Java 简明教程

Java - Streams

Stream 是 Java 8 中引入的新抽象层。使用流，你可以使用类似于 SQL 语句那样声明式的方式处理数据。例如，考虑以下 SQL 语句。

SELECT max(salary), employee_id, employee_name FROM Employee

上述 SQL 表达式自动返回薪水最高的员工的详细信息，而无需开发人员进行任何计算。使用 Java 中的集合框架，开发人员必须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使用，Java 开发人员必须编写可能很容易出错的并行代码处理。

为了解决此类问题，Java 8 引入了流的概念，使开发人员能够以声明方式处理数据，并利用多核架构，而无需针对它编写任何特定代码。

What is Stream in Java?

流表示来自某个源的对象序列，它支持聚合操作。以下是流的特性：

Sequence of elements - 流按顺序方式提供特定类型的元素集合。流按需获取/计算元素。它从不存储元素。
Source – 流将 Collections 、 Arrays 或 I/O 资源作为输入源。
Aggregate operations - 流支持过滤、映射、限制、缩减、查找、匹配等聚合操作。
Pipelining - 大多数流操作返回流本身，以便它们的可以进行管道处理。这些操作被称为中间操作，它们的函数是获取输入、处理这些输入并将输出返回到目标。collect() 方法是终端操作，通常存在于管道处理操作的末尾，以标记流的末尾。
Automatic iterations - 流操作在内部对提供的源元素进行迭代，这与需要显式迭代的集合相反。

Generating Streams in Java

在 Java 8 中， Collection interface 有两种生成流的方法。

stream() - 返回一个将集合视为源的顺序流。
parallelStream() - 返回一个将集合视为源的并行流。

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

forEach Method

流提供了一个新的 'forEach' 方法来迭代流中的每个元素。以下代码片段展示了如何使用 forEach 打印 10 个随机数。

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

map Method

'map' 方法用于将每个元素映射到它对应的结果。以下代码片段使用 map 打印数字的唯一平方。

List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);

//get list of unique squares
List<Integer> squaresList = numbers.stream().map( i -> i*i).distinct().collect(Collectors.toList());

filter Method

'filter' 方法用于基于某个条件排除元素。以下代码片段使用 filter 打印空字符串的计数。

List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");

//get count of empty string
int count = strings.stream().filter(string -> string.isEmpty()).count();

limit Method

'limit' 方法用于减小流的大小。以下代码片段展示了如何使用 limit 打印 10 个随机数。

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

sorted Method

'sorted' 方法用于对流进行排序。以下代码片段展示了如何以排序顺序打印 10 个随机数。

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

Parallel Processing

parallelStream 是用于并行处理的流的替代方案。看看以下使用 parallelStream 打印空字符串的计数的代码片段。

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");

//get count of empty string
long count = strings.parallelStream().filter(string -> string.isEmpty()).count();

在顺序流和并行流之间切换非常容易。

Collectors

收集器用于组合对流元素进行处理的结果。收集器可用于返回列表或字符串。

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);

Statistics

从 Java 8 开始，已经引入了统计收集器，能够在执行流处理时计算所有统计信息。

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());

Java Streams Example

在任意编辑器中创建以下 Java 程序，例如，在 C:\> JAVA 中创建。

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();
   }
}

Verify the Result

按照如下方式使用 javac 编译器编译类 −

C:\JAVA>javac Java8Tester.java

现在按照如下方式运行 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: 2
Strings of length 3: 3
Filtered List: [abc, bc, efg, abcd, jkl]
Merged String: abc, bc, efg, abcd, jkl
Squares List: [9, 4, 49, 25]
List: [1, 2, 13, 4, 15, 6, 17, 8, 19]
Highest number in List : 19
Lowest number in List : 1
Sum of all numbers : 85
Average of all numbers : 9
Random Numbers:
-1279735475
903418352
-1133928044
-1571118911
628530462
18407523
-881538250
-718932165
270259229
421676854
Using Java 8:
List: [abc, , bc, efg, abcd, , jkl]
Empty Strings: 2
Strings of length 3: 3
Filtered List: [abc, bc, efg, abcd, jkl]
Merged String: abc, bc, efg, abcd, jkl
Squares List: [9, 4, 49, 25]
List: [1, 2, 13, 4, 15, 6, 17, 8, 19]
Highest number in List : 19
Lowest number in List : 1
Sum of all numbers : 85
Average of all numbers : 9.444444444444445
Random Numbers:
-1009474951
-551240647
-2484714
181614550
933444268
1227850416
1579250773
1627454872
1683033687
1798939493
Empty Strings: 2