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https://issues.apache.org/jira/browse/IMPALA-7867?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16691195#comment-16691195
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Paul Rogers edited comment on IMPALA-7867 at 11/19/18 2:43 AM:
---------------------------------------------------------------
[~marcelk], thanks much for the note. The guarantees you site for {{ArrayList}}
are accurate. This is why, when _creating_ a list, {{ArrayList}} is often a
good choice.
However, when _declaring_ a list, the interface is sufficient. Though a
variable is declared as {{List}}, its implementation is actually {{ArrayList}}
if declared that way.
Not sure if this is a difference between Java and C++ behavior, but it is
pretty standard Java practice to hind the implementation choice and expose only
the generic interface.
Example:
{code:java}
public class Foo {
// Declaration is generic, implementation is ArrayList
private final List<Bar> myList_ = new ArrayList<>();
// Interface is generic
List<Bar> getList() { return myList_; }
// Implementation in terms of generic interface, implementation
// is O(1) as guaranteed by ArrayList.
void Bar getBar(int i) {
return myList_.get(i);
}
}
{code}
Later, we could decide, say, to change the semantics so that the list is
immutable and use a new {{ImmutableArrayList}} as the implementation. Consumers
of the class don't care, they still see {{List}} (but now backed by a different
implementation.)
For more information, please see the [collection
docs|https://docs.oracle.com/javase/8/docs/technotes/guides/collections/overview.html].
Does this clear up the confusion?
was (Author: paul.rogers):
[~marcelk], thanks much for the note. The guarantees you site for {{ArrayList}}
are accurate. This is why, when _creating_ a list, {{ArrayList}} is often a
good choice.
However, when _declaring_ a list, the interface is sufficient. Though a
variable is declared as {{List}}, its implementation is actually {{ArrayList}}
if declared that way.
Not sure if this is a difference between Java and C++ behavior, but it is
pretty standard Java practice to hind the implementation choice and expose only
the generic interface.
Example:
{code:java}
public class Foo {
// Declaration is generic, implementation is ArrayList
private final List<Bar> myList_ = new ArrayList<>();
// Interface is generic
List<Bar> getList() { return myList_; }
// Implementation in terms of generic interface, implementation
// is O(1) as guaranteed by ArrayList.
void Bar getBar(int i) {
return myList_.get(i);
}
}
{code}
Does this clear up the confusion?
> Expose collection interfaces, not implementations
> -------------------------------------------------
>
> Key: IMPALA-7867
> URL: https://issues.apache.org/jira/browse/IMPALA-7867
> Project: IMPALA
> Issue Type: Improvement
> Components: Frontend
> Affects Versions: Impala 3.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Minor
>
> When using Java collections, a common Java best practice is to expose the
> collection interface, but hide the implementation choice. This pattern allows
> us to start with a generic implementation (an {{ArrayList}}, say), but evolve
> to a more specific implementation to achieve certain goals (a {{LinkedList}}
> or {{ImmutableList}}, say.)
> For whatever reason, the Impala FE code exposes {{ArrayList}}, {{HashMap}}
> and other implementation choices as variable types and in method signatures.
> This ticket tracks a gradual process of revising the declarations and
> signatures to use the interfaces {{List}} instead of the implementation
> {{ArrayList}}.
> Also, the FE code appears to predate Java 7, so that declarations of lists
> tend to be in one of two forms (with or without Guava):
> {code:java}
> foo1 = new ArrayList<Bar>();
> foo2 = Lists.newArrayList();
> {code}
> Since Java 7, the preferred form is:
> {code:java}
> foo = new ArrayList<>();
> {code}
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