DoubleStream mapToLong() in Java

Namachila November 8, 2025

0 3 2 minutes read

DoubleStream mapToLong() returns a LongStream consisting of the results of applying the given function to the elements of this stream.

Note : DoubleStream mapToLong() is a intermediate operation. These operations are always lazy. Intermediate operations are invoked on a Stream instance and after they finish their processing, they give a Stream instance as output.
Syntax :

LongStream mapToLong(DoubleToLongFunction mapper)

Parameters :

DoubleStream : A sequence of primitive double-valued elements. This is the double primitive specialization of Stream.
mapper : A stateless function to apply to each element.

Return Value : The function returns a LongStream consisting of the results of applying the given function to the elements of this stream.

Example 1 :

// Java code for LongStream mapToLong
// (DoubleToLongFunction mapper)
import java.util.*;
import java.util.stream.LongStream;
import java.util.stream.DoubleStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating a DoubleStream
        DoubleStream stream = DoubleStream.of(3.3, 6.5, 16.4,
                                              8.7, 10.4);
  
        // Using LongStream mapToLong(DoubleToLongFunction mapper)
        // to return a LongStream consisting of the
        // results of applying the given function to
        // the elements of this stream.
        LongStream stream1 = stream.mapToLong(num -> (long)num);
  
        // Displaying the elements in stream1
        stream1.forEach(System.out::println);
    }
}

Output :

Example 2 :

// Java code for LongStream mapToLong
// (DoubleToLongFunction mapper)
import java.util.*;
import java.util.stream.LongStream;
import java.util.stream.DoubleStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating a DoubleStream
        DoubleStream stream = DoubleStream.of(3.3, 6.5, 16.4,
                                              8.7, 10.4);
  
        // Using LongStream mapToLong(DoubleToLongFunction mapper)
        // to return a LongStream consisting of the
        // results of applying the given function to
        // the elements of this stream.
        LongStream stream1 = stream.mapToLong(num -> (long)num - 10);
  
        // Displaying the elements in stream1
        stream1.forEach(System.out::println);
    }
}

Output :

-7
-4
6
-2
0

Example 3 :

// Java code for LongStream mapToLong
// (DoubleToLongFunction mapper)
import java.util.*;
import java.util.stream.LongStream;
import java.util.stream.DoubleStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating a DoubleStream
        DoubleStream stream = DoubleStream.of(3.3, 6.5, 16.4,
                                              8.7, 10.4);
  
        // Using LongStream mapToLong(DoubleToLongFunction mapper)
        // to return a LongStream consisting of the
        // results of applying the given function to
        // the elements of this stream.
        LongStream stream1 = stream.mapToLong(num -> (long)(2 * num * num));
  
        // Displaying the elements in stream1
        stream1.forEach(System.out::println);
    }
}