Java 8 List<V> into Map<K, V>
I want to translate a List of objects into a Map using Java 8's streams and lambdas.
This is how I would write it in Java 7 and below.
private Map<String, Choice> nameMap(List<Choice> choices) {
final Map<String, Choice> hashMap = new HashMap<>();
for (final Choice choice : choices) {
hashMap.put(choice.getName(), choice);
}
return hashMap;
}
I can accomplish this easily using Java 8 and Guava but I would like to know how to do this without Guava.
In Guava:
private Map<String, Choice> nameMap(List<Choice> choices) {
return Maps.uniqueIndex(choices, new Function<Choice, String>() {
@Override
public String apply(final Choice input) {
return input.getName();
}
});
}
And Guava with Java 8 lambdas.
private Map<String, Choice> nameMap(List<Choice> choices) {
return Maps.uniqueIndex(choices, Choice::getName());
}
基于Collectors文档，它非常简单：
Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(Choice::getName,
Function.identity()));
如果您的密钥不能保证对列表中的所有元素都是唯一的，则应将其转换为Map<String, List<Choice>>而不是Map<String, Choice>
Map<String, List<Choice>> result =
choices.stream().collect(Collectors.groupingBy(Choice::getName));
使用getName（）作为键和选择本身作为映射的值：
Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(Choice::getName, c -> c));
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