The abstract supertype of all types representing definite values. Any two values which are assignable to Object may be compared for value equality using the == and != operators, even if the values are of different concrete type:

true == false
1 == "hello world"
"hello"+" "+"world" == "hello world"
Singleton("hello world") == ["hello world"]

However, since Null is not a subtype of Object, the value null cannot be compared to any other value using the == operator. Thus, value equality is not defined for optional types. This neatly bypasses the problem of deciding the value of the expression null==null, which is simply illegal.

A concrete subclass of Object must refine equals() and hash (or inherit concrete refinements), providing a concrete definition of value equality for the class.

In extreme cases it is acceptable for two values to be equal even when they are not instances of the same class. For example, the Integer value 1 and the Float value 1.0 are considered equal. Except in these extreme cases, instances of different classes are considered unequal.

By: Gavin
See also Basic, Null
hashSource Codeshared formal Integer hash

The hash value of the value, which allows the value to be an element of a hash-based set or key of a hash-based map. Implementations must respect the constraint that:

  • if x==y then x.hash==y.hash.

Therefore, a class which refines equals() must also refine hash.

Because the Integer type is platform-dependent a compiler for a given platform is permitted to further manipulate the calculated hash for an object, and the resulting hash may differ between platforms.

stringSource Codeshared default String string

A developer-friendly string representing the instance. Concatenates the name of the concrete class of the instance with the hash of the instance. Subclasses are encouraged to refine this implementation to produce a more meaningful representation.

equalsSource Codeshared formal Boolean equals(Object that)

Determine if two values are equal. Implementations should respect the constraints that:

  • if x===y then x==y (reflexivity),
  • if x==y then y==x (symmetry),
  • if x==y and y==z then x==z (transitivity).

Furthermore it is recommended that implementations ensure that if x==y then x and y have the same concrete class.

A class which explicitly refines equals() is said to support value equality, and the equality operator == is considered much more meaningful for such classes than for a class which simply inherits the default implementation of identity equality from Identifiable.