自己实现的Collector(模板模式)

最新推荐文章于 2025-12-16 16:02:25 发布

翻译最新推荐文章于 2025-12-16 16:02:25 发布 · 218 阅读

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本文介绍了一个自定义的Collector实现，用于收集Java Stream中的元素到List中，并展示了如何设计一个包含菜品名称、是否素食、热量和类型等属性的Dish对象。通过比较自定义Collector与内置Collectors.toList()的使用，验证了两者的等效性。

domain对象

public class Dish {
    public Dish(String name, boolean vegetarian, int calories, Type type) {
        this.name = name;
        this.vegetarian = vegetarian;
        this.calories = calories;
        this.type = type;
    }

    public String getName() {
        return name;
    }


    public int getCalories() {
        return calories;
    }

    public boolean isVegetarian() {
        return vegetarian;
    }

    public Type getType() {
        return type;
    }

    private final String name;

    private final boolean vegetarian;

    private final int calories;

    private final Type type;


    public enum Type {MEAT, FISH, OTHER}
}

自己实现的Collector

import java.util.*;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;

public class ToListCollector<T> implements java.util.stream.Collector<T, List<T>, List<T>> {
    @Override
    public Supplier<List<T>> supplier() {
        return ArrayList::new;
//        return () -> new ArrayList<>();
    }

    @Override
    public BiConsumer<List<T>, T> accumulator() {
//        return List::add;
        return (list, item) -> list.add(item);
    }

    @Override
    public Function<List<T>, List<T>> finisher() {
        return Function.identity();
    }

    @Override
    public BinaryOperator<List<T>> combiner() {
        return (list1, list2) -> {
            list1.addAll(list2);
            return list1;
        };
    }

    @Override
    public Set<Characteristics> characteristics() {
        return Collections.unmodifiableSet(
                EnumSet.of(Characteristics.IDENTITY_FINISH, Characteristics.CONCURRENT)
        );
    }
}

测试用例

import chapter4.Dish;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Test {
    private static List<Dish> menu = Arrays.asList(
            new Dish("pork", false, 800, Dish.Type.MEAT)
            , new Dish("beef", false, 700, Dish.Type.MEAT)
            , new Dish("chicken", false, 400, Dish.Type.MEAT)
            , new Dish("french fries", true, 530, Dish.Type.OTHER)
            , new Dish("rice", true, 350, Dish.Type.OTHER)
            , new Dish("season fruit", true, 120, Dish.Type.OTHER)
            , new Dish("pizza", true, 550, Dish.Type.OTHER)
            , new Dish("prawns", false, 300, Dish.Type.FISH)
            , new Dish("prawns", false, 300, Dish.Type.FISH)
            , new Dish("salmon", false, 450, Dish.Type.FISH) );


    public static void main(String[] args) {
       List<Dish> list1 = menu.stream().collect(Collectors.toList());
       List<Dish> list2 = menu.stream().collect(new ToListCollector<Dish>());
        System.out.println(list1.equals(list2));
    }
}