Java8新特性之Stream API详解

Java8新特性之Stream API详解

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 getEmployees(){

List 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 list = EmployeeData.getEmployees();

// 返回一个顺序流

Stream stream = list.stream();

// 返回一个并行流

Stream 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 stream = Arrays.stream(list2);

stream.forEach(System.out::println);

}

// 通过Stream.of 创建

@Test

public void test3() {

Stream stream = Stream.of(1OiiKmQbTX, 2, 3, 4, 5, 6);

stream.forEach(System.out::println);

}

// Stream.filter进行过滤

@Test

public void test4() {

List list = EmployeeData.getEmployees();

list.stream().filter(employee -> employee.getAge() > 25).forEach(System.out::println);

}

// 使用StreamAPI筛选

@Test

public void test5() {

List 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 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() {

OiiKmQbTXList 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 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 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 list = EmployeeData.getEmployees();

boolean allMatch = list.stream().allMatch(http://e -> e.getSalary() > 5000);

System.out.println(allMatch);

}

// StreamAPI查找：返回第一个元素

@Test

public void test11() {

List list = EmployeeData.getEmployees();

Optional 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 list = EmployeeData.getEmployees();

Stream salaryStream = list.stream().map(e -> e.getSalary());

Optional 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 list = EmployeeData.getEmployees();

Optional min = list.stream().min((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()));

System.out.println(min);

}

// StreamAPI:归约

@Test

public void test14() {

List list = EmployeeData.getEmployees();

Stream salaryStream = list.stream().map(e -> e.getSalary());

Optional sum = salaryStream.reduce((s1, s2) -> s1 + s2);

System.out.println(sum.get());

}

// StreamAPI：收集

@Test

public void test15(){

List list = EmployeeData.getEmployees();

List employeeList = list.stream().filter(e -> e.getSalary() > 5000).collect(Collectors.toList());

for (Employee employee : employeeList) {

System.out.println(employee);

}

}

}

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