Java Stream.reduce()用法及代码示例

很多时候，我们需要执行将流减少到单个结果值的操作，例如，最大值，最小值，总和，乘积等。减少是组合所有元素的重复过程。

reduce操作将二进制运算符应用于流中的每个元素，其中运算符的第一个参数是前一个应用程序的返回值，第二个参数是当前流元素。

用法:

T reduce(T identity, BinaryOperator<T> accumulator);

Where, identity is initial value 
of type T and accumulator is a 
function for combining two values.

sum()，min()，max()，count()等是归约运算的一些示例。 reduce()明确要求您指定如何减少流中生成的数据。

让我们看一些示例，以更好地理解reduce()函数：
示例1:

// Implementation of reduce method 
// to get the longest String 
import java.util.*; 
  
class GFG { 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        // creating a list of Strings 
        List<String> words = Arrays.asList("GFG", "Geeks", "for", 
                                           "GeeksQuiz", "GeeksforGeeks"); 
  
        // The lambda expression passed to 
        // reduce() method takes two Strings 
        // and returns the longer String. 
        // The result of the reduce() method is 
        // an Optional because the list on which 
        // reduce() is called may be empty. 
        Optional<String> longestString = words.stream() 
                                   .reduce((word1, word2) 
                             -> word1.length() > word2.length() 
                                           ? word1 : word2); 
  
        // Displaying the longest String 
        longestString.ifPresent(System.out::println); 
    } 
}

输出：

GeeksforGeeks

示例2:

// Implementation of reduce method 
// to get the combined String 
import java.util.*; 
  
class GFG { 
  
    // Driver code 
    public static void main(String[] args) 
    { 
  
        // String array 
        String[] array = { "Geeks", "for", "Geeks" }; 
  
        // The result of the reduce() method is 
        // an Optional because the list on which 
        // reduce() is called may be empty. 
        Optional<String> String_combine = Arrays.stream(array) 
                                           .reduce((str1, str2) 
                                           -> str1 + "-" + str2); 
  
        // Displaying the combined String 
        if (String_combine.isPresent()) { 
            System.out.println(String_combine.get()); 
        } 
    } 
}

输出：

Geeks-for-Geeks

示例3:

// Implementation of reduce method 
// to get the sum of all elements 
import java.util.*; 
  
class GFG { 
  
    // Driver code 
    public static void main(String[] args) 
    { 
  
        // Creating list of integers 
        List<Integer> array = Arrays.asList(-2, 0, 4, 6, 8); 
  
        // Finding sum of all elements 
        int sum = array.stream().reduce(0, 
                (element1, element2) -> element1 + element2); 
  
        // Displaying sum of all elements 
        System.out.println("The sum of all elements is " + sum); 
    } 
}

输出：

The sum of all elements is 16

示例4:

// Implementation of reduce method 
// to get the product of all numbers 
// in given range. 
import java.util.*; 
import java.util.stream.IntStream; 
  
class GFG { 
  
    // Driver code 
    public static void main(String[] args) 
    { 
  
        // To get the product of all elements 
        // in given range excluding the 
        // rightmost element 
        int product = IntStream.range(2, 8) 
                     .reduce((num1, num2) -> num1 * num2) 
                     .orElse(-1); 
  
        // Displaying the product 
        System.out.println("The product is : " + product); 
    } 
}

输出：

The product is : 5040

相关用法

注：本文由纯净天空筛选整理自Sahil_Bansall大神的英文原创作品 Stream.reduce() in Java with examples。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。