tostring() is implicitly called… how?

Question

In the following code, how is toString() is implicitly called?

class Payload {
    private int weight;
    public Payload (int w) {
        weig         


        

Answer 1

This is just a language feature which is available for free. See Concatenating strings section:

Such a concatenation can be a mixture of any objects. For each object that is not a String, its toString() method is called to convert it to a String.