我将从Java 8中的Stream API开始.
这是我使用的Person对象:
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;
}
}
这是我的代码,它初始化一个对象列表Person,它获取由特定名字过滤的对象数,最大年龄和最小高度,平均权重,最后创建一个包含这些值的对象数组:
ListpersonsList = new ArrayList (); 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));
是否可以单独调用该stream()方法并直接返回对象数组？
Object[] result = personsList.stream()...count()...max()...min()...average()
我之前问过非常类似的问题:Java 8 Streams:如何调用一次Collection.stream()方法并检索几个聚合值的数组,但这次我不能使用该summaryStatistics()方法,因为我使用不同的字段(年龄,身高,体重)检索聚合值.
编辑2016-01-07
我测试了TriCore和的解决方案Tagir Valeev,并计算了每个解决方案的运行时间.
似乎TriCore解决方案比效率更高Tagir Valeev.
Tagir Valeev与我的解决方案(使用多个Streams)相比,解决方案似乎没有节省太多时间.
这是我的测试类:
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 personsList = new ArrayList();
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 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 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 firstNameFilter;
public PersonStatistics(Predicate 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 personsList) {
// @formatter:off
Collector 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 Collector multiCollector(Collector... collectors) {
@SuppressWarnings("unchecked")
Collector[] cs = (Collector[]) collectors;
// @formatter:off
return Collector. 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 Collector filtering(Predicate filter, Collector downstream) {
BiConsumer accumulator = downstream.accumulator();
Set 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);
}
}
Lukas Eder.. 9
使用标准JDK 8 API解决这个问题有点棘手,它没有提供很多组合Collector类型的方法.如果您愿意使用像jOOλ这样的第三方库,您可以写:
Tuple4, Optional, Optional> 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);
以上产量:
(2, Optional[35], Optional[1.8], Optional[75.0])
注意,它使用的是新Agg.filter()方法,类似于JDK 9 Collectors.filtering()方法,其工作方式如下:
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