How to collect a stream into a CopyOnWriteArrayList - intellij-idea

Question

I'm getting "Incompatible types, required: CopyOnWriteArrayList, found: Object" with the following. I'm using IntelliJ 2016.1.1.
CopyOnWriteArrayList<Foo> l = fields.stream()
.distinct()
.collect(toCollection(CopyOnWriteArrayList::new));

Answer 1

The problem is that fields has an inappropriate type, most likely, it has a raw type, which will turn the generic invocations of the Stream chain into unchecked operations returning their erased type, which is Object for the terminal collect call.
Using the right type, this works without problems, i.e.
List<String> fields=Arrays.asList("foo", "bar", "baz", "foo");
CopyOnWriteArrayList<String> l =
fields.stream()
.distinct()
.collect(Collectors.toCollection(CopyOnWriteArrayList::new));
works. But note that building a CopyOnWriteArrayList this way is rather expensive as the name “copy on write” already suggests. Copying the entire contents on each insertion leads to quadratic time complexity.
The solution is to collect into a temporary collection, better suited to incremental building, before converting to the desired target type. That copying step might look like overhead, but it’s linear overhead, compared to the quadratic complexity of collecting directly into the CopyOnWriteArrayList.
CopyOnWriteArrayList<String> l =
fields.stream()
.distinct()
.collect(Collectors.collectingAndThen(
Collectors.toList(), CopyOnWriteArrayList::new));
Note that in this specific case, distinct implicitly builds a Set behind the scenes, so we can improve the performance by building the Set explicitly in place of the temporary List and remove the distinct step:
CopyOnWriteArrayList<String> l =
fields.stream()
.collect(Collectors.collectingAndThen(
Collectors.toCollection(LinkedHashSet::new),
CopyOnWriteArrayList::new));
which leads to the conclusion that for this specific use case, we can have it all simpler and potentially even more efficient:
CopyOnWriteArrayList<String> l = new CopyOnWriteArrayList<>(new LinkedHashSet<>(fields));

Answer 2

It seams like your fields object is not of type Foo , otherwise it should work find below working code.
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.stream.Collectors;
public class Foo {
private String name;
Foo(String name){
this.name=name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Override
public String toString() {
return "Foo [name=" + name + "]";
}
public static void main(String[] args) {
List<Foo> fields = new ArrayList<>();
fields.add(new Foo("aa"));
fields.add(new Foo("bb"));
CopyOnWriteArrayList<Foo> l = fields.stream().distinct().collect(Collectors.toCollection(CopyOnWriteArrayList::new));
System.out.println("l"+l);
}
}
PS:If your fields is non generic then also this will give error