Collect method for streams doesn't work as expected

Question

I'm testing the collect method for streams. In this code I'm trying to build a single string from a stream of string by using a StringBuilder in the first case and concatenation of String in the second case. I am not understanding why the second case produces an empty string.

    String[] strs = new String[] {"aaaaa", "bbbbbbb", "c", "dd"};
    
    
    StringBuilder sb = Arrays.stream(strs).collect(StringBuilder::new, StringBuilder::append, StringBuilder::append);
    
    System.out.println(sb);
    
    
    String s = Arrays.stream(strs).collect(String::new, String::concat, String::concat);
    
    System.out.println(s);

Answer 1

Well, the reason it doesn't work is that String is immutable.

String::new is a Supplier<String> that returns an empty String .

And your accumulator and combiner have no way of mutating it, since String is immutable.

Your code has the same effect as:

String s = new String ();
for (String str : strs) {
    s.concat(str);
}

The calls to concat produce new String s, and don't affect the value of the result String .

Since String is immutable, you can use immutable reduction (ie use reduce instead of collect ):

String s = Arrays.stream(strs).reduce("",String::concat);

This will have the same result as:

String s = "";
for (String str : strs) {
    s = s.concat(str);
}