Why doesn't the String method concat work in a for loop?

Question

I am in the process of learning. I will be using StringBuilder for this problem, but this is bothering me.

The objective is to make a String from an array of Strings. The concat method doesn't work but the addition operator works.

This works:

public static void main(String[] args) {
    String[] arr= {"1","2","3"};
    String output ="";
    for(String str: arr)
        output+=str;
    System.out.println(output);
}

But this doesn't concat method doesn't allow me to concatenate a String:

public static void main(String[] args) {
    String[] arr= {"1","2","3"};
    String output ="";
    for(String str: arr)
        output.concat(str);
    System.out.println(output);
}

Answer 1

The concat method doesn't mutate the current String, but in fact returns a new String containing the result.

Use it like this:

output = output.concat(str);

String.concat documentation.

Answer 2

You need to assign the return of concat.

for(String str: arr)
    output = output.concat(str);

The method concat() returns a String of the concatenated results you just need to assign it to output.