[英]Java flatMap - whats the difference stream.of() and collection.stream()
[英]Java 8 Streams: How to call once the Collection.stream() method and retrieve an array of several aggregate values with different fields
我將從Java 8中的Stream API開始。
這是我使用的Person對象:
public class Person {
private String firstName;
private String lastName;
private int age;
private double height;
private double weight;
public Person(String firstName, String lastName, int age, double height, double weight) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
this.height = height;
this.weight = weight;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public int getAge() {
return age;
}
public double getHeight() {
return height;
}
public double getWeight() {
return weight;
}
}
這是我的代碼,它初始化一個對象列表Person,它獲取由特定名字過濾的對象數,最大年齡和最小高度,平均權重,最后創建一個包含這些值的對象數組:
List<Person> personsList = new ArrayList<Person>();
personsList.add(new Person("John", "Doe", 25, 1.80, 80));
personsList.add(new Person("Jane", "Doe", 30, 1.69, 60));
personsList.add(new Person("John", "Smith", 35, 174, 70));
long count = personsList.stream().filter(p -> p.getFirstName().equals("John")).count();
int maxAge = personsList.stream().mapToInt(Person::getAge).max().getAsInt();
double minHeight = personsList.stream().mapToDouble(Person::getHeight).min().getAsDouble();
double avgWeight = personsList.stream().mapToDouble(Person::getWeight).average().getAsDouble();
Object[] result = new Object[] { count, maxAge, minHeight, avgWeight };
System.out.println(Arrays.toString(result));
是否可以單獨調用stream()
方法並直接返回對象數組?
Object[] result = personsList.stream()...count()...max()...min()...average()
我之前問過非常類似的問題: Java 8 Streams:如何調用一次Collection.stream()方法並檢索幾個聚合值的數組,但這次我不能使用summaryStatistics()
方法因為我使用了不同的字段(年齡,高度) ,weight)檢索聚合值。
編輯2016-01-07
我測試了TriCore
和Tagir Valeev
的解決方案,並計算了每個解決方案的運行時間。
似乎TriCore
解決方案比Tagir Valeev
更有效。
與我的解決方案(使用多個Streams)相比, Tagir Valeev
的解決方案似乎沒有節省太多時間。
這是我的測試類:
public class StreamTest {
public static class Person {
private String firstName;
private String lastName;
private int age;
private double height;
private double weight;
public Person(String firstName, String lastName, int age, double height, double weight) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
this.height = height;
this.weight = weight;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public int getAge() {
return age;
}
public double getHeight() {
return height;
}
public double getWeight() {
return weight;
}
}
public static abstract class Process {
public void run() {
StopWatch timer = new StopWatch();
timer.start();
doRun();
timer.stop();
System.out.println(timer.getTime());
}
protected abstract void doRun();
}
public static void main(String[] args) {
List<Person> personsList = new ArrayList<Person>();
for (int i = 0; i < 1000000; i++) {
int age = random(15, 60);
double height = random(1.50, 2.00);
double weight = random(50.0, 100.0);
personsList.add(new Person(randomString(10, Mode.ALPHA), randomString(10, Mode.ALPHA), age, height, weight));
}
personsList.add(new Person("John", "Doe", 25, 1.80, 80));
personsList.add(new Person("Jane", "Doe", 30, 1.69, 60));
personsList.add(new Person("John", "Smith", 35, 174, 70));
personsList.add(new Person("John", "T", 45, 179, 99));
// Query with mutiple Streams
