Stream mapToLong() in Java with examples

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

Stream mapToLong(ToLongFunction mapper) is an 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(ToLongFunction<? super T> mapper)

Where, LongStream is a sequence of primitive 
long-valued elements and T is the type 
of stream elements. mapper is a stateless function 
which is applied to each element and the function
returns the new stream.

Example 1 : mapToLong() function with operation of returning stream satisfying the given function.

// Java code for Stream mapToLong
// (ToLongFunction mapper) to get a
// LongStream by applying the given function
// to the elements of this stream.
import java.util.*;

class GFG {

    // Driver code
    public static void main(String[] args)
    {

        System.out.println("The stream after applying "
                        + "the function is : ");

        // Creating a list of Strings
        List<String> list = Arrays.asList("25", "225", "1000",
                                                  "20", "15");

        // Using Stream mapToLong(ToLongFunction mapper)
        // and displaying the corresponding LongStream
        list.stream().mapToLong(num -> Long.parseLong(num))
            .filter(num -> Math.sqrt(num) / 5 == 3 )
            .forEach(System.out::println);
    }
}

Output :

The stream after applying the function is : 
225

Example 2 : mapToLong() function with returning number of set-bits in string length.

// Java code for Stream mapToLong
// (ToLongFunction mapper) to get a
// LongStream by applying the given function
// to the elements of this stream.
import java.util.*;

class GFG {

    // Driver code
    public static void main(String[] args)
    {
        // Creating a list of Strings
        List<String> list = Arrays.asList("Data Structures", "JAVA", "OOPS",
                                             "GeeksforGeeks", "Algorithms");

        // Using Stream mapToLong(ToLongFunction mapper)
        // and displaying the corresponding LongStream
        // which contains the number of one-bits in
        // binary representation of String length
        list.stream().mapToLong(str -> Long.bitCount(str.length()))
            .forEach(System.out::println);
    }
}

Output :

4
1
1
3
2