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https://issues.apache.org/jira/browse/GROOVY-12585?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Paul King updated GROOVY-12585:
-------------------------------
Description:
h3. Problem
Groovy turns a collection into another collection type when it is assigned or
cast ({{T x = value}}, {{(T) value}}, through
{{DefaultTypeTransformation.castToType}}), and when it is coerced with {{as}}
(through {{DefaultGroovyMethods.asType(Collection, Class)}}). The two paths
know different default implementations, and neither covers the common
collection interfaces. So:
* some assignments fail where the equivalent {{as}} works;
* some {{as}} coercions return a proxy that does not behave like the requested
type;
* an array source and a collection source give different results for the same
target;
* a single-element list assigned to some collection classes gives an empty
collection.
Results below are the same with indy and classic call sites, on JDK 17 and 21.
Groups 1 and 2 fail the same way in every release checked, from 2.4.12 to 6.0.0.
h4. 1. Assignment fails where {{as}} works
||Target||{{T x = [3, 1, 2, 1]}}||{{[3, 1, 2, 1] as T}}||
|{{SortedSet}}|{{GroovyCastException}}|{{TreeSet [1, 2, 3]}}|
|{{Queue}}|{{GroovyCastException}}|{{LinkedList [3, 1, 2, 1]}}|
|{{Stack}}|{{GroovyCastException}}|{{Stack [3, 1, 2, 1]}}|
|{{List}} (source {{[3, 1, 2] as
LinkedHashSet}})|{{GroovyCastException}}|{{ArrayList [3, 1, 2]}}|
{{asType}} has a table of defaults for {{List}}, {{Set}}, {{SortedSet}},
{{Queue}} and {{Stack}}. {{castToType}} has none for a {{Collection}} source:
it gives a {{LinkedHashSet}} when the target accepts one. Otherwise it tries
the target's constructor with the elements, then with the collection, which
fails for an interface and for a class without a copy constructor such as
{{Stack}}.
An array source goes through a different branch: a no-arg constructor plus
{{addAll}}. So {{Stack s = new Integer[]\{3, 1, 2\}}} works while {{Stack s =
[3, 1, 2]}} fails.
h4. 2. Assignment fails, and {{as}} returns a proxy that does not behave like
the type
||Target||{{T x = value}}||{{value as T}}||
|{{NavigableSet}}|{{GroovyCastException}}|proxy over the list: {{([3, 1, 2, 1]
as NavigableSet).size() == 4}}, {{first() == 3}}|
|{{Deque}}|{{GroovyCastException}}|proxy over the list: {{peekFirst()}} throws
{{MissingMethodException}}|
|{{BlockingQueue}}|{{GroovyCastException}}|proxy over the list|
|{{SortedMap}}, {{NavigableMap}}|{{GroovyCastException}}|dynamic proxy over the
map: {{firstKey()}} throws {{UnsupportedOperationException}}|
|{{ConcurrentMap}}|{{GroovyCastException}}|dynamic proxy over the map|
|{{Iterable}} (array source)|{{GroovyCastException}}|proxy over the array|
h4. 3. Array and collection sources give different result types
||Expression||Result||
|{{Set s = new Integer[]\{3, 1, 2\}}}|{{LinkedHashSet [3, 1, 2]}}|
|{{new Integer[]\{3, 1, 2\} as Set}}|{{HashSet [1, 2, 3]}} (order lost)|
|{{[3, 1, 2] as Set}}|{{LinkedHashSet [3, 1, 2]}}|
|{{HashSet h = [3, 1, 2]}}|{{LinkedHashSet [3, 1, 2]}}|
|{{[3, 1, 2] as HashSet}}|{{HashSet [1, 2, 3]}}|
h4. 4. A single element is passed to a capacity constructor
Because {{castToType}} tries the target's constructor with the elements first,
a one-element list picks a capacity constructor:
||Expression||Result||
|{{Vector v = [5]}}|empty {{Vector}} (capacity 5)|
|{{PriorityQueue q = [5]}}|empty {{PriorityQueue}}|
|{{ArrayDeque d = [5]}}|empty {{ArrayDeque}}|
|{{[5] as Vector}}|{{Vector [5]}}|
h3. Already working (no change intended)
{{Collection}}, {{Iterable}} (collection source), {{Set}}, {{AbstractSet}},
{{List}} (list or array source), {{HashSet}}, {{LinkedHashSet}}, {{TreeSet}},
{{ArrayList}}, {{LinkedList}}, {{Vector}} (more than one element),
{{ArrayDeque}}, {{PriorityQueue}}, {{ConcurrentLinkedQueue}},
{{CopyOnWriteArrayList}}, {{Map}}, {{HashMap}}, {{LinkedHashMap}},
{{AbstractMap}}, {{TreeMap}}, {{ConcurrentHashMap}}, {{ConcurrentSkipListMap}}.
