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Java 8 新特性|java.util.stream.Collectors

程序员文章站 2022-05-22 19:21:30
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Java 8 流的新类 java.util.stream.Collectors 实现了 java.util.stream.Collector 接口,同时又提供了大量的方法对流 ( stream ) 的元素执行 map and reduce 操作,或者统计操作。

1、 Collectors.averagingDouble()

Collectors.averagingDouble() 方法将流中的所有元素视为 double 类型并计算他们的平均值。该方法返回的是同一个 Collectors 实例,因此可以进行链式操作。

Collectors.averagingDouble() 接受一个参数,这个参数是一个 lambda 表达式,用于对所有的元素执行一个 map 操作。

Java 所有集合的 stream().collect() 可以接受一个收集器实例作为其参数并返回该收集器的计算结果

参考下面的代码,collect() 方法会把所有的元素收集起来然后传递给 Collectors.averagingDouble(d->d*2) 收集器,对每个元素执行 *2 操作后计算平均值

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class AveragingDoubleTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.averagingDouble(d->d*2));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.AveragingDoubleTest
5.0

Process finished with exit code 0

2、 Collectors.averagingInt()

该方法和Collectors.averagingDouble()一样,不同的是它把流中的所有元素看成是 int 类型,并返回一个浮点类型的平均值

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class AveragingIntTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.averagingInt(v->v*2));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.AveragingIntTest
5.0

Process finished with exit code 0

3、 Collectors.averagingLong()

该方法也和Collectors.averagingDouble()类似,不同的是它把流中的所有元素看成是 long 类型,并返回一个 double 类型的平均值

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class AveragingLongTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.averagingLong(v->v*2));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.AveragingLongTest
5.0

Process finished with exit code 0

4、 Collectors.averagingDouble()

Collectors.collectingAndThen()函数应该最像 map and reduce 了,它可接受两个参数,第一个参数用于 reduce 操作,而第二参数用于 map 操作。

也就是,先把流中的所有元素传递给第二个参数,然后把生成的集合传递给第一个参数来处理。

例如下面的代码,先把 [1,2,3,4] 这个集合传递给 s-> ss lambda 表达式,计算得出结果为 [1,4,9,16] ,然后再把 [1,4,9,16] 传递给 v->v2 表达式,计算得出 [2,8,18,32] ,然后传递给 Collectors.averagingLong() 计算得到结果为 25.0

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class CollectingAndThenTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.collectingAndThen(Collectors.averagingLong(v->v*2),
                s-> s*s));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.CollectingAndThenTest
25.0

Process finished with exit code 0

5、 Collectors.counting()

Collectors.counting() 用于统计流中元素的个数。

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class CountingTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        long result= list.stream().collect(Collectors.counting());
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.CountingTest
4

Process finished with exit code 0

6、 Collectors.joining()

Collectors.joining() 方法用某个指定的拼接字符串把所有元素拼接成一个字符串,并添加可选的前缀和后缀

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class JoiningTest {

    public static void main(String[] args) {
        List<String> list = Arrays.asList("A","B","C","D");
        String result= list.stream().collect(Collectors.joining(",","(",")"));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.JoiningTest
(A,B,C,D)

Process finished with exit code 0

7、 Collectors.maxBy() 和 Collectors.minBy()

Collectors.maxBy()和Collectors.minBy() 两个方法分别用于计算流中所有元素的最大值和最小值。

两个方法都可以接受一个比较器作为参数,用于如何计算最大值或最小值

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class MaxByMinByTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(30,10,20,35);
        //Get Max
        list.stream().collect(Collectors.maxBy(new MaxByMinByTest().new IntegerComp()))
                .ifPresent(i->System.out.println(i));
        //Get Min
        list.stream().collect(Collectors.minBy(new MaxByMinByTest().new IntegerComp()))
                .ifPresent(i->System.out.println(i));
    }
    class IntegerComp implements Comparator<Integer> {
        @Override
        public int compare(Integer i1, Integer i2) {
            if(i1 >=i2 ){
                return 1;
            }else{
                return -1;
            }
        }
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.MaxByMinByTest
35
10

Process finished with exit code 0

8、 Collectors.summingInt()

