java – How does this code work?

Question:

public class Test1 {

    public static void main(String[] args) {
        String s1 = "hello";
        int mas[] = new int[3];

        test(mas);
        test2(s1);

        System.out.println(mas[0]);
        System.out.println(s1);
    }

    static void test(int massive[]) {
        massive[0] = 1;
    }

    static void test2(String string) {
        string.concat("111");
    }
}

In both cases, for reference types, a copy of the object reference is passed. But only in the case of an array, we change this object. And in the case of a string, a new object is not created in place of a variable (I remember that the string itself cannot be changed).

Why? What do I not understand?

Answer:

The String.concat() method concatenates the current string with an argument and returns the resulting string. The original string is not modified .

It looks like this schematically

class String {
  public String concat(String arg) {
    return this + arg;
  }
}