Immutable Fields

From X10Wiki

Parameters, fields, and local variables are immutable by default. Immutable fields are equivalent to properties, and thus there is no special syntax for properties. To make a variable mutable, prefix it with the var qualifier. If we decide fields should be mutable by default, then they can be made immutable with a val qualifier.

Note: We are not completely certain which of these should be immutable by default, and which not. We should check on the Frequency in Practice of Immutable Fields.

The final qualifier is retained. A final field may not be overridden in a subclass. A non-final field can be, but its type must be equivalent for mutable fields and a subtype for immutable fields.

Note: This differs from Java, where all field declarations generate a new field. However, it seems to work naturally this way, and having multiple fields of the same name is a recipe for confusion anyway.