Collectors.summingInt() 方法将流中的所有元素视为 int 类型,并计算所有元素的总和 ( sum )

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class SummingIntTest {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(30,10,20,35);
        int result = list.stream().collect(Collectors.summingInt(i->i));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.SummingIntTest
95

Process finished with exit code 0 

9、 Collectors.summingLong()

Collectors.summingLong() 将流中的所有元素视为 long 类型,并计算所有元素的总和

package com.sjh.test.java8.collectors;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class SummingLongTest {

    public static void main(String[] args) {
        List<Long> list = new ArrayList<>();
        list.add((long)340);
        list.add((long)240);
        list.add((long)360);
        long result = list.stream().collect(Collectors.summingLong(l->l));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.SummingLongTest
940

Process finished with exit code 0

10、 Collectors.summingDouble()

Collectors.summingDouble() 将流中的所有元素视为 double 类型,并计算所有元素的总和

package com.sjh.test.java8.collectors;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class SummingDoubleTest {

    public static void main(String[] args) {
        List<Double> list = Arrays.asList(340.5,234.56,672.76);
        Double result = list.stream().collect(Collectors.summingDouble(d->d));
        System.out.println(result);
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.SummingDoubleTest
1247.82

Process finished with exit code 0

summingInt()、summingLong()、summingDouble()这三个函数的结果的类型,就是它们如何看待元素的类型。

11、 Collectors.toList()

Collectors.toList() 将流中的所有元素导出到一个列表 ( List ) 中

package com.sjh.test.java8.collectors;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class ToListTest {

    public static void main(String[] args) {
        List<String> list = Stream.of("AA","BB","CC").collect(Collectors.toList());
        list.forEach(s->System.out.println(s));
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.ToListTest
AA
BB
CC

Process finished with exit code 0

12、 Collectors.toSet()

Collectors.toSet() 把流中的所有元素导出到一个集合 ( Set ) 中,并排除重复的元素 ( Set 的特性 )

package com.sjh.test.java8.collectors;

import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class ToSetTest {

    public static void main(String[] args) {
        Set<String> set = Stream.of("AA","AA","BB").collect(Collectors.toSet());
        set.forEach(s->System.out.println(s));
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.ToSetTest
AA
BB

Process finished with exit code 0

13、 Collectors.toMap()

Collectors.toMap() 将流中的所有元素导出到一个哈希表 ( Map ) 中。该方法接受两个参数,第一个参数用于生成键 ( key ) ,第二个参数用于生成值 ( value )。两个参数都是 Lambda 表达式。

package com.sjh.test.java8.collectors;

import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class ToMapTest {

    public static void main(String[] args) {
        Map<String,String> map = Stream.of("AA","BB","CC").collect(Collectors.toMap(k->k, v->v+v));
        map.forEach((k,v)->System.out.println("key:"+k +" value:"+v));
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.ToMapTest
key:CC value:CCCC
key:BB value:BBBB
key:AA value:AAAA

Process finished with exit code 0

14、 Collectors.mapping()

Collectors.mapping() 一般用于多重 map and reduce 中。Java 文档中描述的原型如下:

mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)

第一个参数用于 map ,第二个参数用于 reduce

package com.sjh.test.java8.collectors;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class MappingTest {

    public static void main(String[] args) {
        List<Person> list = Person.getList();
        Map<Integer, String> nameByAge
                = list.stream().collect(Collectors.groupingBy(Person::getAge,
                Collectors.mapping(Person::getName, Collectors.joining(","))));
        nameByAge.forEach((k,v)->System.out.println("Age:"+k +" Persons: "+v));
    }
}
class Person {
    private String name;
    private int age;
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    public static List<Person> getList() {
        List<Person> list = new ArrayList<>();
        list.add(new Person("JayLa", 30));
        list.add(new Person("LayMan", 20));
        list.add(new Person("Tom", 20));
        list.add(new Person("XinYi", 30));
        return list;
    }
}

运行结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.collectors.MappingTest
Age:20  Persons: LayMan,Tom
Age:30  Persons: JayLa,XinYi

Process finished with exit code 0

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Java 8 新特性|java.util.stream.Collectors