Explanation
An array is a collection of data structures and we often used before jdk1.8, handling collections and arrays is not very convenient. But by the JDK1.8 after using Stream processing set will make the code more clear and concise. As a developer, in fact, is very necessary to learn the use of the new features, even said that the project does not use. Although JDK1.8 has been released a few years, but still found that many people do not use the new API JDK1.8 brought us. For example, in an article written before still use SimpleDateFormat? Java8 are released N years, it LocalDateTime turn , explained the new API on the JDK1.8 time, you can also take a look.
Stream class common api
Here are the more common api, you want to see more specific API, you can refer to jdk1.8 official documents, where a more detailed description.
initialization
Stream to initialize a conventional two approaches:
Method One: List of stream () method
@Test
public void listInit(){
List<String> list = new ArrayList<>();
list.add("apple");
list.add("banana");
Stream<String> stream = list.stream();
}
复制代码Method two: Stream.of () method
@Test
public void streamOfInit(){
// 第一种
Stream<String> stream = Stream.of("a", "b", "c", "d");
// 第二种
String [] strings = {"a","b","c"};
Stream<String> stream1 = Stream.of(strings);
}
复制代码filter filter
Selection of the required elements, such as: the example below to filter out the string dog
/**
* 过滤
*/
@Test
public void filter() {
String[] strings = {"apple", "banana", "cat", "dog"};
List<String> list = Stream.of(strings).filter(e -> !StringUtils.equals(e, "dog")).collect(Collectors.toList());
System.out.println(JSON.toJSONString(list));
}
复制代码Output:
["apple","banana","cat"]
复制代码map to generate a new set of
There are two classes
@Data
@AllArgsConstructor
public static class Person {
private Integer id;
private String name;
}
@Data
public static class Man {
private Integer id;
private String name;
}
复制代码To be converted into a List List
@Test
public void map2() {
List<Person> list = new ArrayList<>();
list.add(new Person(1, "happyjava1"));
list.add(new Person(2, "happyjava2"));
List<Man> manList = list.stream().map(e -> {
Man man = new Man();
man.setId(e.getId());
man.setName(e.getName());
return man;
}).collect(Collectors.toList());
System.out.println(JSON.toJSONString(manList));
}
复制代码Output
[{"id":1,"name":"happyjava1"},{"id":2,"name":"happyjava2"}]
复制代码Collectors.toMap spliced into the list of required map
This is often used when the query list data to the database
@Test
public void testCollectToMap() {
List<Person> list = new ArrayList<>();
list.add(new Person(1, "happyjava1"));
list.add(new Person(2, "happyjava2"));
Map<Integer, Person> personMap = list.stream().collect(Collectors.toMap(Person::getId, e -> e));
System.out.println(JSON.toJSONString(personMap));
Map<Integer, String> nameMap = list.stream().collect(Collectors.toMap(Person::getId, Person::getName));
System.out.println(JSON.toJSONString(nameMap));
}
复制代码Output
{1:{"id":1,"name":"happyjava1"},2:{"id":2,"name":"happyjava2"}}
{1:"happyjava1",2:"happyjava2"}
复制代码sort sort
sort method without parameters to support natural ordering of basic data types, type of packaging, String, etc. type
@Test
public void sort() {
// 自然顺序排序 基本数据类型 字符串
List<String> collect = Stream.of("1", "5", "2", "9", "3", "4").sorted().collect(Collectors.toList());
System.out.println(JSON.toJSONString(collect));
List<Person> list = new ArrayList<>();
list.add(new Person(1, "happyjava1"));
list.add(new Person(2, "happyjava2"));
// 自然顺序排序
List<Person> personList = list.stream().sorted((o1, o2) -> {
if(o1.getId() > o2.getId()){
return 1;
} else if(o1.getId().equals(o2.getId())){
return 0;
} else {
return -1;
}
}).collect(Collectors.toList());
System.out.println(JSON.toJSONString(personList));
}
复制代码Output
["1","2","3","4","5","9"]
[{"id":1,"name":"happyjava1"},{"id":2,"name":"happyjava2"}]
复制代码skip and limit
This corresponds to the MySQL limit n, m, and MongoDB the skip, limit usage is consistent. It means that across a few rows, meaning to take a few lines of records.
@Test
public void skipAndLimit(){
List<String> list = Stream.of("1", "5", "2", "9", "3", "4").skip(2).limit(3).collect(Collectors.toList());
System.out.println(JSON.toJSONString(list));
}
复制代码Output
["2","9","3"]
复制代码to sum up
Stream commonly used operations are listed here, Stream of course there are many other uses, also can not list them here, you need the reader to learn.
Reproduced in: https: //juejin.im/post/5d07a0406fb9a07f014ef6e6