java Stream API Java8新特性之Stream API详解
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一、前言
StreamAPI在Java8版本中使用，关注的是对数据的筛选、查找、存储等
它可以做的事情有：过滤、排序、映射、归约
二、使用流程
Stream实例化中间操作（过滤、排序、映射、规约）终止操作（匹配查找、归约、收集）
三、案例演示
public class EmployeeData {
public static List<Employee> getEmployees(){
List<Employee> list = new ArrayList<>();
list.add(new Employee(1001, "马化腾", 34, 6000.38));
list.add(new Employee(1002, "马云", 12, 9876.12));
list.add(new Employee(1003, "刘强东", 33, 3000.82));
list.add(new Employee(1004, "雷军", 26, 7657.37));
list.add(new Employee(1005, "李彦宏", 65, 5555.32));
list.add(new Employee(1006, "比尔盖茨", 42, 9500.43));
list.add(new Employee(1007, "任正非", 26, 4333.32));
list.add(new Employee(1008, "扎克伯格", 35, 2500.32));
return list;
}
}
package JDK_8;
import org.junit.Test;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class StreamAPI {
// 方式1：使用集合创建Stream
@Test
public void test1() {
List<Employee> list = EmployeeData.getEmployees();
// 返回一个顺序流
Stream<Employee> stream = list.stream();
// 返回一个并行流
Stream<Employee> employeeStream = list.parallelStream();
}
// 方式2：使用数组创建流对象
@Test
public void test2() {
Employee e1 = new Employee(1003, "WZY", 26, 3000.69);
Employee e2 = new Employee(1007, "王紫玉", 25, 8888);
Employee[] list2 = new Employee[]{e1, e2};
Stream<Employee> stream = Arrays.stream(list2);
stream.forEach(System.out::println);
}
// 通过Stream.of 创建
@Test
public void test3() {
Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5, 6);
stream.forEach(System.out::println);
}
// Stream.filter进行过滤
@Test
public void test4() {
List<Employee> list = EmployeeData.getEmployees();
list.stream().filter(employee -> employee.getAge() > 25).forEach(System.out::println);
}
// 使用StreamAPI筛选
@Test
public void test5() {
List<Employee> list = EmployeeData.getEmployees();
list.add(new Employee(1004, "wzy", 25, 18888));
list.add(new Employee(1004, "wzy", 25, 18888));
list.add(new Employee(1004, "wzy", 25, 18888));
list.add(new Employee(1004, "wzy", 25, 18888));
list.stream().forEach(System.out::println);
System.out.println();
list.stream().distinct().forEach(System.out::println);
}
@Test
public void test6() {
List<String> list = Arrays.asList("a", "b", "c");
// map(Function<? super T,? extends R> mapper)
// 返回由给定函数应用于此流的元素的结果组成的流。
list.stream().map(s -> s.toUpperCase()).forEach(System.out::println);
}
// 使用StreamAPI筛选：工资大于6000的员工
@Test
public void test7() {
List<Employee> list = EmployeeData.getEmployees();
list.stream().filter(s -> s.getSalary() > 6000).forEach(System.out::println);
}
// StreamAPI映射：map,接收参数，将参数转换为其他形式的信息；
@Test
public void test8() {
List<Employee> list = EmployeeData.getEmployees();
list.stream().map(e -> e.getAge()).forEach(System.out::println);
}
// StreamAPI排序,sorted帮助实现Comparable接口进行对象比较
@Test
public void test9() {
List<Employee> list = EmployeeData.getEmployees();
list.stream().sorted((e1, e2) -> {
int ageValue = Integer.compare(e1.getAge(), e2.getAge());
if (ageValue != 0) {
return ageValue;
} else {
return Double.compare(e1.getSalary(), e2.getSalary());
}
}).forEach(System.out::println);
}
// StreamAPI匹配:所有元素是否满足下列条件
@Test
public void test10() {
List<Employee> list = EmployeeData.getEmployees();
boolean allMatch = list.stream().allMatch(e -> e.getSalary() > 5000);
System.out.println(allMatch);
}
// StreamAPI查找：返回第一个元素
@Test
public void test11() {
List<Employee> list = EmployeeData.getEmployees();
Optional<Employee> first = list.stream().findFirst();
System.out.println(first);
}
// StreamAPI查找：返回薪水数最大的员工
@Test
public void test12() {
List<Employee> list = EmployeeData.getEmployees();
Stream<Double> salaryStream = list.stream().map(e -> e.getSalary());
Optional<Double> max = salaryStream.max(Double::compare);
System.out.println(max);
}
// StreamAPI查找：薪水最小的员工对像
@Test
public void test13() {
List<Employee> list = EmployeeData.getEmployees();
Optional<Employee> min = list.stream().min((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()));
System.out.println(min);
}
// StreamAPI:归约
@Test
public void test14() {
List<Employee> list = EmployeeData.getEmployees();
Stream<Double> salaryStream = list.stream().map(e -> e.getSalary());
Optional<Double> sum = salaryStream.reduce((s1, s2) -> s1 + s2);
System.out.println(sum.get());
}
// StreamAPI：收集
@Test
public void test15(){
List<Employee> list = EmployeeData.getEmployees();
List<Employee> employeeList = list.stream().filter(e -> e.getSalary() > 5000).collect(Collectors.toList());
for (Employee employee : employeeList) {
System.out.println(employee);
}
}
}
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