In this article, we will discuss Stream’s mapToDouble() method in detail with examples and explanation
1. Stream mapToDouble() method :
- This Stream method is an intermediate operation which returns a
DoubleStreamconsisting of the results of applying the given function to the elements of this stream - Stream’s mapToDouble() method is stateless which means it is non-interfering with other elements in the stream
- Method signature :- DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)
- Where DoubleStream is a sequence of primitive double-valued elements and T is the type of Stream elements
2. Stream mapToDouble() method examples :
2.1 Convert String to double values :
- A list contains String elements
- We are going to convert these String elements present in List to primitive double-valued sequence using Stream’s mapToDouble() method which returns DoubleStream using 3 different approaches
- 1st approach – using lambda expression i.e.; mapToDouble(str -> Double.parseDouble(str))
- 2nd approach – using Method reference i.e.; mapToDouble(Double::parseDouble)
- 3rd approach – implementing Anonymous Function for ToDoubleFunction<String>
- Finally, we can use Stream’s forEach() method to iterate/print primitive double values to the console
ConvertStringToDouble.java
package net.bench.resources.stream.maptodouble.example;
import java.util.Arrays;
import java.util.List;
import java.util.function.ToDoubleFunction;
import java.util.stream.DoubleStream;
public class ConvertStringToDouble {
public static void main(String[] args) {
// list of Strings
List<String> numbers = Arrays.asList(
"1.34",
"19.32",
"0.97",
"10.87",
"5.67"
);
// 1.1 string to double using mapToDouble() - lambda expression
DoubleStream doubleStreamLEx = numbers
.stream()
.mapToDouble(str -> Double.parseDouble(str)); // mapToDouble()
// 1.2 print to console
System.out.println("1. String to double using mapToDouble() -"
+ " Lambda Expression :- \n");
doubleStreamLEx.forEach(num -> System.out.println(num));
// 2.1 string to double using mapToDouble() - method reference
DoubleStream doubleStreamMRef = numbers
.stream()
.mapToDouble(Double::parseDouble); // mapToDouble()
// 2.2 print to console
System.out.println("\n2. String to double using mapToDouble() -"
+ " Method Reference :- \n");
doubleStreamMRef.forEach(System.out::println);
// 3.1 string to long using mapToDouble() - Anonymous Function
DoubleStream dblStreamAnonymousFn = numbers
.stream()
.mapToDouble(new ToDoubleFunction<String>() {
@Override
public double applyAsDouble(String str) {
return Double.parseDouble(str);
}
});
// 3.2 print to console
System.out.println("\n3. String to double using mapToDouble() -"
+ " Anonymous Function :- \n");
dblStreamAnonymousFn.forEach(System.out::println);
}
}
Output:
1. String to double using mapToDouble() - Lambda Expression :-
1.34
19.32
0.97
10.87
5.67
2. String to double using mapToDouble() - Method Reference :-
1.34
19.32
0.97
10.87
5.67
3. String to double using mapToDouble() - Anonymous Function :-
1.34
19.32
0.97
10.87
5.67
2.2 Get double numbers divisible by 1.5 :
- A list contains numbers in String format
- We are converting these String elements present in the List to double numbers using Stream’s mapToDouble() method which returns DoubleStream and then filtering those values which are divisible by 1.5
- Mapping :- using mapToDouble() method i.e.; mapToDouble(str -> Double.parseDouble(str))
- Filtering :- using filter() method i.e.; filter(num -> num % 1.5 == 0)
- Finally, we can use Stream’s forEach() method to iterate/print converted/filtered double values to the console
GetDoubleNumbersDivisibleBy1Point5.java
package net.bench.resources.stream.maptodouble.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.DoubleStream;
public class GetDoubleNumbersDivisibleBy1Point5 {
public static void main(String[] args) {
// list of Strings
List<String> numbers = Arrays.asList(
"1.50",
"19.50",
"0.90",
"10.50",
"5.67"
);
// 1.1 print original numbers
System.out.println("Original numbers in String format :- \n");
numbers.forEach(System.out::println);
// 2. string to double using mapToDouble() and filter()
DoubleStream doubleStreamLEx = numbers
.stream() // get Sequential stream
.mapToDouble(str -> Double.parseDouble(str)) // mapToDouble()
.filter(num -> num % 1.5 == 0); // filter()
// 2.1 print to console
System.out.println("\nMapped double numbers divisible by 1.5 :- \n");
doubleStreamLEx.forEach(num -> System.out.println(num));
}
}
Output:
Original numbers in String format :-
1.50
19.50
0.90
10.50
5.67
Mapped double numbers divisible by 1.5 :-
1.5
19.5
10.5
2.3 Get Square of double numbers :
- A list contains numbers in String format
- We are converting these String elements present in the List to double numbers and squaring them using Stream’s mapToDouble() method which returns DoubleStream
- Finally, we can use Stream’s forEach() method to iterate/print converted/squared double values to the console
GetSquaresForDoubleNumbers.java
package net.bench.resources.stream.maptodouble.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.DoubleStream;
public class GetSquaresForDoubleNumbers {
public static void main(String[] args) {
// list of Strings
List<String> numbers = Arrays.asList(
"1.34",
"19.32",
"0.97",
"10.87",
"5.67"
);
System.out.println("1. Original numbers in String format :- \n");
numbers.forEach(System.out::println);
// 1.1 string to double using mapToDouble() - lambda expression
DoubleStream doubleStreamLEx = numbers
.stream()
.mapToDouble(
str -> Double.parseDouble(str) * Double.parseDouble(str)
); // mapToDouble()
// 1.2 print to console
System.out.println("\n2. Squares of double numbers using mapToDouble() -"
+ " Lambda Expression :- \n");
