"""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.""" see (`class Basic`, `class Null`) by ("Gavin") tagged("Basic types") shared abstract class Object() extends Anything() { "Determine if two values are equal. For any two non-null objects `x` and `y`, `x.equals(y)` may be written as: x == y 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]]. Note that an implementation of `equals()` that always returns [[false]] does satisfy the constraints given above. Therefore, in very rare cases where there is no reasonable definition of value equality for a class, for example, [[function references|Callable]], it is acceptable for `equals()` to be defined to return `false` for every argument." shared formal Boolean equals(Object that); "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`. In general, `hash` values vary between platforms and between executions of the same program. Note that when executing on a Java Virtual Machine, the 64-bit [[Integer]] value returned by an implementation of `hash` is truncated to a 32-bit integer value by taking the exclusive disjunction of the 32 lowest-order bits with the 32 highest-order bits, before returning the value to the caller." see (`function identityHash`) shared formal Integer hash; "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." shared default String string => className(this) + "@" + Integer.format(hash, #10); }