On JDK 21, also {{SequencedCollection}}, {{SequencedSet}} and {{SequencedMap}}.
h3. Proposal
One table of default implementations, used by both {{castToType}} and
{{asType}}. A value that is already an instance of the target is returned
unchanged, as now.
||Target||Default||
|{{List}}, {{Collection}}, {{Iterable}}|{{ArrayList}}|
|{{Set}}|{{LinkedHashSet}}|
|{{SortedSet}}, {{NavigableSet}}|{{TreeSet}}|
|{{Queue}}, {{Deque}}|{{LinkedList}} (accepts {{null}} elements, unlike
{{ArrayDeque}}; matches today's {{as Queue}})|
|{{BlockingQueue}}|{{LinkedBlockingQueue}}|
|{{SortedMap}}, {{NavigableMap}}|{{TreeMap}}|
|{{ConcurrentMap}}|{{ConcurrentHashMap}}|
For a concrete collection class, assignment and cast try its copy constructor,
then its no-arg constructor followed by {{addAll}}, and only then the elements
as constructor arguments. This is the order {{as}} already uses, and it fixes
group 4. As with {{as}} today (GROOVY-7867), a constructor that throws is
skipped in favour of the next step, and its exception is kept as a suppressed
exception if every step fails.
{{as}} with a concrete class keeps its current path. So the open question is
answered by keeping today's behaviour: {{HashSet h = list}} still gives a
{{LinkedHashSet}} (a subclass, so order is kept), and {{list as HashSet}} still
gives a {{HashSet}}.
h3. Behaviour changes
* Assignments in groups 1 and 2 that now fail would succeed.
* {{as NavigableSet}}, {{as Deque}}, {{as BlockingQueue}}, {{as SortedMap}},
{{as NavigableMap}} and {{as ConcurrentMap}} would return a real collection
instead of a proxy, as would {{as Iterable}} and {{as Deque}} with an array
source. Code relying on the proxy keeping a reference to the original
collection would see a copy.
* {{array as Set}} would keep element order ({{LinkedHashSet}} instead of
{{HashSet}}).
* A single-element list assigned to a collection class would hold the element
instead of giving an empty collection (group 4).
* {{TreeSet}}/{{TreeMap}} targets throw {{ClassCastException}} or
{{NullPointerException}} for non-{{Comparable}} or {{null}} elements and keys,
as {{as SortedSet}} and {{TreeSet x = ...}} already do.
* A map coerced with {{as}} to an interface outside the table, such as
{{Runnable}} or {{Comparator}}, is still a proxy over the map's closures.
h3. Out of scope
The type checker has its own rules for list and map literals, and they are to
be raised in their own issues:
* GROOVY-XXXXX: under {{@TypeChecked}} and {{@CompileStatic}}, a literal
assigned to the interfaces in the table is a compile error, for example
{{SortedSet s = [3, 1]}}. A literal assigned to a concrete collection class is
matched against a constructor taking the elements, so {{LinkedList l = [3, 4,
5]}} fails with "Cannot find matching constructor java.util.LinkedList(int,
int, int)". Under {{@CompileStatic}}, {{Vector v = [5]}} calls {{Vector(int)}}
and stays empty. Until that issue is fixed, {{Vector v = [5]}} gives {{[5]}} in
dynamic and {{@TypeChecked}} code but {{[]}} under {{@CompileStatic}}.
