Java8 Streams流

流（Stream）是Java 8中引入的一个新的抽象层。使用流，您可以以类似于SQL语句的声明方式处理数据。例如，考虑以下SQL语句。

SELECT max(salary), employee_id, employee_name FROM Employee

上述的SQL表达式会自动返回最高薪水的雇员详细信息，而无需在开发者端进行任何计算。使用Java的集合框架，开发者需要使用循环并进行重复检查。另一个问题是效率；由于多核处理器很容易获取，Java开发者需要编写并行处理代码，这可能容易出错。

为了解决这些问题，Java 8引入了流(Stream)的概念，它可以让开发者以声明式的方式处理数据并充分利用多核架构，而无需为此编写任何特定的代码。

什么是流(Stream)

流(Stream)表示从源中获取的对象的序列，可以支持聚合操作。以下是流(Stream)的特点：

生成流(Stream)

在Java 8中，Collection接口有两个方法可以生成流(Stream)。

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());

forEach

Stream提供了一个新的方法‘forEach’，用于迭代流的每个元素。以下代码段展示了如何使用forEach打印10个随机数。

Random random = new Random();random.ints().limit(10).forEach(System.out::println);

map

‘map’方法用于将每个元素映射到其对应的结果。以下代码段使用map打印出数字的唯一平方数。

List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);//get list of unique squaresList<Integer> squaresList = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList());

过滤器

“filter”方法用于根据条件消除元素。下面的代码段使用filter打印空字符串的数量。

List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");//get count of empty stringint count = strings.stream().filter(string -> string.isEmpty()).count();

限制

‘limit’方法用于减小流的大小。下面的代码段展示了如何使用limit方法打印出10个随机数。

Random random = new Random();random.ints().limit(10).forEach(System.out::println);

按顺序排列

‘sorted’方法用于对流进行排序。下面的代码片段展示了如何以排序的方式打印出10个随机数。

Random random = new Random();random.ints().limit(10).sorted().forEach(System.out::println);

并行处理

parallelStream是流的替代品，用于并行处理。看下面的代码片段，使用parallelStream打印空字符串的数量。

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");//get count of empty stringlong count = strings.parallelStream().filter(string -> string.isEmpty()).count();

在顺序流和并行流之间切换非常容易。

收集器

收集器被用于将流中元素的处理结果组合起来。收集器可以用于返回一个列表或一个字符串。

List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());System.out.println("Filtered List: " + filtered);String mergedString = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining(", "));System.out.println("Merged String: " + mergedString);

统计数据

在Java 8中，引入了统计数据收集器，用于在流处理时计算所有统计数据。

List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);IntSummaryStatistics stats = numbers.stream().mapToInt((x) -> x).summaryStatistics();System.out.println("Highest number in List : " + stats.getMax());System.out.println("Lowest number in List : " + stats.getMin());System.out.println("Sum of all numbers : " + stats.getSum());System.out.println("Average of all numbers : " + stats.getAverage());

