Collect primitive array values into a collection of type Map

Question

How can I convert an array of primitive int, long, or double values into a collection of type Map?

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class PrimitiveStreamCollection {
    private static final String[] words = {
        "zero", "one", "two", "three", "four",
        "five", "six", "seven", "eight", "nine"
    };
    
    private static final int[] numbers = { 8, 6, 7, 5, 3, 0, 9 };

    public static Map<Integer, String> collect(int[] values) {
        return // Collect the array of values into a Map<Integer, String>
    }
    
    public static void main(String[] args) {
        Map<Integer, String> map = collect(numbers);
        System.out.println(map);
        // {0=zero, 3=three, 5=five, 6=six, 7=seven, 8=eight, 9=nine}
    }
}

Answer 1

In order to convert an array of primitive values to a collection; the values need to either be boxed into corresponding object types, or the collect method for the primitive stream needs to be called.

Boxing

Here are the 8 primitive types and their corresponding wrapper classes:

• bool → Boolean
• byte → Byte
• char → Character
• double → Double
• float → Float
• int → Integer
• long → Long
• short → Short

See also: https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html

In the code below, the int[] is transformed into an IntStream which is then boxed into a Stream<Integer> and collected with the Collectors.toMap collector.

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class PrimitiveStreamCollection {
    private static final String[] words = {
        "zero", "one", "two", "three", "four",
        "five", "six", "seven", "eight", "nine"
    };

    private static final int[] numbers = { 8, 6, 7, 5, 3, 0, 9 };

    public static Map<Integer, String> collectBoxed(int[] values) {
        return Arrays.stream(values)
            .boxed()
            .collect(
                Collectors.toMap(
                    Function.identity(),
                    value -> words[value]));
    }

    public static void main(String[] args) {
        Map<Integer, String> boxedMap = collectBoxed(numbers);
        System.out.println(boxedMap);
    }
}

Primitive streams

The Arrays.stream method has been overridden to accept only arrays of double, int, and long.

• double[] → DoubleStream
• int[] → IntStream
• long[] → LongStream

In the code below, the collect method is called directly on the primitive stream object. It takes three arguments:

• supplier - in this case a HashMap<Integer, String>
• accumulator - how to apply the iteree to the supplier
• combiner - how multiple values are combined

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class PrimitiveStreamCollection {
    private static final String[] words = {
        "zero", "one", "two", "three", "four",
        "five", "six", "seven", "eight", "nine"
    };

    private static final int[] numbers = { 8, 6, 7, 5, 3, 0, 9 };

    public static Map<Integer, String> collectUnboxed(int[] values) {
        return Arrays.stream(values)
            .collect(
                HashMap::new,
                (acc, value) -> acc.put(value, words[value]),
                HashMap::putAll);
    }

    public static void main(String[] args) {
        Map<Integer, String> unboxedMap = collectUnboxed(numbers);
        System.out.println(unboxedMap);
    }
}