Lambdas and Streams best-practices

Lambda and Stream best practices extracted from the Modern Java in Action - Lambda, streams, functional and reactive programming and some others. Also, this repo includes some tips when using lambda and streams as a Java developer.

What are lambdas and streams?

List of (non exhautive) intermediate operations of Stream:

Operation	Type	Return Type	Argument of the Operation	Function descriptor
filter	Intermediate	Stream<T>	Predicate<T>	T -> boolean
map	Intermediate	Stream<R>	Function<T, R>	T -> R
limit	Intermediate	Stream<T>	long
distinct	Intermediate	Stream<T>

List of (non exhautive) terminal operations of Stream:

Operation	Type	Return Type	Purpose
forEach	Terminal	void	Consumes each element from a stream and applies a lambda to each of them.
count	Terminal	long	Returns the number of elements in a stream.
collect	Terminal	Stream<T>	Returns a collection from a stream (such as Map, List or event Integer).

Get the name of all dishes with calories greater than 300

Stop doing this:

List<String> highCaloricDishes = new ArrayList<>();
Iterator<String> iterator = menu.iterator();
while(iterator.hasNext()) {
  Dish dish = iterator.next();
  if(dish.getCalories() > 300) {
    highCaloricDishes.add(d.getName());
  }
}

Do this:

List<String> highCaloricDish =
  menu.stream()
  .filter(dish -> dish.getCalories() > 300)
  .collect(toList());

---

Print all the dishes in the collection

Stop doing this:

Iterator<String> iterator = menu.iterator();
while (iterator.hasNext()) {
  Dish dish = iterator.next();
  System.out.println(dish);
}

Do this:

menu.forEach(System.out::println);

---

Count the number of distinct dishes (at most 3)

Stop doing this:

Set<Dish> dishes = new HashSet<>();
int count = 0;
for(Dish dish : menu) {
   if (dish.getCalories() > 300) {
      dishes.add(dish);
      count++;
   }
   if (count == 3) break;
}

Do this:

long count = menu.stream()
  .filter(dish -> dish.getCalories() > 300)
  .distinct()
  .limit(3)
  .count();

---

Get vegetarian dishes from all available dishes

Stop doing this:

List<Dish> vegetarianDishes = new ArrayList<>();
for(Dish d: menu) {
  if(d.isVegetarian()){
  vegetarianDishes.add(d);
  }
}

Do this:

List<Dish> vegetarianDishes =
  menu.stream()
  .filter(Dish::isVegetarian)
  .collect(toList());

---

Get all the dishes with the calories lower than 320 (presume the collection is already sorted)

Stop doing this:

List<Dish> dishesWithLowerThan320Calories = new ArrayList<>();
for(Dish dish : menu) {
   if (dish.getCalories() < 320) {
       dishesWithLowerThan320Calories.add(dish);
   }	
}

Do this:

List<Dish> slicedMenu
 	= specialMenu.stream()
 	.takeWhile(dish -> dish.getCalories() < 320)
 	.collect(toList());

Get all the dishes with the calories greater than or equal to 320 (presume the collection is already sorted)

Stop doing this:

List<Dish> dishesWithLowerThan320Calories = new ArrayList<>();
for(Dish dish : menu) {
   if (dish.getCalories() >= 320) {
       dishesWithLowerThan320Calories.add(dish);
   }	
}

Do this:

List<Dish> slicedMenu
 	= specialMenu.stream()
 	.dropWhile(dish -> dish.getCalories() < 320)
 	.collect(toList());

---

Get a list of the first 3 dishes having calories greater than 300

Stop doing this:

List<Dish> dishes = new ArrayList<>();
int count = 0;
for(Dish dish : menu) {
   if (dish.getCalories() > 300) {
      dishes.add(dish);
   }
   if (count == 3) {
      break;
   }
}

Do this:

List<Dish> dishes = specialMenu
  .stream()
  .filter(dish -> dish.getCalories() > 300)
  .limit(3)
  .collect(toList()); 

---

Filter dishes have calories greater than 300, skip 3 first ones and return the rest

Stop doing this:

List<Dish> dishes = new ArrayList<>();
int skipCount = 0;
for(Dish dish : menu) {
  if (dish.getCalories() > 300 && skipCount == 3) {
     dishes.add(dish);
  }
  if (skipCount < 3) {
    skipCount++; 
  }
}

Do this:

List<Dish> dishes = menu.stream()
                        .filter(dish -> dish.getCalories() > 300)
                        .skip(3)
                        .collect(Collectors.toList());

---

Get the first 2 meat dishes

Stop doing this:

List<Dish> dishes = new ArrayList<>();
int count = 0;
for(Dish dish : menu) {
  if (dish.getType() == Dish.Type.MEAT) {
    dishes.add(dish);
    count++;
  }
  if (count == 2) {
    break;
  }
}

Do this:

List<Dish> dishes = menu.stream()
                        .filter(dish -> dish.getType() == Dish.Type.MEAT)
                        .limit(2)
                        .collect(Collectors.toList());

---

Group all dishes by its type

Stop doing this:

Map<Dish.Type, List<Dish>> typeToDish = new HashMap<>();
for(Dish dish : menu) {
   List<Dish> dishes = typeToDish.get(dish.getType());
   if (dishes == null) {
      dishes = new ArrayList<>();
   }
   dishes.add(dish);
   typeToDish.put(dish.getType(), dishes);
}

Do this:

 Map<Dish.Type, List<Dish>> typeToDish = menu.stream().collect(groupingBy(Dish::getType));