Java8 mapToObj method

Hi Friends,

I am gonna to explain about mapToObj in Java8, its very basic topic.

In Java 8, mapToObj is a method available in the primitive streams (IntStream, LongStream, DoubleStream) to map each element of the stream to an object. This is particularly useful when you need to transform primitive values into their corresponding object forms or other complex types.

Usage

Here’s a detailed look at mapToObj for IntStream:

IntStream intStream = IntStream.of(1, 2, 3, 4, 5);
Stream<String> stringStream = intStream.mapToObj(Integer::toString);

In this example:

• IntStream.of(1, 2, 3, 4, 5) creates a stream of integers.
• mapToObj(Integer::toString) maps each integer to its string representation.

General Syntax

The general syntax of mapToObj is:

mapToObj(IntFunction<R> mapper)

Where IntFunction<R> is a functional interface that takes an int as input and produces an object of type R.

Example

Here’s an example to demonstrate mapToObj with a more practical scenario:

import java.util.stream.IntStream;
import java.util.stream.Stream;

public class MapToObjExample {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5};

        Stream<String> stringStream = IntStream.of(numbers)
                                               .mapToObj(n -> "Number " + n);

        stringStream.forEach(System.out::println);
    }
}

In this example:

• IntStream.of(numbers) creates a stream of integers from the array numbers.
• mapToObj(n -> "Number " + n) maps each integer to a string prefixed with "Number ".
• forEach(System.out::println) prints each string.

Explanation with Filtering Example

Let’s revisit the code you mentioned and explain it further in the context of filtering numbers that start with “1”:

import java.util.Arrays;

public class StartsWithOne {
    public static void main(String[] args) {
        int[] arr = {10, 12, 23, 34, 45, 51, 13, 16};
        Arrays.stream(arr)
              .mapToObj(s -> s + "")
              .filter(s -> s.startsWith("1"))
              .forEach(System.out::println);
    }
}

Detailed steps:

1. Convert Array to Stream: Arrays.stream(arr) creates a stream from the array arr. If arr is an int[], it produces an IntStream.
2. Mapping to Strings: .mapToObj(s -> s + "") converts each integer in the stream to its string representation.
3. Filtering: .filter(s -> s.startsWith("1")) filters out the strings that start with "1".
4. Printing: .forEach(System.out::println) prints each element of the filtered stream.

This process ensures that only the numbers starting with “1” (when converted to strings) are printed.

Example : Filtering and Printing Even Numbers as Strings

import java.util.stream.IntStream;

public class FilterEvenNumbers {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5, 6};

        IntStream.of(numbers)
                 .filter(n -> n % 2 == 0)
                 .mapToObj(Integer::toString)
                 .forEach(System.out::println);
    }
}

Output:

2
4
6

Example : Converting Integers to Their Square Strings

This example converts each integer in the stream to a string representing its square.

import java.util.stream.IntStream;
import java.util.stream.Stream;

public class SquareStrings {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5};

        Stream<String> squareStrings = IntStream.of(numbers)
          .mapToObj(n -> "Square of " + n + " is " + (n * n));

        squareStrings.forEach(System.out::println);
    }
}

Thanks,

Happy Learning.