Java 8 stream流:
直接上代码:
一、新建User类准备数据
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.math.BigDecimal;
/**
* @author super zheng
* @date 2019/8/4 18:06
* @Description: 这里使用 lombok
*/
@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
public class User {
// 姓名
private String name;
// 年龄
private Integer age;
// 性别
private Integer sex;
// 所在省市
private String address;
// 钱
private BigDecimal money;
}
二、新建测试类,运用 stream方法
package com.aossci.microservices;
import com.aossci.microservices.lamdba.User;
import org.apache.commons.lang.StringUtils;
import org.junit.Test;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.toList;
/**
* @author super zheng
* @date 2019/8/4 18:08
* @Description:
*/
public class LamdbaTest {
List<User> list= Arrays.asList(
new User("钢铁侠",40,0,"华盛顿",new BigDecimal(1)),
new User("钢铁侠",40,0,"华盛顿",new BigDecimal(1)),
new User("钢铁侠",40,0,"华盛顿",new BigDecimal(1)),
new User("蜘蛛侠",20,0,"华盛顿",new BigDecimal(2)),
new User("赵丽颖",30,1,"湖北武汉市",new BigDecimal(3)),
new User("詹姆斯",35,0,"洛杉矶",new BigDecimal(4)),
new User("李世民",60,0,"山西省太原市",new BigDecimal(5)),
new User("蔡广坤",20,1,"陕西西安市",new BigDecimal(6)),
new User("葫芦娃的爷爷",70,0,"山西省太原市",new BigDecimal(7))
);
@Test
public void testFilter(){
// 测试过滤条件
List<User> filterList = list.stream().filter(user -> user.getAge() >= 40)
.collect(toList());
filterList.forEach(System.out::println);
}
@Test
public void testDistinct(){
// 测试去重
List<User> collect = list.stream().filter(user -> user.getAge() >= 40)
.distinct().collect(toList());
collect.forEach(System.out::println);
}
@Test
public void testSort(){
// 默认为升序
List<User> collect = list.stream().distinct().sorted(Comparator.comparingInt(User::getAge))
.collect(toList());
collect.forEach(System.out::println);
System.out.println("--------------------------");
// 降序排列
List<User> collect2 = list.stream().distinct().sorted(Comparator.comparingInt(User::getAge)
.reversed()).collect(toList());
collect2.forEach(System.out::println);
}
@Test
public void testLimit(){
// 默认升序 limit找出前面几个
List<User> collect = list.stream().distinct().sorted(Comparator.comparingInt(User::getAge))
.limit(1).collect(toList());
collect.forEach(System.out::println);
}
@Test
public void testSkip(){
// skip的意思是跳过,也就是去除前n个元素
Set<User> collect = list.stream().distinct().sorted(Comparator.comparingInt(User::getAge))
.skip(3).collect(Collectors.toSet());
collect.forEach(System.out::println);
}
@Test
public void testMap(){
// map(T->R)
// map是将T类型的数据转为R类型的数据,比如我们想要设置一个新的list,存储用户所有的城市信息。
// .map(User::getAddress) 相当于 .map(user -> user.getAddress)
// 此处 map 将 List<User> 转换成 List<String>
List<String> collect = list.stream().map(User::getAddress).distinct().collect(toList());
collect.forEach(System.out::println);
}
@Test
public void testFlatMap(){
// flatMap(T -> Stream<R>)
// 将流中的每一个元素 T 映射为一个流,再把每一个流连接成为一个流
List<String> flatList = new ArrayList<>();
flatList.add("唱,跳");
flatList.add("rape,篮球,music");
flatList.add(null);
//这里原集合中的数据由逗号分割,使用split进行拆分后,得到的是Stream<String[]>,
// 字符串数组组成的流,要使用flatMap的
// Arrays::stream
//将Stream<String[]>转为Stream<String>,
flatList = flatList.stream().filter(s-> StringUtils.isNotEmpty(s)).map(s -> s.split(",")).flatMap(Arrays::stream).collect(toList());
flatList.forEach(System.out::println);
}
@Test
public void testArrays(){
List<String> flatList = new ArrayList<>();
flatList.add("唱,跳");
flatList.add("rape,篮球,music");
List<String> strings = new ArrayList<>();
for (String s : flatList) {
strings.addAll(Arrays.asList(s.split(",")));
}
strings.forEach(System.out::println);
}
@Test
public void testAllMatch(){
//allMatch(T->boolean)
//
//检测是否全部满足参数行为,假如这些用户是网吧上网的用户名单,那就需要检查是不是每个人都年满18周岁了。
boolean b = list.stream().allMatch(user -> user.getAge() >= 18);
// true
System.out.println(b);
}
@Test
public void testAnyMatch(){
// anyMatch(T->boolean)
//
//检测是否有任意元素满足给定的条件,比如,想知道同学名单里是否有女生。
boolean b = list.stream().anyMatch(user -> user.getSex()==0);
System.out.println(b);
}
@Test
public void testNoneMatch(){
//noneMatch(T -> boolean)
//
//流中是否有元素匹配给定的 T -> boolean 条件。
boolean isLSJ = list.stream().noneMatch(user -> user.getAddress().contains("山西"));
System.out.println(isLSJ);
}
@Test
public void testFindFirst(){
