I have the following Java6 and Java8 code:
List<ObjectType1> lst1 = // a list of ObjectType1 objects
List<ObjectType2> lst2 = // a list of ObjectType1 objects, same size of lst1
List<ObjectType3> lst3 = new ArrayLis<ObjectType3>(lst1.size());
for(int i=0; i < lst1.size(); i++){
lst3.add(new ObjectType3(lst1.get(i).getAVal(), lst2.get(i).getAnotherVal()));
}
Is there any way in Java8 to handle the previous for in a more concise way using Lambda?
A Stream is tied to a given iterable/Collection so you can't really "iterate" two collections in parallel.
One workaround would be to create a stream of indexes but then it does not necessarily improve over the for loop. The stream version could look like:
List<ObjectType3> lst3 = IntStream.range(0, lst1.size())
.mapToObj(i -> new ObjectType3(lst1.get(i).getAVal(), lst2.get(i).getAnotherVal()))
.collect(toList());
IntStream.range(0, lst1.size()).parallel() thus you may eventually speedup your execution time! This is a good point, isn't it? :)You could create a method that transforms two collections into a new collection, like this:
public <T, U, R> Collection<R> singleCollectionOf(final Collection<T> collectionA, final Collection<U> collectionB, final Supplier<Collection<R>> supplier, final BiFunction<T, U, R> mapper) {
if (Objects.requireNonNull(collectionA).size() != Objects.requireNonNull(collectionB).size()) {
throw new IllegalArgumentException();
}
Objects.requireNonNull(supplier);
Objects.requireNonNull(mapper);
Iterator<T> iteratorA = collectionA.iterator();
Iterator<U> iteratorB = collectionB.iterator();
Collection<R> returnCollection = supplier.get();
while (iteratorA.hasNext() && iteratorB.hasNext()) {
returnCollection.add(mapper.apply(iteratorA.next(), iteratorB.next()));
}
return returnCollection;
}
The important part here is that it will map the obtained iteratorA.next() and iteratorB.next() into a new object.
It is called like this:
List<Integer> list1 = IntStream.range(0, 10).boxed().collect(Collectors.toList());
List<Integer> list2 = IntStream.range(0, 10).map(n -> n * n + 1).boxed().collect(Collectors.toList());
singleCollectionOf(list1, list2, ArrayList::new, Pair::new).stream().forEach(System.out::println);
In your example it would be:
List<ObjectType3> lst3 = singleCollectionOf(lst1, lst2, ArrayList::new, ObjectType3::new);
Where for example Pair::new is a shorthand for the lamdda (t, u) -> new Pair(t, u).
I haven't found a way to update 1 stream to another, however, I accomplished a similar feat using a Map. :)
Map<Integer, String> result = new HashMap<>();
for(int index = 100; index > 0; index--){
result.put(index, String.valueOf(index));
}
result.keySet().stream()
.filter(key -> key%3 == 0)
.sorted()
.forEach(key -> result.put(key, "Fizz"));
result.keySet().stream()
.filter(key -> key%5 == 0)
.sorted()
.forEach(key -> result.put(key, "Buzz"));
result.keySet().stream()
.filter(key -> key%3 == 0 && key%5 == 0)
.sorted()
.forEach(key -> result.put(key, "FizzBuzz"));
result.keySet().stream().forEach(key -> System.out.println(result.get(key)));
ObjectType3?