Java 8-流 - 芒果文档

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📜 Java 8-流

📅 最后修改于: 2021-01-05 04:40:05 🧑 作者: Mango

Stream是Java 8中引入的新的抽象层。使用stream，可以以类似于SQL语句的声明性方式处理数据。例如，考虑以下SQL语句。

SELECT max(salary), employee_id, employee_name FROM Employee

上面的SQL表达式自动返回受薪员工的最大详细信息，而无需在开发人员端进行任何计算。使用Java中的collections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。由于可以轻松获得多核处理器，因此Java开发人员必须编写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，该流使开发人员可以声明性地处理数据并利用多核体系结构，而无需为其编写任何特定的代码。

什么是流?

流表示来自源的一系列对象，支持聚合操作。以下是流的特征-

元素顺序-流以顺序方式提供一组特定类型的元素。流按需获取/计算元素。它从不存储元素。
源-流将集合，数组或I / O资源作为输入源。
聚合操作-Stream支持聚合操作，例如过滤，映射，限制，缩小，查找，匹配等。
流水线化-大多数流操作返回流本身，以便其结果可以流水线化。这些操作称为中间操作，其函数是获取输入，处理它们并将输出返回给目标。 collect()方法是一种终端操作，通常出现在流水线操作的结尾以标记流的结尾。
自动迭代-与需要显式迭代的Collections相比，Stream操作在提供的源元素内部进行迭代。

产生流

使用Java 8，Collection接口具有两种生成Stream的方法。

stream() -返回一个以集合为源的顺序流。
parallelStream() -返回一个以集合为源的并行Stream。

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

每次

Stream提供了一种新的“ forEach”方法来迭代流的每个元素。以下代码段显示了如何使用forEach打印10个随机数。

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

地图

‘map’方法用于将每个元素映射到其相应的结果。以下代码段使用map打印唯一的数字平方。

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

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

过滤

“过滤器”方法用于根据标准消除元素。以下代码段使用过滤器打印空字符串的计数。

Liststrings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");

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

限制

“限制”方法用于减小流的大小。以下代码段显示了如何使用limit打印10个随机数。

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

已排序

“排序”方法用于对流进行排序。以下代码段显示了如何按排序顺序打印10个随机数。

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

并行处理

parallelStream是并行处理流的替代方法。看一下下面的代码段，该代码段使用parallelStream打印一些空字符串。

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

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

在顺序和并行流之间切换非常容易。

收藏家

收集器用于合并对流元素的处理结果。收集器可用于返回列表或字符串。

Liststrings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List 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

在Java 8中，引入了统计信息收集器以在完成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());

流示例

使用您选择的任何编辑器(例如，C：\> JAVA)创建以下Java程序。

Java8Tester.java

现场演示

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 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 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 numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
        
      //get list of square of distinct numbers
      List squaresList = getSquares(numbers);
      System.out.println("Squares List: " + squaresList);
      List 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 strings) {
      int count = 0;

      for(String string: strings) {
        
         if(string.isEmpty()) {
            count++;
         }
      }
      return count;
   }
    
   private static int getCountLength3UsingJava7(List strings) {
      int count = 0;
        
      for(String string: strings) {
        
         if(string.length() == 3) {
            count++;
         }
      }
      return count;
   }
    
   private static List deleteEmptyStringsUsingJava7(List strings) {
      List filteredList = new ArrayList();
        
      for(String string: strings) {
        
         if(!string.isEmpty()) {
             filteredList.add(string);
         }
      }
      return filteredList;
   }
    
   private static String getMergedStringUsingJava7(List 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 getSquares(List numbers) {
      List squaresList = new ArrayList();
        
      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 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 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 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

现在如下运行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