英文:
Convert a stream of Objects to a Map
问题
以下是您要翻译的内容：
我想要将一系列的对象转换为一个映射。键是对象本身，值是 Function.identity()。我的目标是为每个人创建一个增量索引。
public class Person {
private String firstName;
private String lastName;
}
/* 期望的结果
键:[Person1], 值:1
键:[Person2], 值:2
键:[Person3], 值:3
*/
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
return persons.filter(p -> "John".equalsIgnoreCase(p.getFirstName()))
.collect(Collectors.toMap(Function.identity(), person -> 1));
}
我的问题是，在应用 .filter() 后，我无法将对象作为键（甚至值）放入 .toMap() 方法中。
英文:
I would like to convert a stream of Objects to a Map. The key is the object itself and the value is Function.identity(). My goal is to create an incremental index for every Person.
public class Person {
private String firstName;
private String lastName;
}
/* Expected Result
Key:[Person1], value:1
Key:[Person2], value:2
Key:[Person3], value:3
*/
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
return persons.filter(p -> "John".equalIgnoreCase(p.getFirstName)
.collect(Collectors.toMap(Person, Function.identity()));
}
My problem is that after applying the .filter(), I can't put my object as a key (or even value) in .toMap() method.
答案1
得分: 2
你可以分两步完成：
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
List<Person> filtered = persons.filter(p -> "John".equalsIgnoreCase(p.getFirstName))
.collect(Collectors.toList());
return IntStream.range(0, filtered.size())
.boxed()
.collect(Collectors.toMap(filtered::get, i -> i + 1));
}
英文:
You could make it in two steps:
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
List<Person> filterd = persons.filter(p -> "John".equalIgnoreCase(p.getFirstName))
.collect(Collectors.toList());
return IntStream.range(0, filterd.size())
.boxed()
.collect(Collectors.toMap(filterd::get, i -> i + 1));
}
答案2
得分: 0
你可以尝试类似这样的代码：
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
AtomicInteger counter = new AtomicInteger(0);
return persons.filter(p -> "John".equalsIgnoreCase(p.getFirstName()))
.collect(Collectors.toMap(p -> p, p -> counter.getAndIncrement()));
}
英文:
You can try something like this:
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
AtomicInteger counter = new AtomicInteger(0);
return persons.filter(p -> "John".equalIgnoreCase(p.getFirstName())
.collectors.toMap(p->p, counter.getAndIncrement());
}
答案3
得分: 0
你可以使用 AtomicInteger 来生成一个递增值作为 Map 的值：
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
final AtomicInteger atomicInteger = new AtomicInteger();
return persons
.filter(p -> "John".equalsIgnoreCase(p.getFirstName()))
.collect(Collectors.toMap(Function.identity(), notUsed -> atomicInteger.incrementAndGet()));
}
注意在使用并行流时要小心，因为它依赖于处理的顺序。但是它会产生唯一的值，因为 AtomicInteger 是线程安全的。
英文:
You could use AtomicInteger to generate an increasing value as value to your Map:
public Map<Person, Integer> getMapOfPersons(Stream<Person> persons) {
final AtomicInteger atomicInteger = new AtomicInteger();
return persons
.filter(p -> "John".equalsIgnoreCase(p.getFirstName()))
.collect(Collectors.toMap(Function.identity(), notUsed -> atomicInteger.incrementAndGet()));
}
Beware of using this with parallel streams since it relies on the order of the processing. It will, however, render unique values since AtomicInteger is thread safe.
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