I'm starting with the Stream API in Java 8.
Here is my Person object I use:
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;
}
}
Here is my code which initializes a list of objects Person and which gets the number of objects filtered by a specific firstname, the maximum age and the minimum height, the weight average, and finally create an array of objects containing these values:
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));
Is it possible to do a single call to the stream() method and to return the array of objects directly ?
Object[] result = personsList.stream()...count()...max()...min()...average()
I asked very similar question previously: Java 8 Streams: How to call once the Collection.stream() method and retrieve an array of several aggregate values but this time I cannot use the summaryStatistics() method because I use different fields (age, height, weight) to retrieve the aggregate values.
EDIT 2016-01-07
I tested the solutions of TriCore and Tagir Valeev, and I computed the running time for each solution.
It seems that the TriCore solution is more efficient than Tagir Valeev.
Tagir Valeev's solution seems not save much time compared to my solution (using multiple Streams).
Here is my test class:
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);
}
}
Here is the collector
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};
}
}
You can use this collector as follows
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));
}
}
Lukas Eder
This is a bit tricky to solve with standard JDK 8 API, which doesn't offer many ways to compose Collector types. If you're willing to use a third party library like jOOλ, you could write:
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);
The above yields:
(2, Optional[35], Optional[1.8], Optional[75.0])
Note, it's using the new Agg.filter() method, which is similar to the JDK 9 Collectors.filtering() method and works like this:
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()
);
}
How does collect(collector1, collector2, ...) work?
If you don't want to use the above third-party library, you can write your own Collector combining utility. An example that combines two collectors into a Tuple2 collector:
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)
)
);
}
Disclaimer: I work for the company behind jOOλ.
Without third-party libraries you may create a universal collector which combines the results of any number of specified collectors into single Object[] array:
/**
* 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());
}
For your concrete problem you'll also need a 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()]));
}
Now you can build a collector which will generate the final result:
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));
来源：https://stackoverflow.com/questions/34621835/java-8-streams-how-to-call-once-the-collection-stream-method-and-retrieve-an