new Process() {
protected void doRun() {
queryJava8(personsList);
}
}.run();
// Query with 'TriCore' method
new Process() {
protected void doRun() {
queryJava8_1(personsList);
}
}.run();
// Query with 'Tagir Valeev' method
new Process() {
protected void doRun() {
queryJava8_2(personsList);
}
}.run();
}
// --------------------
// JAVA 8
// --------------------
private static void queryJava8(List<Person> personsList) {
long count = personsList.stream().filter(p -> p.getFirstName().equals("John")).count();
int maxAge = personsList.stream().mapToInt(Person::getAge).max().getAsInt();
double minHeight = personsList.stream().mapToDouble(Person::getHeight).min().getAsDouble();
double avgWeight = personsList.stream().mapToDouble(Person::getWeight).average().getAsDouble();
Object[] result = new Object[] { count, maxAge, minHeight, avgWeight };
System.out.println("Java8: " + Arrays.toString(result));
}
// --------------------
// JAVA 8_1 - TriCore
// --------------------
private static void queryJava8_1(List<Person> personsList) {
Object[] objects = personsList.stream().collect(Collector.of(() -> new PersonStatistics(p -> p.getFirstName().equals("John")),
PersonStatistics::accept, PersonStatistics::combine, PersonStatistics::toStatArray));
System.out.println("Java8_1: " + Arrays.toString(objects));
}
public static class PersonStatistics {
private long firstNameCounter;
private int maxAge = Integer.MIN_VALUE;
private double minHeight = Double.MAX_VALUE;
private double totalWeight;
private long total;
private final Predicate<Person> firstNameFilter;
public PersonStatistics(Predicate<Person> firstNameFilter) {
Objects.requireNonNull(firstNameFilter);
this.firstNameFilter = firstNameFilter;
}
public void accept(Person p) {
if (this.firstNameFilter.test(p)) {
firstNameCounter++;
}
this.maxAge = Math.max(p.getAge(), maxAge);
this.minHeight = Math.min(p.getHeight(), minHeight);
this.totalWeight += p.getWeight();
this.total++;
}
public PersonStatistics combine(PersonStatistics personStatistics) {
this.firstNameCounter += personStatistics.firstNameCounter;
this.maxAge = Math.max(personStatistics.maxAge, maxAge);
this.minHeight = Math.min(personStatistics.minHeight, minHeight);
this.totalWeight += personStatistics.totalWeight;
this.total += personStatistics.total;
return this;
}
public Object[] toStatArray() {
return new Object[] { firstNameCounter, maxAge, minHeight, total == 0 ? 0 : totalWeight / total };
}
}
// --------------------
// JAVA 8_2 - Tagir Valeev
// --------------------
private static void queryJava8_2(List<Person> personsList) {
// @formatter:off
Collector<Person, ?, Object[]> collector = multiCollector(
filtering(p -> p.getFirstName().equals("John"), Collectors.counting()),
Collectors.collectingAndThen(Collectors.mapping(Person::getAge, Collectors.maxBy(Comparator.naturalOrder())), Optional::get),
Collectors.collectingAndThen(Collectors.mapping(Person::getHeight, Collectors.minBy(Comparator.naturalOrder())), Optional::get),
Collectors.averagingDouble(Person::getWeight)
);
// @formatter:on
Object[] result = personsList.stream().collect(collector);
System.out.println("Java8_2: " + Arrays.toString(result));
}
/**
* Returns a collector which combines the results of supplied collectors
* into the Object[] array.