Optional<User> fristUser = list.stream().findFirst();
fristUser.ifPresent(System.out::println);
}
@Test
public void testFindAny(){
//findAny():找到任意一个元素
Optional<User> any = list.stream().findAny();
any.ifPresent(System.out::println);
//这里我们发现findAny返回的也总是第一个元素,
// 那么为什么还要进行区分呢?因为在并行流 parallelStream() 中找到的确实是任意一个元素。
Optional<User> any1 = list.parallelStream().findAny();
any1.ifPresent(System.out::println);
}
@Test
public void testCount(){
long count = list.stream().collect(Collectors.counting());
long count1 = list.stream().filter(s->s.getAge()>30).count();
System.out.println(count);
System.out.println(count1);
}
@Test
public void testMax(){
// 求最大年龄
Optional<User> max = list.stream().max(Comparator.comparing(User::getAge));
// 求最小年龄
Optional<User> min = list.stream().min(Comparator.comparing(User::getAge));
max.ifPresent(System.out::println);
min.ifPresent(System.out::println);
}
@Test
public void testSum(){
List<Integer> list2 = new ArrayList<>();
list2.add(1);
list2.add(2);
list2.add(3);
// 将list中数据全部累加起来
Integer reduce = list2.stream().reduce(0, Integer::sum);
System.out.println(reduce);
}
@Test
public void testSums(){
// 求年龄总和
int totalAges = list.stream().collect(Collectors.summingInt(User::getAge));
int totalAge = list.stream().mapToInt(User::getAge).sum();
System.out.println(totalAge);
}
@Test
public void testSumPuls(){
List<String> list2 = new ArrayList<>();
list2.add("1");
list2.add("2");
list2.add("3");
list2.add(null);
// import org.apache.commons.lang.StringUtils;
// .filter(StringUtils::isNumeric) 等价于 .filter(s->StringUtils.isNumeric(s))
IntSummaryStatistics collect = list2.stream().filter(StringUtils::isNumeric)
.collect(Collectors.summarizingInt(Integer::valueOf));
long sum = collect.getSum();
double average = collect.getAverage();
long count = collect.getCount();
int max = collect.getMax();
int min = collect.getMin();
System.out.println(sum);
}
@Test
public void testSumBigDecimal(){
//BigDecimal来记录金钱,假设想得到BigDecimal的总和
BigDecimal reduce = list.stream().map(User::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println(reduce.toString());
}
@Test
public void testAverage(){
// 求平均年龄
Double collect = list.stream().distinct().collect(Collectors.averagingDouble(User::getAge));
System.out.println(collect);
}
@Test
public void testJoins(){
// 给每个指定的属性添加 “,”
String collect = list.stream().map(User::getName).distinct().collect(Collectors.joining(","));
System.out.println(collect);
}
@Test
public void testGroupingBy(){
Map<String, List<User>> cityMap = list.stream()
.collect(Collectors.groupingBy(User::getAddress));
for (Map.Entry<String, List<User>> next : cityMap.entrySet()) {
System.out.println("--------------------------------");
System.out.println(next.getKey());
System.out.println(next.getValue());
System.out.println("--------------------------------");
}
}
@Test
public void testTwoGroupingBy(){
Map<String, Map<Integer, List<User>>> collect = list.stream().collect(
Collectors.groupingBy(User::getAddress, // 一级分组,按所在地区
Collectors.groupingBy(User::getSex)));// 二级分组,按性别
for (Map.Entry<String, Map<Integer, List<User>>> stringMapEntry : collect.entrySet()) {
System.out.println("------------------------------");
System.out.println(stringMapEntry.getKey());
System.out.println(stringMapEntry.getValue());
System.out.println("------------------------------");
}
}
@Test
public void testGroupCount(){
// 统计每个地区的人数
Map<String, Long> collect = list.stream()
.collect(Collectors.groupingBy(User::getAddress, counting()));
for (Map.Entry<String, Long> stringLongEntry : collect.entrySet()) {
System.out.println(stringLongEntry.getKey());
System.out.println(stringLongEntry.getValue());
}
}
@Test
public void testPartitioningBy(){
//partitioningBy 分区
//分区与分组的区别在于,
// 分区是按照 true 和 false 来分的,因此partitioningBy 接受的参数的 lambda 也是 T -> boolean
Map<Boolean, List<User>> collect = list.stream()
.collect(Collectors.partitioningBy(s -> s.getAge() > 30));
for (Map.Entry<Boolean, List<User>> map : collect.entrySet()) {
System.out.println(map.getKey());
System.out.println(map.getValue());
}
}
@Test
public void testRemoveIf(){
List<User> list2 = new ArrayList<>(this.list);
// 根据条件删除
list2.removeIf(user -> user.getAge() <= 40);
list2.forEach(System.out::println);
}
}