0 创建流
public void test1(){
List<String> list = new ArrayList<>();
Stream<String> stream = list.stream();
Integer[] nums = new Integer[];
Stream<Integer> stream1 = Arrays.stream(nums);
Stream<Integer> stream2 = Stream.of(,,,,,);
Stream<Integer> stream3 = Stream.iterate(, (x) -> x + ).limit();
stream3.forEach(System.out::println);
Stream<Double> stream4 = Stream.generate(Math::random).limit();
stream4.forEach(System.out::println);
}
1 合并多个流
//合并多个流
private void merge() {
List<String> list = new ArrayList<String>();
list.add("I am a boy");
list.add("I love the girl");
list.add("But the girl loves another girl"); List<String> result = list.stream() //得到流
.map(line->line.split(" ")) //将每个元素转成String[]
.flatMap(Arrays::stream) //将每个String[] 转成流 ，并合并为一个大流
.distinct() //对元素去重
.collect(Collectors.toList()); //终止流得到集合
result.forEach(System.out::println); //打印
}
2 是否匹配任一元素
//是否匹配任一元素
private void anyMatch() { List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
boolean isHas = persons.stream().anyMatch(e -> e.getAge() == );
System.out.println("isHas : "+isHas);
}
3 是否所有元素都匹配
//是否所有元素都匹配
private void allMatch() { List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
boolean bool = persons.stream().allMatch(e -> e.getAge() == );
System.out.println("isHas : "+bool);
}
4 是否所有元素都不匹配
//是否所有元素都不匹配
private void noneMatch() { List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
boolean bool = persons.stream().noneMatch(e -> e.getAge() == );
System.out.println("isHas : "+bool);
}
5 新增容器Optional ，可以避免空指针异常
private void orElse() {
Optional<Integer> optional2 = Optional.ofNullable(null);
System.out.println( optional2.orElse());
}
6 归并 reduce
//归并
private void testReduce() {
List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
//reduce 不设置初始值
Optional<Integer> optional = persons.stream().map(e -> e.getAge()).reduce((a,b) -> a + b);
System.out.println(optional.get());
//reduce 设置初始值
Integer ageTotal = persons.stream().map(e -> e.getAge()).reduce(,(a,b) -> a + b);
System.out.println(ageTotal);
}
7 map filter等操作
private void commonTest() {
List<Integer> numbers = Arrays.asList(, , , , , , );
//forEach 遍历
numbers.stream().forEach(System.out::println);
//map 映射每个元素到对应的结果
List<Integer> squaresList = numbers.stream().map(i -> i * i).distinct().collect(Collectors.toList());
squaresList.stream().forEach(System.out::println);
//filter 过滤
List<Integer> filetrList = numbers.stream().filter(x -> x>).collect(Collectors.toList());
filetrList.stream().forEach(System.out::println);
//limit 截取多少个
numbers.stream().limit().forEach(System.out::println);
//sorted 排序
numbers.stream().sorted().forEach(System.out::println);
//summaryStatistics 统计
IntSummaryStatistics statistics = numbers.stream().mapToInt(x -> x).summaryStatistics();
System.out.println(statistics.getMax());
System.out.println(statistics.getMin());
}
8 收集器
//使用收集器
private void collector() { List<Person> persons = Arrays.asList(new Person("zhangsan", ),
new Person("lisi", ),new Person("wangwu", ),
new Person("zhaoliu", ),new Person("qianqi", ));
//统计个数
Long count = persons.stream().collect(Collectors.counting());
System.out.println(count); //计算最大值
Optional<Person> optional = persons.stream().collect(Collectors.maxBy(Comparator.comparingInt(Person::getAge)));
System.out.println(optional.get().getName()); //求和
Integer sum = persons.stream().collect(Collectors.summingInt(Person::getAge));
System.out.println(sum); //求平均值
Double average = persons.stream().collect(Collectors.averagingInt(Person::getAge));
System.out.println(average); //统计 最大值 , 最小值 , 平均值 , 和 , 个数
IntSummaryStatistics summary = persons.stream().collect(Collectors.summarizingInt(Person::getAge));
System.out.println(summary.getMax()); //连接字符串
String names = persons.stream().map(x -> x.getName()).collect(Collectors.joining(" | "));
System.out.println(names); //一般归约
Integer sumAge = persons.stream().collect(Collectors.reducing(, Person::getAge, (x , y) -> x + y ));
System.out.println(sumAge); //分组
Map<String, List<Person>> map = persons.stream().collect(Collectors.groupingBy((person) -> {
if(person.getAge() < ) {
return "small";
}else if(person.getAge() < ) {
return "middle";
}else {
return "larger";
}
}));
map.get("middle").forEach( x -> System.out.println(x.getName())); //分组并统计
Map<String, Long> map2 = persons.stream().collect(Collectors.groupingBy((person) -> {
if(person.getAge() < ) {
return "small";
}else if(person.getAge() < ) {
return "middle";
}else {
return "larger";
}
},
Collectors.counting()
));
map2.values().stream().forEach(System.out::println); //分区是分组的特殊情况
Map<Boolean, List<Person>> map3 = persons.stream().collect(Collectors.partitioningBy(x -> x.getAge() > ));
map3.get(false).forEach( x -> System.out.println(x.getName())); }
9 map 的使用，转换类型
//map 可以将一个类型的集合转变为其他类型的集合
public void testMap() {
List<Integer> foo = Arrays.asList(,,,,);
List<Score> scores = foo.stream().map(e -> new Score(e))
.collect(Collectors.toList());
}
class Score {
int value ;
Score(int value){
this.value = value;
}
}
10 对流的操作
skip
filter 从流中排除某些元素。
map
flatMap
limit 截断流，使其元素不超过给定数量。
sort 排序
distinct 筛选，通过流所生成元素的 hashCode() 和 equals() 去除重复元素
allMatch——检查是否匹配所有元素
anyMatch——检查是否至少匹配一个元素
noneMatch——都不匹配或者stream为空时候，返回true
findFirst——返回第一个元素
findAny——返回当前流中的任意元素
count——返回流中元素的总个数
max——返回流中最大值
min——返回流中最小值