* GROOVY-YYYYY: under static checking, a literal returned from a method is
rejected unless its own class is assignable to the return type. {{Set m() \{
[3, 1] \}}} and {{TreeMap m() \{ [b: 2, a: 1] \}}} fail, although the same
literal assigned to a {{Set}} or {{TreeMap}} variable compiles.
* A list or map argument is not converted to a method's parameter type, in any
mode, by design.
was:
h3. Problem
Groovy turns a collection into another collection type when it is assigned or
cast ({{T x = value}}, {{(T) value}}, through
{{DefaultTypeTransformation.castToType}}), and when it is coerced with {{as}}
(through {{DefaultGroovyMethods.asType(Collection, Class)}}). The two paths
know different default implementations, and neither covers the common
collection interfaces. So:
* some assignments fail where the equivalent {{as}} works;
* some {{as}} coercions return a proxy that does not behave like the requested
type;
* an array source and a collection source give different results for the same
target.
Results below are the same with indy and classic call sites, on JDK 17 and 21.
h4. 1. Assignment fails where {{as}} works
||Target||{{T x = [3, 1, 2, 1]}}||{{[3, 1, 2, 1] as T}}||
|{{SortedSet}}|{{GroovyCastException}}|{{TreeSet [1, 2, 3]}}|
|{{Queue}}|{{GroovyCastException}}|{{LinkedList [3, 1, 2, 1]}}|
|{{Stack}}|{{GroovyCastException}}|{{Stack [3, 1, 2, 1]}}|
|{{List}} (source {{[3, 1, 2] as
LinkedHashSet}})|{{GroovyCastException}}|{{ArrayList [3, 1, 2]}}|
{{asType}} has a table of defaults for {{List}}, {{Set}}, {{SortedSet}},
{{Queue}} and {{Stack}}. {{castToType}} has none for a {{Collection}} source:
it gives a {{LinkedHashSet}} when the target accepts one. Otherwise it tries
the target's constructor with the elements, then with the collection, which
fails for an interface and for a class without a copy constructor such as
{{Stack}}.
An array source goes through a different branch: a no-arg constructor plus
{{addAll}}. So Stack s = new Integer[]\{3, 1, 2} works while {{Stack s = [3, 1,
2]}} fails.
h4. 2. Assignment fails, and {{as}} returns a proxy that does not behave like
the type
||Target||{{T x = value}}||{{value as T}}||
|{{NavigableSet}}|{{GroovyCastException}}|proxy over the list: {{([3, 1, 2, 1]
as NavigableSet).size() == 4}}, {{first() == 3}}|
|{{Deque}}|{{GroovyCastException}}|proxy over the list: {{peekFirst()}} throws
{{MissingMethodException}}|
|{{BlockingQueue}}|{{GroovyCastException}}|proxy over the list|
|{{SortedMap}}, {{NavigableMap}}|{{GroovyCastException}}|dynamic proxy over the
map: {{firstKey()}} throws {{UnsupportedOperationException}}|
|{{ConcurrentMap}}|{{GroovyCastException}}|dynamic proxy over the map|
|{{Iterable}} (array source)|{{GroovyCastException}}|proxy over the array|
h4. 3. Array and collection sources give different result types
||Expression||Result||
|{{Set s = new Integer[]{3, 1, 2}}}|{{LinkedHashSet [3, 1, 2]}}|
|{{new Integer[]{3, 1, 2} as Set}}|{{HashSet [1, 2, 3]}} (order lost)|
|{{[3, 1, 2] as Set}}|{{LinkedHashSet [3, 1, 2]}}|
|{{HashSet h = [3, 1, 2]}}|{{LinkedHashSet [3, 1, 2]}}|
|{{[3, 1, 2] as HashSet}}|{{HashSet [1, 2, 3]}}|
h3. Already working (no change intended)
{{Collection}}, {{Iterable}} (collection source), {{Set}}, {{AbstractSet}},
{{List}} (list or array source), {{HashSet}}, {{LinkedHashSet}}, {{TreeSet}},
{{ArrayList}}, {{LinkedList}}, {{Vector}}, {{ArrayDeque}}, {{PriorityQueue}},
{{ConcurrentLinkedQueue}}, {{CopyOnWriteArrayList}}, {{Map}}, {{HashMap}},
{{LinkedHashMap}}, {{AbstractMap}}, {{TreeMap}}, {{ConcurrentHashMap}},
{{ConcurrentSkipListMap}}. On JDK 21, also {{SequencedCollection}},
{{SequencedSet}} and {{SequencedMap}}.
