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.

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.

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.

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, Return(*Arguments)

, it is acceptable for equals() to be defined to return false for every argument.