Where are collections' toString implementations located?

Question

Java

Here is a sample code. A TreeSet collection is created and printed to the console. The output is the string [1, 2, 3, 4, 5, 9].
Where can I see the implementation of the toString() method in TreeSet for example? Passed from ctrl to set.toString(), but the method inherited from Object is shown. Where does this output in square brackets come from?
In general, tell me what and how with these toString(). Either I drove a little, or I really can’t understand.

import java.util.Arrays;
import java.util.Set;
import java.util.TreeSet;

public class App {
    public static void main(String ...args) {
        Set<Integer> set = new TreeSet<>(Arrays.asList(1, 2, 3, 4, 5));
        
        System.out.println(set); // [1, 2, 3, 4, 5]
    }
}

Answer 1

src.zip:java/util/AbstractCollection.java