Java8數(shù)據(jù)流

流/Stream是在JAVA8中引入的一個抽象，可以處理類似SQL語句聲明數(shù)據(jù)。 例如，考慮下面的SQL語句。

SELECT max(salary),employee_id,employee_name FROM Employee

上面的SQL表達(dá)式會自動返回最大薪水員工的細(xì)節(jié)，沒有對開發(fā)者的最終做任何計(jì)算。在Java中使用集合框架，開發(fā)人員必須使用循環(huán)，使檢查反復(fù)。另一個值得關(guān)注的是效率，多核處理器可放心，Java開發(fā)人員必須編寫的并行代碼處理，但是非常容易出錯。

為了解決這樣的問題，JAVA8引入了流的概念，它允許開發(fā)者通過聲明處理數(shù)據(jù)，并可以leverate多核架構(gòu)，而不需要編寫任何特定的代碼。

什么是數(shù)據(jù)流？

流代表從支持聚合操作源的序列的對象。以下是數(shù)據(jù)流的特點(diǎn)。

• 元素序列 - 流提供了一組特定類型的以順序方式元素。流獲取/計(jì)算需求的元素。它不存儲元素。

• 源- 流使用集合，數(shù)組或I/O資源為輸入源。

• 聚合操作 - 數(shù)據(jù)流支持如filter, map, limit, reduced, find, match等聚合操作。

• 管道傳輸 - 大多數(shù)流操作的返回流本身使他們的結(jié)果可以被管道傳輸。這些操作被稱為中間操作以及它們的功能是利用輸入，處理輸入和輸出返回到目標(biāo)。collect()方法是終端操作，這是通常出現(xiàn)在管道傳輸操作結(jié)束標(biāo)記流的結(jié)束。

• 自動迭代 - 流操作內(nèi)部做了反復(fù)對比，其中明確迭代需要集合提供源元素。

生成數(shù)據(jù)流

使用Java8，Collection 接口有兩個方法來生成流。

• stream() -返回順序流考慮集合作為其源。

• parallelStream() - 返回并行數(shù)據(jù)流考慮集合作為其源。

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

ForEach

數(shù)據(jù)流提供了新的forEach方法遍歷該流中的每個元素?？紤]下面的例子打印10個隨機(jī)數(shù)字。

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

map

map方法用于映射每個元素對應(yīng)的結(jié)果?？紤]下面的例子打印唯一的方形數(shù)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方法用于消除基于標(biāo)準(zhǔn)元素?？紤]下面的例子打印空字符串計(jì)數(shù)(總數(shù))。

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 方法用于減少流的大小?？紤]下面的例子打印10個隨機(jī)數(shù)字。

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

sorted

sorted方法用來流排序。考慮下面的例子打印10個隨機(jī)數(shù)字的排序順序。

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

并行處理

parallelStream是流進(jìn)行并行處理的替代方案?？紤]下面的例子打印空字符串計(jì)數(shù)。

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

這是很容易在順序和并行的流之間進(jìn)行切換。

收集器

收集器是用來處理組合在一個數(shù)據(jù)流的元素的結(jié)果。收集器可用于返回一個列表或一個字符串。

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);

統(tǒng)計(jì)

使用Java8，統(tǒng)計(jì)收集器引入計(jì)算所有統(tǒng)計(jì)數(shù)據(jù)時，流處理可以做這些。

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

選擇使用任何編輯器創(chuàng)建以下java程序在C:/> 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();
   }
}

驗(yàn)證結(jié)果

使用javac編譯器編譯如下類

C:\JAVA>javac Java8Tester.java

現(xiàn)在運(yùn)行Java8Tester看到的結(jié)果

C:\JAVA>java Java8Tester

看到結(jié)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