Here's a small example where we override a hashset's equals and hashcode functiosn in order to determind uniqueness ourselves

HashSetPractice.java

import java.util.HashSet;

/**
 * Assume two cows are the same if they have the same name.
 * Write class Cow and the necessary methods to make main this work, and put a print statement inside each method.
 * Then on the lines next to each call to add, write what the methods would print.
 */
class HashSetPractice {
	/*
	 * On the comment lines provided, write *all* that is printed as a
	 * result of each line of code.
	 */
	public static void main(String[] args) {
		HashSet<Cow> herd = new HashSet<Cow>();
		herd.add(new Cow("A")); //Making cow A. Hashing cow A. 
		System.out.println();
		herd.add(new Cow("Alba")); //Making cow Alba. Hashing cow Alba. 
		System.out.println();
		herd.add(new Cow("A")); //Making cow A. Hashing cow A. Checking if cow A equals A. 
		System.out.println();
		herd.add(new Cow("Aga")); //Making cow Aga. Hashing cow Aga. 
		System.out.println();
		herd.add(new Cow("A")); //Making cow A. Hashing cow A. Checking if cow A equals A. 
		System.out.println();
		System.out.println(herd.size()); //3
	}
}

class Cow {
	String name;
	
	Cow(String name) {
		System.out.printf("Making cow %s. ", name);
		this.name = name;
	}
	
	@Override
	public int hashCode() {
		System.out.printf("Hashing cow %s. ", name);
		return this.name.hashCode();
	}
	
	@Override
	public boolean equals(Object object) {
		if (object instanceof Cow) {
			Cow cow = (Cow) object;
			System.out.printf("Checking if cow %s equals %s. ", this.name, cow.name);
			return this.name.equals(cow.name);
		} else {
			return false;
		}
	}
}