Java8数据流 - Java8教程

流/Stream是在JAVA8中引入的一个抽象，可以处理类似SQL语句声明数据。例如，考虑下面的SQL语句。

SELECT max(salary),employee_id,employee_name FROM Employee

上面的SQL表达式会自动返回最大薪水员工的细节，没有对开发者的最终做任何计算。在Java中使用集合框架，开发人员必须使用循环，使检查反复。另一个值得关注的是效率，多核处理器可放心，Java开发人员必须编写的并行代码处理，但是非常容易出错。

为了解决这样的问题，JAVA8引入了流的概念，它允许开发者通过声明处理数据，并可以leverate多核架构，而不需要编写任何特定的代码。

什么是数据流？

流代表从支持聚合操作源的序列的对象。以下是数据流的特点。

元素序列 - 流提供了一组特定类型的以顺序方式元素。流获取/计算需求的元素。它不存储元素。
源- 流使用集合，数组或I/O资源为输入源。
聚合操作 - 数据流支持如filter, map, limit, reduced, find, match等聚合操作。
管道传输 - 大多数流操作的返回流本身使他们的结果可以被管道传输。这些操作被称为中间操作以及它们的功能是利用输入，处理输入和输出返回到目标。collect()方法是终端操作，这是通常出现在管道传输操作结束标记流的结束。
自动迭代 - 流操作内部做了反复对比，其中明确迭代需要集合提供源元素。

生成数据流

使用Java8，Collection 接口有两个方法来生成流。

stream() -返回顺序流考虑集合作为其源。
parallelStream() - 返回并行数据流考虑集合作为其源。

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

ForEach

数据流提供了新的forEach方法遍历该流中的每个元素。考虑下面的例子打印10个随机数字。

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

map

map方法用于映射每个元素对应的结果。考虑下面的例子打印唯一的方形数字。

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

filter方法用于消除基于标准元素。考虑下面的例子打印空字符串计数(总数)。

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

limit 方法用于减少流的大小。考虑下面的例子打印10个随机数字。

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

sorted

sorted方法用来流排序。考虑下面的例子打印10个随机数字的排序顺序。

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

并行处理

parallelStream是流进行并行处理的替代方案。考虑下面的例子打印空字符串计数。

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

这是很容易在顺序和并行的流之间进行切换。

收集器

收集器是用来处理组合在一个数据流的元素的结果。收集器可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

使用Java8，统计收集器引入计算所有统计数据时，流处理可以做这些。

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

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());

Stream 例子

选择使用任何编辑器创建以下java程序在C:/> JAVA

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 seperator){
      StringBuilder stringBuilder = new StringBuilder();
      for(String string: strings){
         if(!string.isEmpty()){
            stringBuilder.append(string);
            stringBuilder.append(seperator);
         }
      }    
      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<Integer> numbers){
      int sum = numbers.get(0);
      for(int i=1;i< numbers.size();i++){
         sum += numbers.get(i).intValue();            
      }        
      return sum;
   }
   private static int getAverage(List<Integer> numbers){
      return getSum(numbers) / numbers.size();
   }
}

验证结果

使用javac编译器编译如下类

C:\JAVA>javac Java8Tester.java

现在运行Java8Tester看到的结果

C:\JAVA>java Java8Tester

看到结果

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