The following lambda expressions take the values of a HashMap, gets a List of Objects in its ArrayList, adds all such Objects to another ArrayList, and then prints each attribute if a co,java,lambda,foreach,java-stream"/>

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The following lambda expressions take the values of a HashMap, gets a List of Objects in its ArrayList, adds all such Objects to another ArrayList, and then prints each attribute if a co

java - 转换列表流 to stream of all Objects in the List

The following lambda expressions take the values of a HashMap, gets a List of Objects in its ArrayList, adds all such Objects to another ArrayList, and then prints each attribute if a co

问题描述

The following lambda expressions take the values of a HashMap, gets a List of Objects in its ArrayList, adds all such Objects to another ArrayList, and then prints each attribute if a condition is met.

This works, but I am a bit frustrated I couldn't figure out how to do this in one step, as in, not using two lambda expressions. 

Map<Integer, Person> people = new HashMap<Integer, Person>();
...
List<Object> objects = new ArrayList<Object>();

people.values().stream()
    .map(p->p.getObjects())
    .forEach(p->objects.addAll(p)); //note: can be multiple
objects.stream()
    .filter(p->p.getClass().toString().contains("Keyword"))
    .forEach(p->System.out.println(p.display()));

So is there a way I can go from line 2 to line 5 directly, which would in effect convert a stream of List of Objects to a stream of all of the Objects themselves?

You could merge your operations to a single stream pipeline as

List<Pet> cats = people.values().stream()
        .flatMap(p -> p.getPets().stream())
        .filter(p -> p.getClass().toString().contains("Cat")) // or Cat.class::isInstance
        .collect(Collectors.toList());

and then perform operations on them as in your code such as

cats.forEach(cat -> System.out.println(cat.getName()));

An overall transformation of your code would look like:

Map<Integer, Person> people = ...;
people.values().stream()
        .flatMap(p -> p.getPets().stream())
        .filter(p -> p.getClass().toString().contains("Cat"))
        .forEach(cat -> System.out.println(cat.getName()));

标签： javalambdaforeachjava-stream

解决方案

您可以将您的操作合并到单个流管道中

List<Pet> cats = people.values().stream()
        .flatMap(p -> p.getPets().stream())
        .filter(p -> p.getClass().toString().contains("Cat")) // or Cat.class::isInstance
        .collect(Collectors.toList());

然后像在您的代码中一样对它们执行操作，例如

cats.forEach(cat -> System.out.println(cat.getName()));

您的代码的整体转换如下所示：

Map<Integer, Person> people = ...;
people.values().stream()
        .flatMap(p -> p.getPets().stream())
        .filter(p -> p.getClass().toString().contains("Cat"))
        .forEach(cat -> System.out.println(cat.getName()));

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