@Since("2.14")
static int hashAllUnordered(Iterable<Object?> objects) {
  int sum = 0;
  int count = 0;
  const int mask = 0x3FFFFFFF;
  for (var object in objects) {
    int objectHash = SystemHash.smear(object.hashCode);
    sum = (sum + objectHash) & mask;
    count += 1;
  }
  return SystemHash.hash2(sum, count, 0);
}