How can I write the following method in Java 8 streams? - java-8

Question

How can I write the following method in Java 8 streams? I couldn't find a way to do it. This is my code:
public static List<ObjectB> getFilteredList(List<ObjectA> list, LocalTime startTime, LocalTime endTime, int quantity) {
List<ObjectA> objectAList = new LinkedList<>();
List<ObjectB> objectBList = new LinkedList<>();
for (ObjectA object : list) {
if (object.getDateTime().toLocalTime().isAfter(startTime) && object.getDateTime().toLocalTime().isBefore(endTime)) {
objectAList.add(object);
}
}
for (ObjectA objectA : objectAList) {
int total = 0;
for (ObjectA object : list) {
if (object.getDateTime().toLocalDate().equals(objectA.getDateTime().toLocalDate())) {
total += object.getQuantity();
}
}
if (total > quantity) {
objectBList.add(new ObjectB(objectA.getDateTime(), objectA.getDescription(), objectA.getQuantity(), true));
} else {
objectBList.add(new ObjectB(objectA.getDateTime(), objectA.getDescription(), objectA.getQuantity(), false));
}
}
return objectBList;}
I have a list of objects with two fields: date and quantity. I need to return a list with one object for each date, but with one more feild - boolean, which should be true if the total sum of all quantites per day is more than 16, and false if it's not.

Answer 1

Let's do this step-by-step.
for (ObjectA object : list) {: a for loop is usually replaced with stream(), so start with list.stream().
if (...) {: condition is usually replaced with filter(), so continue with .filter(object -> object.getDateTime()...)
objectAList.add(object);: adding the results to the container is usually replaced with collect(). You are using LinkedList(), but any other List would be fine here, so we will simply use collect(Collectors.toList()).
So here's first loop:
List<ObjectA> objectAList = list.stream()
.filter(object -> object.getDateTime().toLocalTime().isAfter(startTime) &&
object.getDateTime().toLocalTime().isBefore(endTime))
.collect(Collectors.toList());
Now let's look into the inner loop which calculates the total:
int total = 0;
for (ObjectA object : list) {
if (object.getDateTime().toLocalDate().equals(objectA.getDateTime().toLocalDate())) {
total += object.getQuantity();
}
}
It's also stream-filter-collect sequence, but here you want to collect the sum. So you may use IntStream here which already has the sum() method:
int total = list.stream()
.filter(object -> object.getDateTime().toLocalDate().equals(
objectA.getDateTime().toLocalDate())
.mapToInt(ObjectA::getQuantity).sum();
To make your code less crowded I would extract this to the separate method:
private static int getQuantityByDate(List<ObjectA> list, LocalDate date) {
return list.stream().filter(object -> object.getDateTime().toLocalDate().equals(date))
.mapToInt(ObjectA::getQuantity).sum();
}
Now the next if statement. It just changes the last boolean argument, so I would rewrite it (even without Stream API):
objectBList.add(new ObjectB(objectA.getDateTime(), objectA.getDescription(),
objectA.getQuantity(), total > quantity));
So now we see that the outer loop becomes stream-map-collect chain and could be rewritten this way:
List<ObjectB> objectBList = objectAList.stream()
.map(objectA ->
new ObjectB(objectA.getDateTime(), objectA.getDescription(), objectA.getQuantity(),
getQuantityByDate(list, objectA.getDateTime().toLocalDate()) > quantity))
.collect(Collectors.toList());
Now you can notice that collecting into objectAList is unnecessary as we just use it to create another stream. So we can merge both loops into single pipeline, resulting in the following final code:
private static int getQuantityByDate(List<ObjectA> list, LocalDate date) {
return list.stream().filter(object -> object.getDateTime().toLocalDate().equals(date))
.mapToInt(ObjectA::getQuantity).sum();
}
public static List<ObjectB> getFilteredList(
List<ObjectA> list, LocalTime startTime, LocalTime endTime, int quantity) {
return list.stream()
.filter(object -> object.getDateTime().toLocalTime().isAfter(startTime) &&
object.getDateTime().toLocalTime().isBefore(endTime))
.map(objectA -> new ObjectB(
objectA.getDateTime(), objectA.getDescription(), objectA.getQuantity(),
getQuantityByDate(list, objectA.getDateTime().toLocalDate()) > quantity))
.collect(Collectors.toList());
}