*/
@SafeVarargs
public static <T> Collector<T, ?, Object[]> multiCollector(Collector<T, ?, ?>... collectors) {
@SuppressWarnings("unchecked")
Collector<T, Object, Object>[] cs = (Collector<T, Object, Object>[]) collectors;
// @formatter:off
return Collector.<T, Object[], Object[]> of(
() -> Stream.of(cs).map(c -> c.supplier().get()).toArray(),
(acc, t) -> IntStream.range(0, acc.length).forEach(
idx -> cs[idx].accumulator().accept(acc[idx], t)),
(acc1, acc2) -> IntStream.range(0, acc1.length)
.mapToObj(idx -> cs[idx].combiner().apply(acc1[idx], acc2[idx])).toArray(),
acc -> IntStream.range(0, acc.length)
.mapToObj(idx -> cs[idx].finisher().apply(acc[idx])).toArray());
// @formatter:on
}
/**
* filtering() collector (which will be added in JDK-9, see JDK-8144675)
*/
public static <T, A, R> Collector<T, A, R> filtering(Predicate<? super T> filter, Collector<T, A, R> downstream) {
BiConsumer<A, T> accumulator = downstream.accumulator();
Set<Characteristics> characteristics = downstream.characteristics();
return Collector.of(downstream.supplier(), (acc, t) -> {
if (filter.test(t))
accumulator.accept(acc, t);
} , downstream.combiner(), downstream.finisher(), characteristics.toArray(new Collector.Characteristics[characteristics.size()]));
}
// --------------------
// HELPER METHODS
// --------------------
public static enum Mode {
ALPHA,
ALPHANUMERIC,
NUMERIC
}
private static String randomString(int length, Mode mode) {
StringBuffer buffer = new StringBuffer();
String characters = "";
switch (mode) {
case ALPHA:
characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
break;
case ALPHANUMERIC:
characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
break;
case NUMERIC:
characters = "1234567890";
break;
}
int charactersLength = characters.length();
for (int i = 0; i < length; i++) {
double index = Math.random() * charactersLength;
buffer.append(characters.charAt((int) index));
}
return buffer.toString();
}
private static int random(int min, int max) {
Random rand = new Random();
return rand.nextInt((max - min) + 1) + min;
}
private static double random(double min, double max) {
return min + Math.random() * (max - min);
}
}
使用標准JDK 8 API解決這個問題有點棘手,它沒有提供很多組合Collector
類型的方法。 如果您願意使用像jOOλ這樣的第三方庫,您可以寫:
Tuple4<Long, Optional<Integer>, Optional<Double>, Optional<Double>> result =
Seq.seq(personsList)
.collect(
filter(p -> p.getFirstName().equals("John"), count()),
max(Person::getAge),
min(Person::getHeight),
avg(Person::getWeight)
);
System.out.println(result);
以上產量:
(2, Optional[35], Optional[1.8], Optional[75.0])
注意,它使用新的Agg.filter()
方法,類似於JDK 9 Collectors.filtering()
方法,其工作方式如下:
public static <T, A, R> Collector<T, A, R> filter(
Predicate<? super T> predicate, Collector<T, A, R> downstream) {
return Collector.of(
downstream.supplier(),
(c, t) -> {
if (predicate.test(t))
downstream.accumulator().accept(c, t);
},
downstream.combiner(),
downstream.finisher()
);
}
collect(collector1, collector2, ...)
工作? 如果您不想使用上述第三方庫,則可以編寫自己的Collector
組合實用程序。 將兩個收集器組合到Tuple2
收集器中的Tuple2
:
static <T, A1, A2, D1, D2> Collector<T, Tuple2<A1, A2>, Tuple2<D1, D2>> collectors(
Collector<T, A1, D1> collector1
, Collector<T, A2, D2> collector2
) {
return Collector.<T, Tuple2<A1, A2>, Tuple2<D1, D2>>of(
() -> tuple(
collector1.supplier().get()
, collector2.supplier().get()
),
(a, t) -> {
collector1.accumulator().accept(a.v1, t);
collector2.accumulator().accept(a.v2, t);
},
(a1, a2) -> tuple(
collector1.combiner().apply(a1.v1, a2.v1)
, collector2.combiner().apply(a1.v2, a2.v2)
),
a -> tuple(
collector1.finisher().apply(a.v1)
, collector2.finisher().apply(a.v2)
)
);
}
免責聲明:我為jOOλ背后的公司工作。
