Java8 新特性之流式数据处理

List<Integer> evens = new ArrayList<>();
for (final Integer num : nums) {
    if (num % 2 == 0) {
        evens.add(num);
    }
}

List<Integer> evens = nums.stream().filter(num -> num % 2 == 0).collect(Collectors.toList());

filter()

collect()

中间操作

终端操作

public class Student {
    /** 学号 */
    private long id;
    private String name;
    private int age;
    /** 年级 */
    private int grade;
    /** 专业 */
    private String major;
    /** 学校 */
    private String school;
    // 省略getter和setter
}
// 初始化
List<Student> students = new ArrayList<Student>() {
    {
        add(new Student(20160001, "孔明", 20, 1, "土木工程", "武汉大学"));
        add(new Student(20160002, "伯约", 21, 2, "信息安全", "武汉大学"));
        add(new Student(20160003, "玄德", 22, 3, "经济管理", "武汉大学"));
        add(new Student(20160004, "云长", 21, 2, "信息安全", "武汉大学"));
        add(new Student(20161001, "翼德", 21, 2, "机械与自动化", "华中科技大学"));
        add(new Student(20161002, "元直", 23, 4, "土木工程", "华中科技大学"));
        add(new Student(20161003, "奉孝", 23, 4, "计算机科学", "华中科技大学"));
        add(new Student(20162001, "仲谋", 22, 3, "土木工程", "浙江大学"));
        add(new Student(20162002, "鲁肃", 23, 4, "计算机科学", "浙江大学"));
        add(new Student(20163001, "丁奉", 24, 5, "土木工程", "南京大学"));
    }
};

Stream<T> filter(Predicate<? super T> predicate)

test(T t)

boolean

students

List<Student> whuStudents = students.stream()
                                    .filter(student -> "武汉大学".equals(student.getSchool()))
                                    .collect(Collectors.toList());

DISTINCT

Object.equals(Object)

List<Integer> evens = nums.stream()
                        .filter(num -> num % 2 == 0).distinct()
                        .collect(Collectors.toList());

LIMIT

土木工程

List<Student> civilStudents = students.stream()
                                    .filter(student -> "土木工程".equals(student.getMajor())).limit(2)
                                    .collect(Collectors.toList());

sorted

Comparable

sorted(Comparator<? super T> comparator)

List<Student> sortedCivilStudents = students.stream()
                                            .filter(student -> "土木工程".equals(student.getMajor())).sorted((s1, s2) -> s1.getAge() - s2.getAge())
                                            .limit(2)
                                            .collect(Collectors.toList());

List<Student> civilStudents = students.stream()
                                    .filter(student -> "土木工程".equals(student.getMajor()))
                                    .skip(2)
                                    .collect(Collectors.toList());

SELECT

List<String> names = students.stream()
                            .filter(student -> "计算机科学".equals(student.getMajor()))
                            .map(Student::getName).collect(Collectors.toList());

mapToDouble(ToDoubleFunction<? super T> mapper)

mapToInt(ToIntFunction<? super T> mapper)

mapToLong(ToLongFunction<? super T> mapper)

int totalAge = students.stream()
                    .filter(student -> "计算机科学".equals(student.getMajor()))
                    .mapToInt(Student::getAge).sum();

sum()

String[] strs = {"java8", "is", "easy", "to", "use"};

List<String[]> distinctStrs = Arrays.stream(strs)
                                .map(str -> str.split(""))  // 映射成为Stream<String[]>
                                .distinct()
                                .collect(Collectors.toList());

[j, a, v, a, 8]
[i, s]
[e, a, s, y]
[t, o]
[u, s, e]

List distinctStrs = Arrays.stream(strs)
                                .map(str -> str.split(""))  // 映射成为Stream<String[]>
                                .flatMap(Arrays::stream)  // 扁平化为Stream<String>
                                .distinct()
                                .collect(Collectors.toList());

Stream<String[]>

Steam<String>

flatMapToDouble(Function<? super T,? extends DoubleStream> mapper)

flatMapToInt(Function<? super T,? extends IntStream> mapper)

flatMapToLong(Function<? super T,? extends LongStream> mapper)

reduce

Stream<Student>

// 前面例子中的方法
int totalAge = students.stream()
                .filter(student -> "计算机科学".equals(student.getMajor()))
                .mapToInt(Student::getAge).sum();
// 归约操作
int totalAge = students.stream()
                .filter(student -> "计算机科学".equals(student.getMajor()))
                .map(Student::getAge)
                .reduce(0, (a, b) -> a + b);
// 进一步简化
int totalAge2 = students.stream()
                .filter(student -> "计算机科学".equals(student.getMajor()))
                .map(Student::getAge)
                .reduce(0, Integer::sum);
// 采用无初始值的重载版本，需要注意返回Optional
Optional<Integer> totalAge = students.stream()
                .filter(student -> "计算机科学".equals(student.getMajor()))
                .map(Student::getAge)
                .reduce(Integer::sum);  // 去掉初始值

toSet

toMap

java.util.stream.Collectors

Collectors.reducing()

long count = students.stream().collect(Collectors.counting());
// 进一步简化
long count = students.stream().count();

// 求最大年龄
Optional<Student> olderStudent = students.stream().collect(Collectors.maxBy((s1, s2) -> s1.getAge() - s2.getAge()));
// 进一步简化
Optional<Student> olderStudent2 = students.stream().collect(Collectors.maxBy(Comparator.comparing(Student::getAge)));
// 求最小年龄
Optional<Student> olderStudent3 = students.stream().collect(Collectors.minBy(Comparator.comparing(Student::getAge)));

summingLong

summingDouble

averagingLong

averagingDouble

summarizingLong

summarizingDouble

GROUP BY

Collectors.groupingBy

Function<? super T, ? extends K> classifier

Collector

Collector.counting

ollectors.partitioningBy()

java.util.stream.Collector

public interface Collector<T, A, R> {
    /**
     * A function that creates and returns a new mutable result container.
     *
     * @return a function which returns a new, mutable result container
     */
    Supplier<A> supplier();
    /**
     * A function that folds a value into a mutable result container.
     *
     * @return a function which folds a value into a mutable result container
     */
    BiConsumer<A, T> accumulator();
    /**
     * A function that accepts two partial results and merges them.  The
     * combiner function may fold state from one argument into the other and
     * return that, or may return a new result container.
     *
     * @return a function which combines two partial results into a combined
     * result
     */
    BinaryOperator<A> combiner();
    /**
     * Perform the final transformation from the intermediate accumulation type
     * {@code A} to the final result type {@code R}.
     *
     * <p>If the characteristic {@code IDENTITY_TRANSFORM} is
     * set, this function may be presumed to be an identity transform with an
     * unchecked cast from {@code A} to {@code R}.
     *
     * @return a function which transforms the intermediate result to the final
     * result
     */
    Function<A, R> finisher();
    /**
     * Returns a {@code Set} of {@code Collector.Characteristics} indicating
     * the characteristics of this Collector.  This set should be immutable.
     *
     * @return an immutable set of collector characteristics
     */
    Set<Characteristics> characteristics();
}