Stream是数据渠道,用于操作集合、数组等生成的元素序列。
Stream
Stream操作的三个步骤:
创建Stream
中间操作
终止操作
collection
stream()
parallelStream()
@Test void test11(){ List<String> list = new ArrayList<>(); Stream<String> stringStream = list.stream(); }
Arrays
@Test void test11(){ Person[] person = new Person[10]; Arrays.stream(person); }
of()
@Test void test11(){ Stream<String> stream = Stream.of("a", "b", "c"); }
/** * 迭代 */ @Test void test11(){ Stream<Integer> integerStream = Stream.iterate(0, x -> x + 2); } /** * 生成 */ @Test void test11(){ Stream.generate(() -> Math.random()); }
中间操作不会执行任何操作,只有终止操作才会一次性输出全部值,即“惰性求值”。
filter
lamdba
@Test void test12(){ List<Person> personList = Arrays.asList( new Person("Java旅途",18), new Person("Java旅途",20) ); // 中间操作 Stream<Person> personStream = personList.stream() .filter(e -> e.getAge() > 18); // 终止操作 personStream.forEach(System.out::println); }
limit
@Test void test12(){ List<Person> personList = Arrays.asList( new Person("Java旅途",18), new Person("Java旅途",20) ); personList.stream() .limit(1) .forEach(System.out::println); }
skip(n)
imit(n)
@Test void test12(){ List<Person> personList = Arrays.asList( new Person("Java旅途",18), new Person("Java旅途",20) ); personList.stream() .skip(1) .forEach(System.out::println); }
distinct
hashCode()
equals()
@Test void test12(){ List<Person> personList = Arrays.asList( new Person("Java旅途",18), new Person("Java旅途",20), new Person("Java旅途",20) ); personList.stream() .distinct() .forEach(System.out::println); }
注意:使用distinct的时候需要重写实体的hashCode()和equals()方法。
map
/** * 获取personList的所有name */ @Test void test13(){ List<Person> personList = Arrays.asList( new Person("Java旅途",18), new Person("Java旅途",20), new Person("Java旅途",20) ); personList.stream() .map(Person::getName) .forEach(System.out::println); }
flatMap
// 将字符串转换为字符,并将字符放进list返回 static Stream<Character> filterCharcter(String string){ List<Character> list = new ArrayList<>(); for (Character ch : string.toCharArray()){ list.add(ch); } return list.stream(); } @Test void test13(){ List<String> list = Arrays.asList("aaa","bbb","ccc"); list.stream() .flatMap(LamdbaApplicationTests::filterCharcter) .forEach(System.out::println); }
sorted()
sorted(comparator com)
/** * 定制排序,e1和e2按age排序,age一样按name排 */ @Test void test14(){ List<Person> personList = Arrays.asList( new Person("Java旅途",18), new Person("Java旅途",20) ); personList.stream() .sorted((e1,e2) -> { if(e1.getAge() == e2.getAge()){ return e1.getName().compareTo(e2.getName()); }else{ return e1.getAge()+"".compareTo(e2.getAge()+""); } }).forEach(System.out::println); }
完善一下Person类
Person
@Data public class Person implements Serializable { private static final long serialVersionUID = -7008474395345458049L; private String name; private int age; private Status status; public Person() { } public Person(String name, int age) { this.name = name; this.age = age; } public Person(String name, int age, Status status) { this.name = name; this.age = age; this.status = status; } public enum Status { FRER, BUSY; } }
allMatch
/** * 是否所有元素都是Fire状态,是返回true */ @Test void test15(){ List<Person> personList1 = Arrays.asList( new Person("Java旅途",18, Person.Status.FRER), new Person("Java旅途",20, Person.Status.BUSY) ); boolean b = personList1.stream() .allMatch(e -> e.getStatus().equals(Person.Status.FRER)); System.out.println(b); }
anyMatch
@Test void test15(){ List<Person> personList1 = Arrays.asList( new Person("Java旅途",18, Person.Status.FRER), new Person("Java旅途",20, Person.Status.BUSY) ); boolean b = personList1.stream() .anyMatch(e -> e.getStatus().equals(Person.Status.FRER)); System.out.println(b); }
noneMatch
@Test void test15(){ List<Person> personList1 = Arrays.asList( new Person("Java旅途",18, Person.Status.FRER), new Person("Java旅途",20, Person.Status.BUSY) ); boolean b = personList1.stream() .noneMatch(e -> e.getStatus().equals(Person.Status.FRER)); System.out.println(b); }
findFirst
@Test void test15(){ List<Person> personList1 = Arrays.asList( new Person("Java旅途",18, Person.Status.FRER), new Person("Java旅途",20, Person.Status.BUSY) ); Optional b = personList1.stream() .findFirst(); System.out.println(b.get()); }
findAny
@Test void test15(){ List<Person> personList1 = Arrays.asList( new Person("Java旅途",18, Person.Status.FRER), new Person("Java旅途",20, Person.Status.BUSY) ); Optional b = personList1.stream() .findAny(); System.out.println(b.get()); }
count
max
min
reduce(T identity, BinaryOperator)
reduce(BinaryOperator)
/** * reduce 第一个参数是起始值 */ @Test void test16(){ List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10); Integer sum = list.stream() .reduce(0,(x,y) -> x+y); System.out.println(sum); Optional<Integer> reduce = list.stream() .reduce((x, y) -> x + y); System.out.println(reduce.get()); }
collect
Collector
/** * 取出名字放在一个list中 */ @Test void test16(){ List<Person> personList1 = Arrays.asList( new Person("Java旅途",18, Person.Status.FRER), new Person("Java旅途",20, Person.Status.BUSY) ); List<String> collect = personList1.stream() .map(Person::getName) .collect(Collectors.toList()); collect.forEach(System.out::println); }
Optional类是一个容器类,代表一个值存在或不存在,原来用null表示一个值不存在,现在Optional可以更好的表达这个概念。并且可以避免空指针异常。
Optional.of(T t) ——创建一个Optional实例
Optional.empty()——创建一个空的optional实例
Optional.ofNullable(T t)——若t不为null,创建optional实例,否则创建空实例
isPresent()——判断是否包含值
orElse(T t)——如果调用对象包含值,返回该值,否则返回 t
orElseGet(Supplier s)——如果调用对象包含值,返回该值,否则返回 s 获取的值
map(Function f)——如果有值对其处理,并返回处理后的Optional,否则返回Optional.empty()
flatMap(Function mapper)——与map类似,要求返回值必须是Optional
原文链接:https://www.cnblogs.com/zhixie/p/13320744.html