使用Java流的一对一地图 [英] One-to-one map with Java streams

问题描述

使用Stream API获取一对一映射时遇到一些麻烦。基本上，假设你有一个班级。

Having a little trouble using the Stream API to get a one to one mapping. Basically, say you've got a class.

public class Item {
    private final String uuid;

    private Item(String uuid) {
        this.uuid = uuid;
    }

    /**
    * @return universally unique identifier
    */
    public String getUuid() {
        return uuid;
    }
}

我想要映射< String，Item> 以便于查找。但鉴于 Stream< Item> ，似乎没有一种简单的方法可以达到 Map< String，Item> 。

I'd like a Map<String, Item> for easy look up. But given a Stream<Item> there doesn't seem a simple way to arrive at that Map<String, Item>.

显然， Map< String，List< Item>> 并不是一件事：

public static Map<String, List<Item>> streamToOneToMany(Stream<Item> itemStream) {
    return itemStream.collect(groupingBy(Item::getUuid));
}

这是更安全更普遍的情况，但我们做在这种情况下知道只会一对一。我找不到任何可编译的东西 - 我特意试图将下游参数砸到 Collectors.groupingBy 。类似于：

That's the safer more general case, but we do know in this situation that there will only ever be one-to-one. I can't find anything that compiles though -- I've specifically been trying to muck with the downstream parameter to Collectors.groupingBy. Something like:

// DOESN'T COMPILE
public static Map<String, Item> streamToOneToOne(Stream<Item> itemStream) {
    return itemStream.collect(groupingBy(Item::getUuid, Function.identity()));
}

我缺少什么？

推荐答案

使用 收集器#toMap（功能，功能） ，从每个项目的 uuid 和项目作为值本身。

Use the Collectors#toMap(Function, Function), generating the key from each Item's uuid and the Item as the value itself.

public static Map<String, Item> streamToOneToOne(Stream<Item> itemStream) {
    return itemStream.collect(Collectors.toMap(Item::getUuid, Function.identity()));
}

来自javadoc的注释

Note from the javadoc




如果映射的密钥包含重复项（根据
Object.equals（Object）），则集合操作时，抛出IllegalStateException 。如果映射的键可能有
重复，请使用 toMap（Function，Function，BinaryOperator）。

If the mapped keys contains duplicates (according to Object.equals(Object)), an IllegalStateExceptionis thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

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