问题
I often want to map one list into another list. For example if I had a list of people, and I wanted a list of their names, I would like to do:
GOAL
List<Person> people = ... ;
List<String> names = people.map(x -> x.getName());
Something like this is possible with Java 8:
JAVA 8 VERSION
List<String> names = people.stream()
.map(x -> x.getName())
.collect(Collectors.toList());
But this is clearly not as nice. In fact, I think using Guava is cleaner:
GUAVA VERSION
List<String> names = Lists.transform(people, x -> x.getName());
However, I do like chaining. So, is my goal possible?
I have heard people say that Java 8 default methods are similar to C# extension methods. With a C# extension method, I could easily add a helper method to IEnumerable<T>:
public static IEnumerable<TRet> Map<T, TRet>(this IEnumerable<T> list, Func<T, TRet> selector)
{
return list.Select(selector);
}
However I can't figure out how to use default methods to extend an existing interface.
Also, this is obviously a trivial example. In general, I would like to be able to extend the List<T> and Iterable<T> interfaces to make interacting with the streams api easier.
回答1:
No; you can't do that.
Default methods are not the same as extension methods; they can only be defined within the original interface.
回答2:
If you want to have a lightweight view to a List applying a Function and supporting chaining you can do it like this:
import java.util.*;
import java.util.function.Function;
import java.util.stream.Stream;
public class MappingList<E> extends AbstractList<E> {
// using this helper class we avoid carrying <S> with the public API
static final class Source<E,S> {
final List<S> list;
final Function<? super S, ? extends E> mapper;
Source(List<S> l, Function<? super S, ? extends E> m) {
list=l;
mapper=m;
}
E get(int index) { return mapper.apply(list.get(index)); }
<T> Source map(Function<? super E, ? extends T> f) {
Objects.requireNonNull(f);
return new Source<>(list, mapper.andThen(f));
}
Stream<E> stream() { return list.stream().map(mapper); }
Stream<E> parallelStream() { return list.parallelStream().map(mapper); }
}
final Source<E,?> source;
private MappingList(Source<E,?> s) {
Objects.requireNonNull(s);
source=s;
}
@Override
public E get(int index) {
return source.get(index);
}
@Override
public int size() {
return source.list.size();
}
@Override
public Stream<E> stream() {
return source.stream();
}
@Override
public Stream<E> parallelStream() {
return source.parallelStream();
}
public <T> MappingList<T> map(Function<? super E, ? extends T> f) {
return new MappingList<>(source.map(f));
}
public static <S,T> MappingList<T> map(
List<S> l, Function<? super S, ? extends T> f) {
Objects.requireNonNull(l);
if(l instanceof MappingList)
return ((MappingList<S>)l).map(f);
return new MappingList<>(new Source<>(l, f));
}
}
It supports a GUAVA style creation of a mapped list while still allowing to use the Stream API with the mapped list evaluating all values lazily:
public static void main(String[] arg) {
List<String> strings=Arrays.asList("a", "simple", "list");
List<Integer> ints=MappingList.map(strings, s->compute(s));
List<Integer> results=MappingList.map(ints, i->compute(i));
for(int result:results) {
System.out.println("first result: "+result);
System.out.println("Not computing any more values");
break;
}
System.out.println();
System.out.println(" interacting with stream API:");
System.out.println(results.stream().filter(i-> i>500).findFirst());
}
public static int compute(String s) {
System.out.println("doing computation for "+s);
return Integer.parseInt(s, 36);
}
public static int compute(int i) {
System.out.println("doing computation for "+i);
return i*i;
}
doing computation for a doing computation for 10 first result: 100 Not computing any more values interacting with stream API: doing computation for a doing computation for 10 doing computation for simple doing computation for 1724345618 Optional[410277188]
If you want to create a List with pre-calculated values out of it you can simply use new ArrayList<>(mappedList).
回答3:
Eating my own dog's food and implementing what I suggested in a comment (UNTESTED, but it should work -- note that you should use super where appropriate, this is not my forte):
public final class ListTransformer<T>
{
private final List<T> inputList;
public static <X> ListTransformer<X> transform(final List<X> inputList)
{
return new ListTransformer<X>(inputList);
}
private ListTransformer(final List<T> inputList)
{
this.inputList = inputList;
}
public <U> List<U> using(final Function<T, U> f)
{
return inputList.stream().map(f).collect(Collectors.toList());
}
}
Usage:
import static wherever.is.ListTransformer.transform;
//
final List<String> names = transform(personList).using(x -> x.getName());
回答4:
C# extension methods are great and provide and easy mechanism to extend C# collections with own methods.
But now there are streams in java. Based on @fge response I came with this snippet to write own extension methods on streams:
public final class StreamExtender<T>
{
private final Stream<T> _inputStream;
public static <T> StreamExtender<T> extend(final Stream<T> inputStream)
{
return new StreamExtender<>(inputStream);
}
private StreamExtender(final Stream<T> inputStream)
{
this._inputStream = inputStream;
}
public <U> List<U> extensionMethod(final Function<T, U> f)
{
// your own code.
return _inputStream.map(f).collect(Collectors.toList());
}
}
And to see it in action:
Integer[] array = { 1, 2, 3};
List<String> result = StreamExtender.extend(stream(array)).extensionMethod(Object::toString);
来源：https://stackoverflow.com/questions/22308823/extending-listt-in-java-8