Melwin Jose: 2016

Experimenting how to use Objects as elements in HashSet.
The code below explores how hashCode() and equals() functions are used in HashSet and how to override them so that you can tweak it for your own needs.

Comments contain the explanation and check the output to see when hashCode() and equals() functions are called.

	import java.util.HashSet;
	import java.util.Iterator;
	import java.util.Set;

	public class LearningHashSet {

	private static class Element {
	int roll_no;
	public Element(int r){
	roll_no=r;
	}

	// hashCode() is used to find the bucket
	@Override
	public int hashCode() {
	System.out.println("hashCode for "+roll_no);
	return roll_no%10;
	}

	// equals() is used to check if objects
	// are equal
	@Override
	public boolean equals(Object obj) {
	System.out.println("equals("+roll_no+","+((Element)obj).roll_no+")");
	return roll_no==(((Element)obj).roll_no);
	}

	/*
	The contract between equals() and hashCode() is:
	- If two objects are equal, then they must have the same hash code.
	- If two objects have the same hash code, they may or may not be equal.

	NOTE : HashSet is backed by HashMap, so the Keys are the Elements
	and values are all references to the SAME dummy objects.
	Highlighted in the image
	*/

	/* Default equals()
	public boolean equals(Object obj) {
	// checks if obj are equal
	return super.equals(obj);
	}
	*/
	}

	public static void main(String[] args) {
	Set<Element> ts = new HashSet<Element>();

	System.out.println("Going to add : new Element(1)");
	if(ts.add(new Element(1))) System.out.println("new Element(1) added");
	// hashCode = 1, goes to bucket-1

	System.out.println("\nGoing to add : new Element(11)");
	if(ts.add(new Element(11))) System.out.println("new Element(11) added");
	// hashCode = 1, goes to bucket-1
	// Since the objects the different insertion is successful

	System.out.println("\nGoing to add : new Element(2)");
	if(ts.add(new Element(2))) System.out.println("new Element(2) added");
	// hahsCode = 2, goes to bucket-2

	System.out.println("\nGoing to add : Element three");
	Element three = new Element(3);
	if(ts.add(three)) System.out.println("three added");
	// hahsCode = 3, goes to bucket-3

	System.out.println("\nGoing to add : Element three");
	if(ts.add(three)) System.out.println("three added");
	// hahsCode = 3, goes to bucket-3
	// but since the elements are equal(same roll-no), insertion fails
	// Would also FAIL with the default equals(), as the same objects are used

	System.out.println("\nGoing to add : new Element(3)");
	if(ts.add(new Element(3))) System.out.println("new Element(1) added");
	// hahsCode = 3, goes to bucket-3
	// but since the elements are equal(same roll-no), insertion fails
	// Would PASS with the default equals(), as different objects are used

	System.out.print("\nSet : ");
	Iterator<Element> i = ts.iterator();
	while(i.hasNext()){
	Element current = i.next();
	System.out.print(current.roll_no+" ");
	}
	}

	}