h3. Proposal
One table of default implementations, used by both {{castToType}} and
{{asType}}. A value that is already an instance of the target is returned
unchanged, as now.
||Target||Default||
|{{List}}, {{Collection}}, {{Iterable}}|{{ArrayList}}|
|{{Set}}|{{LinkedHashSet}}|
|{{SortedSet}}, {{NavigableSet}}|{{TreeSet}}|
|{{Queue}}, {{Deque}}|{{LinkedList}} (accepts {{null}} elements, unlike
{{ArrayDeque}}; matches today's {{as Queue}})|
|{{BlockingQueue}}|{{LinkedBlockingQueue}}|
|{{SortedMap}}, {{NavigableMap}}|{{TreeMap}}|
|{{ConcurrentMap}}|{{ConcurrentHashMap}}|
For a concrete target class without a copy constructor, fall back to its no-arg
constructor plus {{addAll}} for a collection source too, as is already done for
an array source.
h3. Behaviour changes
* Group 1 assignments that now fail would succeed.
* {{as NavigableSet}}, {{as Deque}}, {{as BlockingQueue}}, {{as SortedMap}},
{{as NavigableMap}} and {{as ConcurrentMap}} would return a real collection
instead of a proxy. Code relying on the proxy keeping a reference to the
original collection would see a copy.
* {{array as Set}} would keep element order ({{LinkedHashSet}} instead of
{{HashSet}}).
* {{TreeSet}}/{{TreeMap}} targets throw {{ClassCastException}} or
{{NullPointerException}} for non-{{Comparable}} or {{null}} elements and keys,
as {{as SortedSet}} and {{TreeSet x = ...}} already do.
Open question: should {{HashSet h = list}} keep giving a {{LinkedHashSet}} (a
subclass, so order is kept), or give the exact class as {{list as HashSet}}
does?
h3. Out of scope
Static compilation rejects a list literal assigned to most concrete collection
classes, for example {{@CompileStatic ... LinkedList l = [3, 4, 5]}} fails with
"Cannot find matching constructor java.util.LinkedList(int, int, int)",
following the list-constructor rule in the type-checking docs. That is a
separate type-checker question, to be raised in its own issue.
> Casting a collection to SortedSet, Queue, Deque and similar interfaces fails,
> or with "as" gives a broken proxy
> ---------------------------------------------------------------------------------------------------------------
>
> Key: GROOVY-12585
> URL: https://issues.apache.org/jira/browse/GROOVY-12585
> Project: Groovy
> Issue Type: Improvement
> Reporter: Paul King
> Priority: Major
> Fix For: 7.x
>
>
> h3. Problem
> Groovy turns a collection into another collection type when it is assigned or
> cast ({{T x = value}}, {{(T) value}}, through
> {{DefaultTypeTransformation.castToType}}), and when it is coerced with {{as}}
> (through {{DefaultGroovyMethods.asType(Collection, Class)}}). The two paths
> know different default implementations, and neither covers the common
> collection interfaces. So:
> * some assignments fail where the equivalent {{as}} works;
> * some {{as}} coercions return a proxy that does not behave like the
> requested type;
> * an array source and a collection source give different results for the same
> target;
> * a single-element list assigned to some collection classes gives an empty
> collection.
> Results below are the same with indy and classic call sites, on JDK 17 and
> 21. Groups 1 and 2 fail the same way in every release checked, from 2.4.12 to
> 6.0.0.
