Java8新特性——stream流

Posted 2020-12-04 精诚所至，金石为开

 

一、基本API初探

package java8.stream;

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

/**
 * @author [e-mail]
 * @version 1.0
 * @name
 * @description
 * @date 2018/7/15
 */
public class BasicTest {
    public static void main(String[] args) {

        //创建串行流
        List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
        List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
        filtered.forEach(System.out::println);

        //使用foreach进行数据迭代 limit 方法用于获取指定数量的流
        Random random = new Random();
        random.ints().limit(10).forEach(System.out::println);

        //Map用于映射每个元素对应的结果，原值为i 映射到i*i .collect(Collectors.toList()):将stream再转换回list集合
        List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
        // 获取对应的平方数
        List<Integer> squaresList = numbers.stream().map( i -> i*i).distinct().limit(2).collect(Collectors.toList());
        squaresList.forEach(System.out::println);

        //filter 方法用于通过设置的条件过滤出元素
        List<String> stringList = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
        // 获取空字符串的数量
        Long count = stringList.stream().filter(string -> string.isEmpty()).count();
        System.out.println("The count of empty string:"+count);


        //sorted用于对流进行排序,默认是从小到大
        List<Integer> array = Arrays.asList(1,3,2,4);
        List<Integer> sortedList = array.stream().sorted().collect(Collectors.toList());
        sortedList.forEach(System.out::println);

        //流并行处理程序parallelStream
        List<String> stringList2 = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl","");
        // 获取空字符串的数量
        Long count2 = stringList2.parallelStream().filter(string -> string.isEmpty()).count();
        System.out.println("The count of empty string:"+count2);


        //collectors 可以返回列表或者字符串
        List<String> stringList3 = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl","");
        String mergedString = stringList3.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining(", "));
        System.out.println("合并字符串: " + mergedString);

        //
        int[] numberList =  {12,3,34,67,100,99};

        IntStream intStream = IntStream.of(numberList);
        IntSummaryStatistics stats = intStream.summaryStatistics();

        System.out.println("列表中最大的数 : " + stats.getMax());
        System.out.println("列表中最小的数 : " + stats.getMin());
        System.out.println("所有数之和 : " + stats.getSum());
        System.out.println("平均数 : " + stats.getAverage());
    }
}

二、分组和合并

 1 /**
 2  * @author [e-mail]
 3  * @version 1.0
 4  * @name
 5  * @description 分组、合并测试dto
 6  * @date 2018/7/15
 7  */
 8 public class Foo {
 9 
10     private int code;
11 
12     private int count;
13 
14     public Foo(int code, int count) {
15         this.code = code;
16         this.count = count;
17     }
18 
19     public int getCode() {
20         return code;
21     }
22 
23     public void setCode(int code) {
24         this.code = code;
25     }
26 
27     public int getCount() {
28         return count;
29     }
30 
31     public void setCount(int count) {
32         this.count = count;
33     }
34 
35     @Override
36     public String toString() {
37         return "Foo{" +
38                 "code=" + code +
39                 ", count=" + count +
40                 \'}\';
41     }
42 }

/**
 * @author [e-mail]
 * @version 1.0
 * @name
 * @description 测试分组和合并  groupingBy方法以及reduce方法
 * @date 2018/7/15
 */
public class TestFoo {
    public static void main(String[] args) {
        Foo foo1 = new Foo(1, 2);
        Foo foo2 = new Foo(2, 23);
        Foo foo3 = new Foo(2, 6);
        List<Foo> list = new ArrayList<>(4);
        list.add(foo1);
        list.add(foo2);
        list.add(foo3);
        Map<Integer, List<Foo>> collect = list.stream().collect(Collectors.groupingBy(Foo::getCode));
        //存储最后的汇总结果集合
        List<Foo> result = new ArrayList<>();
        collect.forEach((k,v)->{
            Optional<Foo> sum = v.stream().reduce(
                    (v1, v2) -> {  //合并
                        v1.setCount(v1.getCount()+v2.getCount());
                        return v1;
                    }
            );
            result.add(sum.orElse(new Foo(0,10)));
        });
        result.forEach(System.out::print);

    }
}

将2号记录的count值进行了合并汇总!23+6=29