Stream API
Generate a brand new stream, which has nothing to do with the data source (the data source is not affected)
a) Create stream method:
1）collection 例如：new ArrayList<>().stream()
2) Arrays.stream()
3) Through the static method of () in the Stream class
4) Create an infinite stream
//创建无线流 — 迭代方式
@Test
public void test1() {
Stream<Integer> stream = Stream.iterate(0,(x) -> x+2);
stream.limit(10).forEach(System.out::println);
}
//创建无线流 ——生成
@Test
public void test2() {
Stream.generate(() ->Math.random()).limit(5).forEach(System.out::println);
}
b) Intermediate operation
1) Screening and slicing
filter-Receive Lambda and exclude certain elements from the stream.
@Test
public void test3() {
//stream有内部迭代
//中间操作
Stream<Employee> stream = emplpoyees.stream().filter((e) ->{
//filter就是中间操作
System.out.println("开始执行判断");
return e.getAge()>35;
});
/**
终止操作：一次性执行全部内容，就是“惰性求值”；
例如：在没有进行终止操作的情况下所有的中间操作都不会执行，只有当执行了终止操作，中间操作才会全部一次性执行！！
**/
stream.forEach(System.out::println);//打印就是终止操作！
}
limit——Truncate the flow so that its elements do not exceed the given number
@Test
public void test3() {
//中间操作
Stream<Employee> stream = emplpoyees.stream().filter((e) ->{
System.out.println("开始执行判断");
return e.getAge()>35;
}).limit(1).forEach(System.out::println);//限制为1条
}
skip(n)-Skip elements. For example, there are five elements in the set. Skip(2) skips the first two and leaves the last three; if there are less than n elements in the stream, an empty stream is returned. Complementary to limit(n)
distinct——Filtering (de-duplication), removing duplicate elements through the elements hashcode() and equals() generated by the stream
2) Map
map——Receive lambda, transform elements into other forms or extract information. Receive a function as a parameter, the function will be applied to each element, and map it into a new element.
flatMap——Receive a function as a parameter, replace each value in the stream with another stream, and then connect all the streams into one stream.
//映射
@Test
public void test4() {
emplpoyees.stream().map(Employee::getName).forEach(System.out::println);
}
3) Sort
sorted(): Natural sorting (Comparable)
@Test
public void test5() {
List<String> list1 = Arrays.asList("bbb","ccc","aaa","ddd");
list1.stream().sorted().forEach(System.out::println);
}
sorted (Comparator com): custom sorting
c) Terminate operation
The following are various methods commonly used to terminate operations:
allMatch
anyMatch
noneMatch
findFirst
findAny
count
max
min
d) Statute
reduce(T identity,BinaryOperator) / reduce(BinaryOperator) can combine the elements in the stream repeatedly to get a value.
@Test
public void test1() {
List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
// T reduce(T identity, BinaryOperator<T> accumulator);
Integer sum = list.stream().reduce(0, (x,y) -> x+y);
System.out.println(sum);//55
//需求：获取员工类中的所有工资的总和（map-reduce规约实现）
Optional<Double> op = emplpoyees.stream().map(Employee::getSarlary).reduce(Double::sum);
System.out.println(op.get());
}
e) Collection
collect-convert the stream to other forms. Receive an implementation of the Collector interface, which is used to summarize the elements in the Stream
//需求：将员工类中的所有名字收集到集合list中
public void test2() {
/**
Employee::getName（方法引用的第三种方式————类：：实例方法名）；实例方法就是非静态方法
Collectors这是一个集合收集的工具类，里边包含了许多静态方法
*/
List<String> list = emplpoyees.stream().map(Employee::getName).collect(Collectors.toList());
list.forEach(System.out::println);
}
//set实现上一个需求（set可以将员工集合中的重复名字的员工去重）
@Test
public void test3() {
Set<String> set = emplpoyees.stream().map(Employee::getName).collect(Collectors.toSet());
set.forEach(System.out::println);
}
//
@Test
public void test4() {
Long count = emplpoyees.stream().collect(Collectors.counting());
System.out.println(count);
Double avg = emplpoyees.stream().collect(Collectors.averagingDouble(Employee::getSarlary));
System.out.println(avg);
Double sum = emplpoyees.stream().collect(Collectors.summingDouble(Employee::getSarlary));
System.out.println(sum);
}
In addition to the above collection methods, there are groupingBy (multi-level grouping), partitioningBy (partitioning), joining (joining) and other methods. API documents can be queried according to requirements.