> h4. 1. Assignment fails where {{as}} works
> ||Target||{{T x = [3, 1, 2, 1]}}||{{[3, 1, 2, 1] as T}}||
> |{{SortedSet}}|{{GroovyCastException}}|{{TreeSet [1, 2, 3]}}|
> |{{Queue}}|{{GroovyCastException}}|{{LinkedList [3, 1, 2, 1]}}|
> |{{Stack}}|{{GroovyCastException}}|{{Stack [3, 1, 2, 1]}}|
> |{{List}} (source {{[3, 1, 2] as
> LinkedHashSet}})|{{GroovyCastException}}|{{ArrayList [3, 1, 2]}}|
> {{asType}} has a table of defaults for {{List}}, {{Set}}, {{SortedSet}},
> {{Queue}} and {{Stack}}. {{castToType}} has none for a {{Collection}} source:
> it gives a {{LinkedHashSet}} when the target accepts one. Otherwise it tries
> the target's constructor with the elements, then with the collection, which
> fails for an interface and for a class without a copy constructor such as
> {{Stack}}.
> An array source goes through a different branch: a no-arg constructor plus
> {{addAll}}. So {{Stack s = new Integer[]\{3, 1, 2\}}} works while {{Stack s =
> [3, 1, 2]}} fails.
> h4. 2. Assignment fails, and {{as}} returns a proxy that does not behave like
> the type
> ||Target||{{T x = value}}||{{value as T}}||
> |{{NavigableSet}}|{{GroovyCastException}}|proxy over the list: {{([3, 1, 2,
> 1] as NavigableSet).size() == 4}}, {{first() == 3}}|
> |{{Deque}}|{{GroovyCastException}}|proxy over the list: {{peekFirst()}}
> throws {{MissingMethodException}}|
> |{{BlockingQueue}}|{{GroovyCastException}}|proxy over the list|
> |{{SortedMap}}, {{NavigableMap}}|{{GroovyCastException}}|dynamic proxy over
> the map: {{firstKey()}} throws {{UnsupportedOperationException}}|
> |{{ConcurrentMap}}|{{GroovyCastException}}|dynamic proxy over the map|
> |{{Iterable}} (array source)|{{GroovyCastException}}|proxy over the array|
> h4. 3. Array and collection sources give different result types
> ||Expression||Result||
> |{{Set s = new Integer[]\{3, 1, 2\}}}|{{LinkedHashSet [3, 1, 2]}}|
> |{{new Integer[]\{3, 1, 2\} as Set}}|{{HashSet [1, 2, 3]}} (order lost)|
> |{{[3, 1, 2] as Set}}|{{LinkedHashSet [3, 1, 2]}}|
> |{{HashSet h = [3, 1, 2]}}|{{LinkedHashSet [3, 1, 2]}}|
> |{{[3, 1, 2] as HashSet}}|{{HashSet [1, 2, 3]}}|
> h4. 4. A single element is passed to a capacity constructor
> Because {{castToType}} tries the target's constructor with the elements
> first, a one-element list picks a capacity constructor:
> ||Expression||Result||
> |{{Vector v = [5]}}|empty {{Vector}} (capacity 5)|
> |{{PriorityQueue q = [5]}}|empty {{PriorityQueue}}|
> |{{ArrayDeque d = [5]}}|empty {{ArrayDeque}}|
> |{{[5] as Vector}}|{{Vector [5]}}|
> h3. Already working (no change intended)
> {{Collection}}, {{Iterable}} (collection source), {{Set}}, {{AbstractSet}},
> {{List}} (list or array source), {{HashSet}}, {{LinkedHashSet}}, {{TreeSet}},
> {{ArrayList}}, {{LinkedList}}, {{Vector}} (more than one element),
> {{ArrayDeque}}, {{PriorityQueue}}, {{ConcurrentLinkedQueue}},
> {{CopyOnWriteArrayList}}, {{Map}}, {{HashMap}}, {{LinkedHashMap}},
> {{AbstractMap}}, {{TreeMap}}, {{ConcurrentHashMap}},
> {{ConcurrentSkipListMap}}. On JDK 21, also {{SequencedCollection}},
> {{SequencedSet}} and {{SequencedMap}}.
