在Java平嵌套集合

流是一系列支持各种方法的对象，这些方法可以通过流水线来产生所需的结果。 Stream 用于按照流水线方法计算元素，而不改变对象的原始值。而且，扁平化意味着将两个或多个集合合并为一个。考虑下图，我们有一个包含 3 个数组的数组，但在展平效果之后，我们将有一个结果数组，其中包含三个数组中的所有元素。

插图：

Input      : arr1[]  = {{1,2,3,4},{5,6,7},{8,9}}; 
Processing : Flatening
Output     : arr1[]  = {1,2,3,4,5,6,7,8,9};

Stream flatMap()方法用于将集合流扁平化为对象流。这些对象是从原始流中的所有集合组合而成的。 flatMap()方法是对流元素的一对多转换，然后将结果元素展平为新的流。基本上Stream.flatMap()方法有助于将Stream>转换为Stream 。

示例 1：使用 flatMap() 方法展平具有相同类型的两个数组的流

Java

// Java Program to flatten a stream of same type two arrays
// using flatMap() method
  
// Importing input output classes
import java.io.*;
// Importing Arrays and Stream classes 
// from java.util package
import java.util.Arrays;
import java.util.stream.Stream;
  
// Main class
class GFG {
    
    // Method 1
    //  To flatten a stream of two arrays of the same type
    public static  Stream flatten(T[] a, T[] b)
    {
        // Stream.flatMap() method coverts
        // Stream> to the  Stream
        Stream stream
            = Stream.of(a, b).flatMap(Arrays::stream);
  
        // Returns the desired stream
        return stream;
    }
  
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Input array of strings
  
        // Array 1 has uppercase characters
        String[] a = { "A", "B", "C" };
  
        // Array 2 has lowercase characters
        String[] b = { "i", "J", "K" };
           
        // Calling the above method in the main() method 
        String[] s = flatten(a, b).toArray(String[] ::new);
  
        // Return string representation of contents
        // of integer array
        System.out.println(Arrays.toString(s));
    }
}

输出

[A, B, C, i, J, K]

示例 2：展平两个相同类型列表的流

Java

// Java Program to Flatten a stream of Two Lists
// of the same type
  
// Importing required libraries
import java.io.*;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
  
// Main class
public class GFG {
  
    // Method 1
    // Flatten a stream of two lists of the same type
    public static  Stream flatten(List a,
                                        List b)
    {
  
        // Stream.flatMap() method coverts
        // Stream> to the  Stream
        // using flatMap(x -> x.stream())
        Stream stream
            = Stream.of(a, b).flatMap(List::stream);
  
        // Return the desired stream
        return stream;
    }
  
    // Method 2
    // Main drier method
    public static void main(String[] args)
    {
        // Input Lists
  
        // List 1
        List a = Arrays.asList("Ma", "Rs", "Xy");
        // List 2
        List b = Arrays.asList("Jw", "Pi", "Br");
  
        // Calling the method 1 and storing it in a single
        // list
        List s
            = flatten(a, b).collect(Collectors.toList());
  
        // Print all the elements in above List object
        System.out.println(s);
    }
}

输出

[Ma, Rs, Xy, Jw, Pi, Br]

示例 3：使用 flatMap() 方法将包含项目列表作为值的地图展平

Java

// Java Program to Flatten a map containing a list of items
// as values using flatMap() method
  
// Importing input output classes
import java.io.*;
// Importing desired classes from java.util package
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;
  
// Main class
public class GFG {
  
    // Method 1
    // To flatten a map containing a list of items as values
    public static  Stream
    flatten(Collection > values)
    {
  
        // Stream.flatMap() method coverts
        // Stream> to the  Stream
        Stream stream
            = values.stream().flatMap(x -> x.stream());
  
        // Return the desired stream
        return stream;
    }
  
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an object of Map class
        // Declaring object of integer and string type
        Map > map = new HashMap<>();
  
        // Adding elements to the above Map object
        // Custom input entries
        map.put(1, Arrays.asList("1", "2", "3"));
        map.put(2, Arrays.asList("4", "5", "6"));
  
        // Creating a List class object holding all elements
        // after flatenning
        List s = flatten(map.values())
                             .collect(Collectors.toList());
  
        // Print and display the above List object
        System.out.println(s);
    }
}

输出

[1, 2, 3, 4, 5, 6]