這是收藏家
public class PersonStatistics {
private long firstNameCounter;
private int maxAge = Integer.MIN_VALUE;
private double minHeight = Double.MAX_VALUE;
private double totalWeight;
private long total;
private final Predicate<Person> firstNameFilter;
public PersonStatistics(Predicate<Person> firstNameFilter) {
Objects.requireNonNull(firstNameFilter);
this.firstNameFilter = firstNameFilter;
}
public void accept(Person p) {
if (this.firstNameFilter.test(p)) {
firstNameCounter++;
}
this.maxAge = Math.max(p.getAge(), maxAge);
this.minHeight = Math.min(p.getHeight(), minHeight);
this.totalWeight += p.getWeight();
this.total++;
}
public PersonStatistics combine(PersonStatistics personStatistics) {
this.firstNameCounter += personStatistics.firstNameCounter;
this.maxAge = Math.max(personStatistics.maxAge, maxAge);
this.minHeight = Math.min(personStatistics.minHeight, minHeight);
this.totalWeight += personStatistics.totalWeight;
this.total += personStatistics.total;
return this;
}
public Object[] toStatArray() {
return new Object[]{firstNameCounter, maxAge, minHeight, total == 0 ? 0 : totalWeight / total};
}
}
您可以按如下方式使用此收集器
public class PersonMain {
public static void main(String[] args) {
List<Person> personsList = new ArrayList<>();
personsList.add(new Person("John", "Doe", 25, 180, 80));
personsList.add(new Person("Jane", "Doe", 30, 169, 60));
personsList.add(new Person("John", "Smith", 35, 174, 70));
personsList.add(new Person("John", "T", 45, 179, 99));
Object[] objects = personsList.stream().collect(Collector.of(
() -> new PersonStatistics(p -> p.getFirstName().equals("John")),
PersonStatistics::accept,
PersonStatistics::combine,
PersonStatistics::toStatArray));
System.out.println(Arrays.toString(objects));
}
}
如果沒有第三方庫,您可以創建一個通用收集器,它將任意數量的指定收集器的結果組合到單個Object[]
數組中:
/**
* Returns a collector which combines the results of supplied collectors
* into the Object[] array.
*/
@SafeVarargs
public static <T> Collector<T, ?, Object[]> multiCollector(
Collector<T, ?, ?>... collectors) {
@SuppressWarnings("unchecked")
Collector<T, Object, Object>[] cs = (Collector<T, Object, Object>[]) collectors;
return Collector.<T, Object[], Object[]> of(
() -> Stream.of(cs).map(c -> c.supplier().get()).toArray(),
(acc, t) -> IntStream.range(0, acc.length).forEach(
idx -> cs[idx].accumulator().accept(acc[idx], t)),
(acc1, acc2) -> IntStream.range(0, acc1.length)
.mapToObj(idx -> cs[idx].combiner().apply(acc1[idx], acc2[idx])).toArray(),
acc -> IntStream.range(0, acc.length)
.mapToObj(idx -> cs[idx].finisher().apply(acc[idx])).toArray());
}
對於您的具體問題,您還需要一個filtering()
收集器(將在JDK-9中添加,請參閱JDK-8144675 ):
public static <T, A, R> Collector<T, A, R> filtering(
Predicate<? super T> filter, Collector<T, A, R> downstream) {
BiConsumer<A, T> accumulator = downstream.accumulator();
Set<Characteristics> characteristics = downstream.characteristics();
return Collector.of(downstream.supplier(), (acc, t) -> {
if(filter.test(t)) accumulator.accept(acc, t);
}, downstream.combiner(), downstream.finisher(),
characteristics.toArray(new Collector.Characteristics[characteristics.size()]));
}
現在您可以構建一個可以生成最終結果的收集器:
Collector<Person, ?, Object[]> collector =
multiCollector(
filtering(p -> p.getFirstName().equals("John"), counting()),
collectingAndThen(mapping(Person::getAge,
maxBy(Comparator.naturalOrder())), Optional::get),
collectingAndThen(mapping(Person::getHeight,
minBy(Comparator.naturalOrder())), Optional::get),
averagingDouble(Person::getWeight));
Object[] result = personsList.stream().collect(collector);
System.out.println(Arrays.toString(result));
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