> h3. Proposal
> One table of default implementations, used by both {{castToType}} and
> {{asType}}. A value that is already an instance of the target is returned
> unchanged, as now.
> ||Target||Default||
> |{{List}}, {{Collection}}, {{Iterable}}|{{ArrayList}}|
> |{{Set}}|{{LinkedHashSet}}|
> |{{SortedSet}}, {{NavigableSet}}|{{TreeSet}}|
> |{{Queue}}, {{Deque}}|{{LinkedList}} (accepts {{null}} elements, unlike
> {{ArrayDeque}}; matches today's {{as Queue}})|
> |{{BlockingQueue}}|{{LinkedBlockingQueue}}|
> |{{SortedMap}}, {{NavigableMap}}|{{TreeMap}}|
> |{{ConcurrentMap}}|{{ConcurrentHashMap}}|
> For a concrete collection class, assignment and cast try its copy
> constructor, then its no-arg constructor followed by {{addAll}}, and only
> then the elements as constructor arguments. This is the order {{as}} already
> uses, and it fixes group 4. As with {{as}} today (GROOVY-7867), a constructor
> that throws is skipped in favour of the next step, and its exception is kept
> as a suppressed exception if every step fails.
> {{as}} with a concrete class keeps its current path. So the open question is
> answered by keeping today's behaviour: {{HashSet h = list}} still gives a
> {{LinkedHashSet}} (a subclass, so order is kept), and {{list as HashSet}}
> still gives a {{HashSet}}.
> h3. Behaviour changes
> * Assignments in groups 1 and 2 that now fail would succeed.
> * {{as NavigableSet}}, {{as Deque}}, {{as BlockingQueue}}, {{as SortedMap}},
> {{as NavigableMap}} and {{as ConcurrentMap}} would return a real collection
> instead of a proxy, as would {{as Iterable}} and {{as Deque}} with an array
> source. Code relying on the proxy keeping a reference to the original
> collection would see a copy.
> * {{array as Set}} would keep element order ({{LinkedHashSet}} instead of
> {{HashSet}}).
> * A single-element list assigned to a collection class would hold the element
> instead of giving an empty collection (group 4).
> * {{TreeSet}}/{{TreeMap}} targets throw {{ClassCastException}} or
> {{NullPointerException}} for non-{{Comparable}} or {{null}} elements and
> keys, as {{as SortedSet}} and {{TreeSet x = ...}} already do.
> * A map coerced with {{as}} to an interface outside the table, such as
> {{Runnable}} or {{Comparator}}, is still a proxy over the map's closures.
> h3. Out of scope
> The type checker has its own rules for list and map literals, and they are to
> be raised in their own issues:
> * GROOVY-XXXXX: under {{@TypeChecked}} and {{@CompileStatic}}, a literal
> assigned to the interfaces in the table is a compile error, for example
> {{SortedSet s = [3, 1]}}. A literal assigned to a concrete collection class
> is matched against a constructor taking the elements, so {{LinkedList l = [3,
> 4, 5]}} fails with "Cannot find matching constructor
> java.util.LinkedList(int, int, int)". Under {{@CompileStatic}}, {{Vector v =
> [5]}} calls {{Vector(int)}} and stays empty. Until that issue is fixed,
> {{Vector v = [5]}} gives {{[5]}} in dynamic and {{@TypeChecked}} code but
> {{[]}} under {{@CompileStatic}}.
> * GROOVY-YYYYY: under static checking, a literal returned from a method is
> rejected unless its own class is assignable to the return type. {{Set m() \{
> [3, 1] \}}} and {{TreeMap m() \{ [b: 2, a: 1] \}}} fail, although the same
> literal assigned to a {{Set}} or {{TreeMap}} variable compiles.
> * A list or map argument is not converted to a method's parameter type, in
> any mode, by design.
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