An example of pathological behaviour of TreeSet when ordering is inconsistent with equals

TreeSetExample.java

import java.util.Comparator;
import java.util.TreeSet;
// Blog post: http://david-soroko.blogspot.co.uk/2015/05/a-kink-in-api.html
public class TreeSetExample {

    public static class Transaction {
        private final String id;
        private final int amount;

        public Transaction(String id, int amount) {
            this.id = id;
            this.amount = amount;
        }

        public int getAmount() {
            return amount;
        }

        @Override
        public int hashCode() {
            return id != null ? id.hashCode() : 0;
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || !(o instanceof Transaction)) return false;
            return id.equals(((Transaction) o).id);
        }
    }

    public static class TransactionComparator implements Comparator<Transaction> {
        @Override
        public int compare(Transaction t1, Transaction t2) {
            return t1.getAmount() - t2.getAmount();
        }
    }

    public static void main(String[] args) {
        TreeSet<Transaction> set = new TreeSet<>(new TransactionComparator());
        Transaction t1 = new Transaction("T1", 100);
        Transaction t2 = new Transaction("T2", 100);

        System.out.printf("set did not already contain t1: %b\n", set.add(t1)); // set did not already contain t1: true
        System.out.printf("set did not already contain t2: %b\n", set.add(t2)); // set did not already contain t2: false
        System.out.printf("|set| = %d\n", set.size()); // |set| = 1

        System.out.printf("t1 is a member of set: %b\n", set.contains(t1)); // t1 is a member of set: true
        System.out.printf("t2 is a member of set: %b\n", set.contains(t2)); // t2 is a member of set: true
    }
}