流示例

使用您选择的任何编辑器，在C:> JAVA中创建以下Java程序。

Java8Tester.java

import java.util.ArrayList;import java.util.Arrays;import java.util.IntSummaryStatistics;import java.util.List;import java.util.Random;import java.util.stream.Collectors;import java.util.Map;public class Java8Tester {   public static void main(String args[]) {      System.out.println("Using Java 7: ");      // Count empty strings      List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");      System.out.println("List: " +strings);      long count = getCountEmptyStringUsingJava7(strings);      System.out.println("Empty Strings: " + count);      count = getCountLength3UsingJava7(strings);      System.out.println("Strings of length 3: " + count);      //Eliminate empty string      List<String> filtered = deleteEmptyStringsUsingJava7(strings);      System.out.println("Filtered List: " + filtered);      //Eliminate empty string and join using comma.      String mergedString = getMergedStringUsingJava7(strings,", ");      System.out.println("Merged String: " + mergedString);      List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);      //get list of square of distinct numbers      List<Integer> squaresList = getSquares(numbers);      System.out.println("Squares List: " + squaresList);      List<Integer> integers = Arrays.asList(1,2,13,4,15,6,17,8,19);      System.out.println("List: " +integers);      System.out.println("Highest number in List : " + getMax(integers));      System.out.println("Lowest number in List : " + getMin(integers));      System.out.println("Sum of all numbers : " + getSum(integers));      System.out.println("Average of all numbers : " + getAverage(integers));      System.out.println("Random Numbers: ");      //print ten random numbers      Random random = new Random();      for(int i = 0; i < 10; i++) {         System.out.println(random.nextInt());      }      System.out.println("Using Java 8: ");      System.out.println("List: " +strings);      count = strings.stream().filter(string->string.isEmpty()).count();      System.out.println("Empty Strings: " + count);      count = strings.stream().filter(string -> string.length() == 3).count();      System.out.println("Strings of length 3: " + count);      filtered = strings.stream().filter(string ->!string.isEmpty()).collect(Collectors.toList());      System.out.println("Filtered List: " + filtered);      mergedString = strings.stream().filter(string ->!string.isEmpty()).collect(Collectors.joining(", "));      System.out.println("Merged String: " + mergedString);      squaresList = numbers.stream().map( i ->i*i).distinct().collect(Collectors.toList());      System.out.println("Squares List: " + squaresList);      System.out.println("List: " +integers);      IntSummaryStatistics stats = integers.stream().mapToInt((x) ->x).summaryStatistics();      System.out.println("Highest number in List : " + stats.getMax());      System.out.println("Lowest number in List : " + stats.getMin());      System.out.println("Sum of all numbers : " + stats.getSum());      System.out.println("Average of all numbers : " + stats.getAverage());      System.out.println("Random Numbers: ");      random.ints().limit(10).sorted().forEach(System.out::println);      //parallel processing      count = strings.parallelStream().filter(string -> string.isEmpty()).count();      System.out.println("Empty Strings: " + count);   }   private static int getCountEmptyStringUsingJava7(List<String> strings) {      int count = 0;      for(String string: strings) {         if(string.isEmpty()) {            count++;         }      }      return count;   }   private static int getCountLength3UsingJava7(List<String> strings) {      int count = 0;      for(String string: strings) {         if(string.length() == 3) {            count++;         }      }      return count;   }   private static List<String> deleteEmptyStringsUsingJava7(List<String> strings) {      List<String> filteredList = new ArrayList<String>();      for(String string: strings) {         if(!string.isEmpty()) {             filteredList.add(string);         }      }      return filteredList;   }   private static String getMergedStringUsingJava7(List<String> strings, String separator) {      StringBuilder stringBuilder = new StringBuilder();      for(String string: strings) {         if(!string.isEmpty()) {            stringBuilder.append(string);            stringBuilder.append(separator);         }      }      String mergedString = stringBuilder.toString();      return mergedString.substring(0, mergedString.length()-2);   }   private static List<Integer> getSquares(List<Integer> numbers) {      List<Integer> squaresList = new ArrayList<Integer>();      for(Integer number: numbers) {         Integer square = new Integer(number.intValue() * number.intValue());         if(!squaresList.contains(square)) {            squaresList.add(square);         }      }      return squaresList;   }   private static int getMax(List<Integer> numbers) {      int max = numbers.get(0);      for(int i = 1;i < numbers.size();i++) {         Integer number = numbers.get(i);         if(number.intValue() > max) {            max = number.intValue();         }      }      return max;   }   private static int getMin(List<Integer> numbers) {      int min = numbers.get(0);      for(int i= 1;i < numbers.size();i++) {         Integer number = numbers.get(i);         if(number.intValue() < min) {            min = number.intValue();         }      }      return min;   }   private static int getSum(List numbers) {      int sum = (int)(numbers.get(0));      for(int i = 1;i < numbers.size();i++) {         sum += (int)numbers.get(i);      }      return sum;   }   private static int getAverage(List<Integer> numbers) {      return getSum(numbers) / numbers.size();   }}

验证结果

使用 javac 编译器编译类，如下所示 –

C:\JAVA>javac Java8Tester.java

现在按照以下步骤运行Java8Tester：

C:\JAVA>java Java8Tester

它应该产生以下结果−

Using Java 7:List: [abc, , bc, efg, abcd, , jkl]Empty Strings: 2Strings of length 3: 3Filtered List: [abc, bc, efg, abcd, jkl]Merged String: abc, bc, efg, abcd, jklSquares List: [9, 4, 49, 25]List: [1, 2, 13, 4, 15, 6, 17, 8, 19]Highest number in List : 19Lowest number in List : 1Sum of all numbers : 85Average of all numbers : 9Random Numbers:-1279735475903418352-1133928044-157111891162853046218407523-881538250-718932165270259229421676854Using Java 8:List: [abc, , bc, efg, abcd, , jkl]Empty Strings: 2Strings of length 3: 3Filtered List: [abc, bc, efg, abcd, jkl]Merged String: abc, bc, efg, abcd, jklSquares List: [9, 4, 49, 25]List: [1, 2, 13, 4, 15, 6, 17, 8, 19]Highest number in List : 19Lowest number in List : 1Sum of all numbers : 85Average of all numbers : 9.444444444444445Random Numbers:-1009474951-551240647-248471418161455093344426812278504161579250773162745487216830336871798939493Empty Strings: 2