Java8新特性——stream流
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 [email protected]
* @version 1.0
* @name
* @description
* @date 2018/7/15
*/
public class BasicTest {
public static void main(String[] args) {
//创建串行流
List strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List 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 numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
// 获取对应的平方数
List 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 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 array = Arrays.asList(1,3,2,4);
List sortedList = array.stream().sorted().collect(Collectors.toList());
sortedList.forEach(System.out::println);
//流并行处理程序parallelStream
List 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 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 [email protected]
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 [email protected]
* @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 list = new ArrayList<>(4);
list.add(foo1);
list.add(foo2);
list.add(foo3);
Map> collect = list.stream().collect(Collectors.groupingBy(Foo::getCode));
//存储最后的汇总结果集合
List result = new ArrayList<>();
collect.forEach((k,v)->{
Optional 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