Java8 of collectors Interface

java8 stream in operation, into the middle of the operation terminal and operation, this embodiment also said main reduction operations of the collect, trap operation, as a higher reduction operations;

Look under the stream of collect operation

	<R> R collect(Supplier<R> supplier,BiConsumer<R, ? super T> accumulator,BiConsumer<R, R> combiner);
	<R, A> R collect(Collector<? super T, A, R> collector);

The first, an abstract method is passed three parameters,

The second operation Collectors collect only a look under the stream parameters in the static factory class, and in this static factory class, most of the realize method three parameters are called, meet almost all our daily operation; therefore, we look at, this static factory class, which achieved;



through code practice what

package com.meinergy.vppd.business.api.test;
 
import java.util.ArrayList;
import java.util.Comparator;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;
 
import org.junit.Test;
 
public class TestJava8 {
    public static List<Emp> list = new ArrayList<>();
    static {
        list.add(new Emp("上海", "小名", 17));
        list.add(new Emp("北京", "小红", 18));
        list.add(new Emp("深圳", "小蓝", 19));
        list.add(new Emp("广州", "小灰", 20));
        list.add(new Emp("杭州", "小黄", 21));
        list.add(new Emp("贵阳", "小白", 22));
    }
 
    @Test
    public void test1() {
        // 转list
        List<String> names = list.stream().map(emp -> emp.getName()).collect(Collectors.toList());
        // 转set
        Set<String> address = list.stream().map(emp -> emp.getName()).collect(Collectors.toSet());
        // 转map，需要指定key和value，Function.identity()表示当前的Emp对象本身
        Map<String, Emp> map = list.stream().collect(Collectors.toMap(Emp::getName, Function.identity()));
        // 计算元素中的个数
        Long count = list.stream().collect(Collectors.counting());
        // 数据求和 summingInt summingLong，summingDouble
        Integer sumAges = list.stream().collect(Collectors.summingInt(Emp::getAge));
        // 平均值 averagingInt,averagingDouble,averagingLong
        Double aveAges = list.stream().collect(Collectors.averagingInt(Emp::getAge));
 
        // 综合处理的，求最大值，最小值，平均值，求和操作
        // summarizingInt，summarizingLong,summarizingDouble
        IntSummaryStatistics intSummary = list.stream().collect(Collectors.summarizingInt(Emp::getAge));
        System.out.println(intSummary.getAverage());// 19.5
        System.out.println(intSummary.getMax());// 22
        System.out.println(intSummary.getMin());// 17
        System.out.println(intSummary.getSum());// 117
 
        // 连接字符串,当然也可以使用重载的方法，加上一些前缀，后缀和中间分隔符
        String strEmp = list.stream().map(emp -> emp.getName()).collect(Collectors.joining());
        String strEmp1 = list.stream().map(emp -> emp.getName()).collect(Collectors.joining("-中间的分隔符-"));
        String strEmp2 = list.stream().map(emp -> emp.getName()).collect(Collectors.joining("-中间的分隔符-", "前缀*", "&后缀"));
        System.out.println(strEmp);// 小名小红小蓝小灰小黄小白
        // 小名-中间的分隔符-小红-中间的分隔符-小蓝-中间的分隔符-小灰-中间的分隔符-小黄-中间的分隔符-小白
        System.out.println(strEmp1);
        // 前缀*小名-中间的分隔符-小红-中间的分隔符-小蓝-中间的分隔符-小灰-中间的分隔符-小黄-中间的分隔符-小白&后缀
        System.out.println(strEmp2);
        // maxBy 按照比较器中的比较结果刷选 最大值
        Optional<Integer> maxAge = list.stream().map(emp -> emp.getAge())
                .collect(Collectors.maxBy(Comparator.comparing(Function.identity())));
        // 最小值
        Optional<Integer> minAge = list.stream().map(emp -> emp.getAge())
                .collect(Collectors.minBy(Comparator.comparing(Function.identity())));
        System.out.println("max:" + maxAge);
        System.out.println("min:" + minAge);
 
        // 归约操作
        list.stream().map(emp -> emp.getAge()).collect(Collectors.reducing((x, y) -> x + y));
        list.stream().map(emp -> emp.getAge()).collect(Collectors.reducing(0, (x, y) -> x + y));
        // 分操作 groupingBy 根据地址，把原list进行分组
        Map<String, List<Emp>> mapGroup = list.stream().collect(Collectors.groupingBy(Emp::getAddress));
        // partitioningBy 分区操作 需要根据类型指定判断分区
        Map<Boolean, List<Integer>> partitioningMap = list.stream().map(emp -> emp.getAge())
                .collect(Collectors.partitioningBy(emp -> emp > 20));
 
    }
 
    static class Emp {
        private String address;
 
        private String name;
 
        private Integer age;
 
        public Emp() {
 
        }
 
        public Emp(String address) {
            this.address = address;
        }
 
        public Emp(String name, Integer age) {
            this.name = name;
            this.age = age;
        }
 
        public Emp(String address, String name, Integer age) {
            super();
            this.address = address;
            this.name = name;
            this.age = age;
        }
 
        public String getAddress() {
            return address;
        }
 
        public void setAddress(String address) {
            this.address = address;
        }
 
        public String getName() {
            return name;
        }
 
        public void setName(String name) {
            this.name = name;
        }
 
        public Integer getAge() {
            return age;
        }
 
        public void setAge(Integer age) {
            this.age = age;
        }
 
        @Override
        public String toString() {
            return "Emp [address=" + address + ", name=" + name + ", age=" + age + "]";
        }
 
    }
}