How to group list of objects using java lambda

Let's consider the following POJO class.

public class Dish {

	private String name;
	private Boolean vegitarian;
	private Integer calories;
	private DishType type;

}

Java 8 stream API provides plenty of features to group objects into different buckets. Let's say, we want to group list of Dish objects based on the amount of calories. We will define an enum constant to declare different calory categories. 

public enum CaloryLevel {
    DIET,
    NORMAL,
    FAT;
}

The Calory level is not an attribute of Dish class. The calory level is defined by using the amount of calories from Dish class which is an attribute of Dish class, ie: 'calories'. If the amount of calory is less than 400, the calory level is 'DIET'. If the amount of calory is between 400 and 700, the calory level is 'NORMAL'. If the calory amount is above 700, the calary level is defined as 'FAT'. The legacy approch of grouping list of objects is as follows. It generally uses a map to put objects into different buckets.

public static void groupDishByCaloriAmountLegacyApproch(List<Dish> menu) {

	Map<CaloryLevel, List<Dish>> dishOverCaloryLevel = new HashMap<CaloryLevel, List<Dish>>();

	for(CaloryLevel caloryLevel : CaloryLevel.values()) {
		dishOverCaloryLevel.put(caloryLevel, new ArrayList<Dish>());
	}

	for(Dish dish : menu) {
		if (dish.getCalaries() <= 400) {
			dishOverCaloryLevel.get(CaloryLevel.DIET).add(dish);
		} else if (dish.getCalaries() <= 700) {
			dishOverCaloryLevel.get(CaloryLevel.NORMAL).add(dish);
		} else {
			dishOverCaloryLevel.get(CaloryLevel.FAT).add(dish);
		}
	}
}

Now, let's see, how we can improve the above code using Java 8 features. We can get the same output by using Java stream API provided methods. See the following code.

Map<CaloryLevel, List<Dish>> dishOverCaloryLevel = menu.stream().collect(Collectors.groupingBy((Dish d) -> {
	if (d.getCalaries() <= 400) {
		return CaloryLevel.DIET;
	} else if (d.getCalaries() <= 700) {
		return CaloryLevel.NORMAL;
	} else
		return CaloryLevel.FAT;
	}
));

Again, the calory level is not an attribute of Dish class. That attribute is external one which is defined by using the amount of calories. If we want to get the calory level by using the amount calories which is defined within the Dish class, we have to write boilaplate codes as in the above method. As a best coding practice, defining the calory level can be added into the Dish class itself as method. The new Dish class is as follows.

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Dish {

	private String name;
	private Boolean vegitarian;
	private Integer calaries;
	private DishType type;

	private CaloryLevel getCaloryLevel() {
		if (this.getCalaries() <= 400) {
			return CaloryLevel.DIET;
		} else if (this.getCalaries() <= 700) {
			return CaloryLevel.NORMAL;
		} else {
			return CaloryLevel.FAT;
		}
	}
}

The complete lambda expresion which was passed into the 'Collectors.groupingBy' method, now it is defined as a method inside the Dish class itself. We can now use Java 8's method referene to simply the code more as follows. Now the code to group list of Dish objects has narrowed down into a single like.

Map<CaloryLevel, List<Dish>> dishOverCaloryLevel1 = menu.stream().collect(Collectors.groupingBy(Dish::getCaloryLevel));