In this article, we will discuss Stream’s flatMapToDouble() method in detail with examples and explanation
1. Stream flatMapToDouble() method :
- This Stream method is an intermediate operation which returns an
DoubleStreamconsisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element - Each mapped stream is
closedafter its contents have been placed into this stream - If a mapped stream is
nullan empty stream is used, instead - Stream’s flatMapToDouble() method is stateless which means it is non-interfering with other elements in the stream
- Method signature :- DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper)
- Where DoubleStream is a sequence of primitive double-valued elements and T is the type of Stream elements
2. Stream flatMapToDouble() method examples :
2.1 Flatten List<List<Double>> to DoubleStream
- A outer main list contains 3 sub-lists and in each sub-list there are 3 or 4 double values
- We are converting outer main list into DoubleStream using flatMapToDouble() method which returns flattened double values
- Finally, we are finding summary statistics of these double values using DoubleStream‘s summaryStatistics() method while peeking into each elements using Stream’s peek() method
package net.bench.resources.stream.flatmaptodouble.example;
import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.stream.DoubleStream;
public class StreamFlatMapToDoubleForList {
public static void main(String[] args) {
// 3 lists defined
List<Double> childList1 = Arrays.asList(19.97, 66.67, 65.34);
List<Double> childList2 = Arrays.asList(79.43, 93.54, 98.76);
List<Double> childList3 = Arrays.asList(43.93, 54.32, 90.12, 86.57);
// Outer main-list contains 3 child sub-list
List<List<Double>> mainList = Arrays.asList(
childList1,
childList2,
childList3
);
// flatMapToDouble()
DoubleStream doubleStream = mainList
.stream()
.flatMapToDouble(list -> list.stream()
.mapToDouble(num -> num));
// iterate/print to console
System.out.println("Flattened list using flatMapToDouble() method :- \n");
// DoubleSummaryStatistics
DoubleSummaryStatistics doubleSummaryStatistics = doubleStream
.peek(System.out::println) // print to console
.summaryStatistics(); // get summary statistics
// summary statistics for flattened list
System.out.println("\nSummary statistics for flattened list :- \n\n"
+ doubleSummaryStatistics);
}
}
Output:
Flattened list using flatMapToDouble() method :-
19.97
66.67
65.34
79.43
93.54
98.76
43.93
54.32
90.12
86.57
Summary statistics for flattened list :-
DoubleSummaryStatistics{count=10, sum=698.650000, min=19.970000,
average=69.865000, max=98.760000}
2.2 Flatten 2-D String[][] array to DoubleStream
- A 2-d String[][] array defined where in each array there are double values defined in String format
- We are converting 2-d String[][] array into DoubleStream using flatMapToDouble() method which returns flattened double values
- Finally, we are finding summary statistics of these double values using DoubleStream‘s summaryStatistics() method while peeking into each elements using Stream’s peek() method
package net.bench.resources.stream.flatmaptodouble.example;
import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.stream.DoubleStream;
public class StreamFlatMapToDoubleForArrays {
public static void main(String[] args) {
// 2d String[][] array
String[][] stringArray2D = {
{"97.19", "76.53", "80.65"},
{"53.79", "83.45", "89.67"},
{"96.43", "65.43", "91.23", "57.86"}
};
// flatMapToDouble()
DoubleStream doubleStream = Arrays.stream(stringArray2D)
.flatMapToDouble(array -> Arrays.stream(array)
.mapToDouble(Double::new));
// iterate/print to console
System.out.println("Flattened 2-d Array using flatMapToDouble() method :- \n");
// DoubleSummaryStatistics
DoubleSummaryStatistics doubleSummaryStatistics = doubleStream
.peek(System.out::println) // print to console
.summaryStatistics(); // get summary statistics
// summary statistics for flattened 2-d Array
System.out.println("\nSummary statistics for flattened 2-d Array :- \n\n"
+ doubleSummaryStatistics);
}
}
Output:
Flattened 2-d Array using flatMapToDouble() method :-
97.19
76.53
80.65
53.79
83.45
89.67
96.43
65.43
91.23
57.86
Summary statistics for flattened 2-d Array :-
DoubleSummaryStatistics{count=10, sum=792.230000, min=53.790000,
average=79.223000, max=97.190000}
2.3 Flatten List<Product[]> to DoubleStream
- There are 3 Product[] array defined
- In each Product[] array, there are 2 Products defined with their attributes such as id, name, quantity and their price
- And all these 3 Product[] array are added to a List
- We are converting List of Product[] array extracting price information (which is of type double) into DoubleStream using flatMapToDouble() method which returns flattened double values
- Also, we are finding summary statistics of Product price using DoubleStream‘s summaryStatistics() method while peeking into each elements using Stream’s peek() method
Product.java
package net.bench.resources.stream.flatmaptodouble.example;
public class Product {
// member variables
private int id;
private String name;
private long quantity;
private double price;
// 4-arg parameterized constructor
// getters and setters
// toString() method
}
StreamFlatMapToDoubleForProductPrice.java
package net.bench.resources.stream.flatmaptodouble.example;
import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.stream.DoubleStream;
public class StreamFlatMapToDoubleForProductPrice {
public static void main(String[] args) {
// Category 1 - Product array
Product[] productArray1 = new Product[] {
new Product(101, "Wheat", 1089L, 36.89),
new Product(102, "Rice", 502L, 58.19)
};
// Category 2 - Product array
Product[] productArray2 = new Product[] {
new Product(103, "Lentils", 803L, 102.45),
new Product(104, "Oil", 208L, 164.75)
};
// Category 3 - Product array
Product[] productArray3 = new Product[] {
new Product(105, "Vegetables", 303L, 45.50),
new Product(106, "Meat", 404L, 731.56)
};
// list of Product[] array
List<Product[]> productList = Arrays.asList(
productArray1,
productArray2,
productArray3
);
// flatMapToDouble()
DoubleStream doubleStream = productList
.stream()
.flatMapToDouble(productArray -> Arrays.stream(productArray)
.mapToDouble(product -> product.getPrice()));
// extract Product Price from all categories
System.out.println("Product Price from all categories "
+ "using flatMapToDouble() method :- \n");
// DoubleSummaryStatistics
DoubleSummaryStatistics doubleSummaryStatistics = doubleStream
.peek(System.out::println) // print to console
.summaryStatistics(); // get summary statistics
// summary statistics for flattened list
System.out.println("\nSummary statistics for Product Price :- \n\n"
+ doubleSummaryStatistics);
}
}
Output:
Product Price from all categories using flatMapToDouble() method :-
36.89
58.19
102.45
164.75
45.5
731.56
Summary statistics for Product Price :-
DoubleSummaryStatistics{count=6, sum=1139.340000, min=36.890000,
average=189.890000, max=731.560000}
References:
- https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
Happy Coding !!
Happy Learning !!