Java8中带有流的嵌套列表
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Nested lists with streams in Java8
提问by sk555
I have a list of objects A. Each object A in this list contains list of object B and the object B contains list of Object C. The object C contains an attribute name that i want to use to filter using java 8.
我有一个对象 A 的列表。这个列表中的每个对象 A 都包含对象 B 的列表，对象 B 包含对象 C 的列表。对象 C 包含一个属性名称，我想用它来使用 java 8 进行过滤。
how to write the code below in java 8 using streams to avoid nested loop :
如何使用流在 java 8 中编写以下代码以避免嵌套循环：
C c1 = null;
String name = "name1"
for (A a: listOfAObjects) {
for (B b: a.getList()) {
for (C c: b.getPr()) {
if (c.getName().equalsIgnoreCase(name)) {
c1= c;
break;
}
}
}
}
采纳答案by YCF_L
You can use two flatMapthen a filterthen you can pick the first one or if no result return null:
您可以使用两个flatMap然后一个filter然后您可以选择第一个或者如果没有结果返回null：
C c1 = listOfAObjects.stream()
.flatMap(a -> a.getList().stream())
.flatMap(b -> b.getPr().stream())
.filter(c -> c.getName().equalsIgnoreCase(name))
.findFirst()
.orElse(null);
回答by Eugene
listOfObjectsA.stream()
.flatMap(a -> a.getListOfObjectsB.stream())
.flatMap(b -> b.getListOfObjectsC().stream())
.filter(c -> name.equals(c.getName()))
.findAny()
.orElse(....)
回答by Arnaud Denoyelle
You can do it with flatMap.
你可以用flatMap.
I made an example with Companywhich contains a list of Person:
我做了一个例子，Company其中包含一个列表Person：
public static void main(String[] args) {
List<Company> companies = Arrays.asList(
new Company(Arrays.asList(new Person("Jon Skeet"), new Person("Linus Torvalds"))),
new Company(Arrays.asList(new Person("Dennis Ritchie"), new Person("Bjarne Stroustrup"))),
new Company(Arrays.asList(new Person("James Gosling"), new Person("Patrick Naughton")))
);
List<String> persons = companies.stream()
.flatMap(company -> company.getPersons().stream())
.map(Person::getName)
.collect(Collectors.toList());
System.out.println(persons);
}
Output :
输出 ：
[Jon Skeet, Linus Torvalds, Dennis Ritchie, Bjarne Stroustrup, James Gosling, Patrick Naughton]
[乔恩·斯基特、莱纳斯·托瓦兹、丹尼斯·里奇、比贾恩·斯特劳斯特鲁普、詹姆斯·高斯林、帕特里克·诺顿]
回答by Alexander Yushko
I had same task but i had one nested class. And I had to filter the objects with filter in the nested collection. As a result, I had to get items that have matches in the collection.
我有同样的任务，但我有一个嵌套类。而且我必须在嵌套集合中使用过滤器过滤对象。因此，我必须获取集合中具有匹配项的项目。
for example:
例如：
public class RootElement {
private String name;
private List<String> nestedElements;
//getters / setters and constructors
}
init collection with elements:
带有元素的初始化集合：
List<RootElement> elements = Arrays.asList(
new RootElement("first", Arrays.asList("one", "two", "three")),
new RootElement("second", Arrays.asList("four", "one", "two")));
filter example:
过滤示例：
String filterParam = "four";
List<RootElement> filtered = elements.stream()
.flatMap(root -> root.getNestedElements()
.stream()
.filter(nested -> nested.equalsIgnoreCase(filterParam))
.map(filteredElement -> new RootElement(root.getName(), root.getNestedElement())))
.collect(Collectors.toList());
Hope it will help someone.
希望它会帮助某人。
