What is the internal working of Set / HashSet in Java ?

Problem statement: Give an explanation for internal working of Set / HashSet in java.
Answer: Reference

What will be the output when you add custom elements in HashSet?

Problem statement: Given code is

1. import java.util.HashSet;
2. class A {}
3. class B {}
4. class C {}
5. public class Test {
6. public static void main(String[] args) {
7. HashSet set = new HashSet();
8. set.add(new A());
9. set.add(new B());
10. set.add(new C());
11. set.add(new A());
12. System.out.println("Elements: \n" + set);
13.   System.out.println("Size: " + set.size());
14. }
15. }

Answer:
Elements: 
[com.practice.core.C@7852e922, com.practice.core.A@15db9742, com.practice.core.B@6d06d69c, com.practice.core.A@4e25154f]
Size: 4

What will be the output when you add String object in HashSet?

Problem statement: Given code is 

1. public class Test {
2. public static void main(String[] args) {
3. HashSet<String> s = new HashSet<>();
4. s.add("A");
5. s.add("B");
6. s.add("C");
7. s.add("A");
8.   System.out.println("Elements: \n" + s);
9.   System.out.println("Size: " + s.size());
10.   System.out.println("Objects: " + s);
11. }
12. }

Answer:
Elements: 
[A, B, C]
Size: 3
Objects: [A, B, C]