What is the best way to convert an int array to String in Java?

Question

What is the best method to convert {1, 2, 3, 4, 5} for "12345" ?

Example:

int[] array = {1, 2, 3, 4, 5};
array.toString(); // Existe alguma função que faça isso?

I know you can do it with a for loop but maybe you have better options.

int[] array = {1, 2, 3, 4, 5};
String s = ""; // Sei que tem StringBuilder também
for (int i : array) {
    s += i;
}

Answer 1

Here’s a code that can help you:

import java.util.stream.IntStream;

public class Main {
    public static void main(String[] args) {
        int[] array = {1, 2, 3, 4, 5};
        String resposta = IntStream.of(array).mapToObj(String::valueOf).reduce("", String::concat);
        System.out.println(resposta);
    }
}

See here working on Ideone.

His idea is to first convert the array into one IntStream and then in a Stream of Strings, using the String.valueOf(int) to convert int in String. Finally, the reduce concatenates all the Stringresulting s (using the method concat in one.

Although to be honest, probably the solution using a for and a StringBuilder will be the one with the best performance.

Answer 2

I think the best way in both legibility and efficiency is to use StringBuilder even, thanks for the help there people.

int[] array = {1, 2, 3, 4, 5};
StringBuilder sb = new StringBuilder();
for (int i : array) sb.append(i);

Answer 3

If you are using Java 8, you can use stream:

    String str = Arrays.stream(intArray).mapToObj(String::valueOf)
        .reduce((a, b) -> a.concat(b)).get();

If you need a char to concatenate you can use the Concat

    a.concat(",").concat(b)

Answer 4

Just to complement the other answers, from Java 8 you can use String.join. But as this method should receive one or more instances of CharSequence and int's are not CharSequence's, we still have to convert the array of int to an array of String:

int[] array = { 1, 2, 3, 4, 5 };
String s = String.join("", IntStream.of(array).mapToObj(String::valueOf).toArray(String[]::new));
System.out.println(s); // 12345

Although in this case, I still prefer the solution that uses a loop and a StringBuilder, that is better than concatenation with + (and both in turn, are faster than streams).

---

Other option (which only works if the array values are all numbers between 0 and 9) is to build a number with the array digits and at the end convert it to String:

int[] array = { 1, 2, 3, 4, 5 };
int n = 0;
for (int i : array) {
    n *= 10;
    n += i;
}
String result = String.format("%0" + array.length + "d", n); // 12345

I used String.format to put the zeros on the left, if the first elements of the array are 0 (because if I just print the number directly, it is shown without the zeroes on the left). I also use the array size (array.length) so that the amount of digits shown is the same amount as in the array (so an array like {0, 0, 0, 1} results in the string 0001).