doubleStreamLEx.forEach(num -> System.out.println(num));
}
}
Output:
1. Original numbers in String format :-
1.34
19.32
0.97
10.87
5.67
2. Squares of double numbers using mapToDouble() - Lambda Expression :-
1.7956000000000003
373.2624
0.9409
118.15689999999998
32.1489
2.4 Get price of all Products from List :
- A list contains Product information with attributes like id, name and its price
- We are intended to extract only price which is of type double from Product list
- We are going to use Method reference i.e.; mapToDouble(Product::getPrice) to get price of all Products
- Finally, we can use Stream’s forEach() method to iterate/print Product price information to the console
Product.java
package net.bench.resources.stream.maptolong.example;
public class Product {
// member variables
private long id;
private String name;
private double price;
// 3-arg parameterized constructor
// getters and setters
// toString()
}
GetPriceOfAllProducts.java
package net.bench.resources.stream.maptodouble.example;
import java.util.Arrays;
import java.util.List;
import java.util.stream.DoubleStream;
public class GetPriceOfAllProducts {
// List of Products
private static List<Product> getProductList() {
return Arrays.asList(
new Product(100000111L, "Rice", 58.19),
new Product(100000222L, "Wheat", 36.89),
new Product(100000333L, "Lentils", 102.45),
new Product(100000444L, "Oil", 164.75),
new Product(100000555L, "Vegetables", 45.56)
);
}
// main() method
public static void main(String[] args) {
// 1. List of Products
List<Product> products = getProductList();
// 1.1 print to console all Product information
System.out.println("All Products :- \n");
products.forEach(System.out::println);
// 2. get price of all Products
DoubleStream productIds = products
.stream() // get Sequential stream
.mapToDouble(Product::getPrice); // mapToDouble()
// 2.1 print to console all Product Id
System.out.println("\nPrice of All Products :- \n");
productIds.forEach(System.out::println);
// 3.1 product price in increasing order
System.out.println("\nProduct Price in Ascending order :- \n");
products
.stream()
.mapToDouble(Product::getPrice)
.sorted()
.forEach(System.out::println);
}
}
Output:
All Products :-
Product [id=100000111, name=Rice, price=58.19]
Product [id=100000222, name=Wheat, price=36.89]
Product [id=100000333, name=Lentils, price=102.45]
Product [id=100000444, name=Oil, price=164.75]
Product [id=100000555, name=Vegetables, price=45.56]
Price of All Products :-
58.19
36.89
102.45
164.75
45.56
Product Price in Ascending order :-
36.89
45.56
58.19
102.45
164.75
2.5 Get summary statistics of Double numbers :
- A list contains numbers in String format
- First, we are going to convert these String elements present in the List to double numbers using Stream’s mapToDouble() method
- Then invoking summaryStatistics() method on the result to get various information like count, sum, min, average and max
- We can also get these information individually/separately using different methods provided in the DoubleSummaryStatistics class like,
1. getSum() method to find sum of elements
2. getCount() method to find number of elements
3. getAverage() method to find average
4. getMax() method to find max element
5. getMin() method to find min element
StatisticsForDoubleNumbers.java
package net.bench.resources.stream.maptodouble.example;
import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.List;
public class StatisticsForDoubleNumbers {
public static void main(String[] args) {
// list of Strings
List<String> numbers = Arrays.asList(
"1.34",
"19.32",
"0.97",
"10.87",
"5.67"
);
// 1.1 get statistics
DoubleSummaryStatistics dblSummaryStatistics = numbers
.stream() // get sequential stream
.mapToDouble(Double::parseDouble) // mapToDouble()
.summaryStatistics(); // summaryStatistics()
// 1.2 Summary statistics
System.out.println(dblSummaryStatistics);
// 2.1 print Sum individually
System.out.println("\nSum = "
+ dblSummaryStatistics.getSum());
// 2.2 print Count individually
System.out.println("\nCount = "
+ dblSummaryStatistics.getCount());
// 2.3 print Average individually
System.out.println("\nAverage = "
+ dblSummaryStatistics.getAverage());
// 2.4 print Max value individually
System.out.println("\nMax value = "
+ dblSummaryStatistics.getMax());
// 2.5 print Min value individually
System.out.println("\nMin value = "
+ dblSummaryStatistics.getMin());
}
}
Output:
DoubleSummaryStatistics{count=5, sum=38.170000, min=0.970000,
average=7.634000, max=19.320000}
Sum = 38.17
Count = 5
Average = 7.634
Max value = 19.32
Min value = 0.97
Related Articles :
- How to create Stream
- Stream filter() method with examples
- Stream map() method with examples
- Stream flapMap method with examples
- Difference between map() and flatMap() in Stream API
- Stream forEach() method with examples
- Stream forEachOrdered() method
- Stream sorted() method with examples
- Stream count() method with examples
- Stream distinct() method with examples
- Stream min() and max() methods
- Stream skip() and limit() methods
- Stream peek() method with examples
- Stream anyMatch() method with examples
- Stream noneMatch() method with examples
- Stream allMatch() method with examples
- Stream findFirst() and findAny() methods
- Stream collect() method with examples
- Stream concat() method with examples
- Stream toArray() method with examples
- Stream reduce() method with examples
- Stream mapToInt() method with examples
- Stream mapToLong() method with examples
- Stream mapToDouble() method with examples
- Stream flatMapToInt() method with examples
- Stream flatMapToLong() method with examples
- Stream flatMapToDouble() method with examples
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 !!