Collecting stream back into the same collection type(将流收集回相同的收集类型)
问题描述
Suppose I have a collection of the unknown type. What I want to do is stream it, do some stuff on the stream, and collect it back into the same collection type as my original collection. For instance:
Collection<? extends Integer> getBigger(Collection<? extends Integer> col, int value) {
return col.stream().filter(v -> v > value).collect(????);
}
The idea of this incomplete code example is to return a List
if col
is of List
class (or any subclass of it), a Set
if col
is of Set
class, etc... The method name and actual operations on the stream here are not important, I've specified them just to illustrate my question.
So, is it possible?
It is not possible without violating the principle on which the Java streams framework has been built on. It would completely violate the idea of abstracting the stream from its physical representation.
The sequence of bulk data operations goes in a pipeline, see the following picture:
The stream is somehow similar to the Schrödinger's cat - it is not materialized until you call the terminal operation. The stream handling is completely abstract and detached from the original stream source.
If you want to work so low-level with your original data storage, don't feel ashamed simply avoiding the streams. They are just a tool, not anything sacred. By introducing streams, the Good Old Collections are still as good as they were, with added value of the internal iteration - the new Iterable.forEach() method.
Added to satisfy your curiosity :)
A possible solution follows. I don't like it myself, and I have not been able to solve all the generics issues there, but it works with limitations.
The idea is creating a collector returning the same type as the input collection. However, not all the collections provide a nullary constructor (with no parameters), and without it the Class.newInstance() method does not work. There is also the problem of the awkwardness of checked exceptions within lambda expression. (It is mentioned in this nice answer here: https://stackoverflow.com/a/22919112/2886891)
public Collection<Integer> getBiggerThan(Collection<Integer> col, int value) {
// Collection below is an example of one of the rare appropriate
// uses of raw types. getClass returns the runtime type of col, and
// at runtime all type parameters have been erased.
@SuppressWarnings("rawtypes")
final Class<? extends Collection> clazz = col.getClass();
System.out.println("Input collection type: " + clazz);
final Supplier<Collection<Integer>> supplier = () -> {
try {
return clazz.newInstance();
}
catch (InstantiationException | IllegalAccessException e) {
throw new RuntimeException(
"A checked exception caught inside lambda", e);
}
};
// After all the ugly preparatory code, enjoy the clean pipeline:
return col.stream()
.filter(v -> v > value)
.collect(supplier, Collection::add, Collection::addAll);
}
As you can see, it works in general, supposed your original collection provides a nullary constructor.
public void test() {
final Collection<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
final Collection<Integer> arrayList = new ArrayList<>(numbers);
final Collection<Integer> arrayList2 = getBiggerThan(arrayList, 6);
System.out.println(arrayList2);
System.out.println(arrayList2.getClass());
System.out.println();
final Collection<Integer> set = new HashSet<>(arrayList);
final Collection<Integer> set2 = getBiggerThan(set, 6);
System.out.println(set2);
System.out.println(set2.getClass());
System.out.println();
// This does not work as Arrays.asList() is of a type
// java.util.Arrays$ArrayList which does not provide a nullary constructor
final Collection<Integer> numbers2 = getBiggerThan(numbers, 6);
}
这篇关于将流收集回相同的收集类型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编程学习网!
本文标题为:将流收集回相同的收集类型
基础教程推荐
- “未找到匹配项"使用 matcher 的 group 方法时 2022-01-01
- 如何使用 Java 创建 X509 证书? 2022-01-01
- Java:带有char数组的println给出乱码 2022-01-01
- 无法使用修饰符“public final"访问 java.util.Ha 2022-01-01
- FirebaseListAdapter 不推送聊天应用程序的单个项目 - Firebase-Ui 3.1 2022-01-01
- 在 Libgdx 中处理屏幕的正确方法 2022-01-01
- 降序排序:Java Map 2022-01-01
- 设置 bean 时出现 Nullpointerexception 2022-01-01
- 减少 JVM 暂停时间 >1 秒使用 UseConcMarkSweepGC 2022-01-01
- Java Keytool 导入证书后出错,"keytool error: java.io.FileNotFoundException &拒绝访问" 2022-01-01