在实体集合中查找所有 id 集合的最有效方法 [英] Most efficient way to find the collection of all ids in a collection of entities

问题描述

我有一个实体:

public class Entity
{
    private long id;    
    private String data;

    public long getId() {
        return id;
    }

    public String getData() {
        return data;
    }
}

和实体集合:

Collection<Entity> entities= ...

找到entities中所有id的Collection的最有效方法是什么?

What is the most efficient way to find the Collection<Long> of all the ids in entities?

推荐答案

假设你有

class Entity {
    final long id;
    final String data;

    public long getId() {
        return id;
    }

    public String getData() {
        return data;
    }

    Entity(long id, String data) {
        this.id = id;
        this.data = data;
    }
}

在 Java 8 中你可以编写

In Java 8 you can write

Collection<Entity> entities = Arrays.asList(new Entity(1, "one"), 
                  new Entity(11, "eleven"), new Entity(100, "one hundred"));
// get a collection of all the ids.
List<Long> ids = entities.stream()
                         .map(Entity::getId).collect(Collectors.toList());

System.out.println(ids);

印刷品

[1, 10, 100]

你可以想象这在 Java 7 或更低版本中相当丑陋.注意 Entity.getId 当应用于 map() 意味着在每个元素上调用这个方法.

As you can imagine this is rather ugly in Java 7 or less. Note the Entity.getId when applied to map() means call this method on each element.

现在，真正有趣的是你可以做到这一点.

Now, the real interesting part is you can do this.

List<Long> ids = entities.parallelStream()
                         .map(Entity::getId).collect(Collectors.toList());

在大多数情况使用并行流会影响性能，但它使尝试和看到非常容易(可能太容易了；)

In most cases using a parallel stream will hurt performance, but it makes trying it and seeing amazingly easy (possibly too easy ;)

最有效的方法是拥有或构建地图.

The most efficient way is to have, or build a Map.

Map<Long, Entity> entitiesMap = ...
// get all ids
Collection<Long> addIds = entitiesMap.keySet();

// look up entities by id.
List<Long> ids = ...
List<Entity> matching = new ArrayList<>();
for(Long id: ids)
    matching.add(entitiesMap.get(id));

这篇关于在实体集合中查找所有 id 集合的最有效方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！