A question About overriding equals and hashcode for an Object

[Ravi Kiran Va]

Hello ,
when going through tutorials in web i found this line :

"The Collection classes that implement interface Set or Map
expect the object or key object respectively to override equals "
I need some clarification on the above statement .
Please let me know if the above is true ,because as far as i am concerned i could easily add Objects to a HashSet
without my Object overriding equals or hashcode .

This is my sample Program :

		Employee emp1 = new Employee();
		emp1.setName("Ravi");

		Employee emp2 = new Employee();
		emp2.setName("Ravi");

		Set set = new HashSet();

		set.add(emp1);
		set.add(emp2);

		Iterator itr = set.iterator();

		while (itr.hasNext()) {
			Employee emp = (Employee) itr.next();
			System.out.println(emp.getName())

public class Employee {

	String name;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
}

Please clarify .

Thanks

[Campbell Ritchie]

Please always tell us where you found such quotes.

Of course you need not override equals() and hashCode(). As long as you don't mind your collection classes working completely incorrectly . . .

If you want them to work properly, however, you will find that quote is absolutely correct.

[Avishkar Nikale]

Overriding equals & hashCode are important when it comes to determining the "identity" of an instance.

You can search the forum & find quite a few topics on the same.

You can also go through SCJP study guide or Head First Java for elaborate details.

[Ravi Kiran Va]

Hi Cambpell ,

Of course you need not override equals() and hashCode(). As long as you don't mind your collection classes working completely incorrectly . . .

This was really funny , could you please add some light on this or provide me a link related to collection classes working of Incorrectly .

Thank you .

[Paul Sturrock]

This should help you understand.

[Campbell Ritchie]

There should be explanations about collections working incorrectly in some of these links: Odersky, Spoon and Venners (courtesy of Garrett Rowe), Angelika Langer and Joshua Bloch's Effective Java™ (you may be able to find a "sample chapter" from the 1st edition on the internet, which describes equals()).

[Ravi Kiran Va]

Thanks Campbell Ritchie and Rob Prime for always as helping hand

[Ravi Kiran Va]

Hi , After going through tutorials you mentioned , i got some idea when to overide equals and HashCode of an Object class .

I assume that , when we are using any Hashing type of Collection class and storing Custom objects under it then we need to override them , if not the searching is not done correctly .

Correct me , if i am wrong .

Thanks in advance .

[Campbell Ritchie]

You should override equals() and hashCode() on every class you intend for serious use, as opposed to trivial training classes.

[Ravi Kiran Va]

on every class you intend for serious use,

Can you please let me know what is the word "for serious use" you mean exactly .

Thanks .

[Dieter Quickfend]

Basically, if you're going to publish your classes and let other people use them... override equals() and hashCode().

[Ravi Kiran Va]

Basically, if you're going to publish your classes and let other people use them... override equals() and hashCode().

, thanks david , every project is developed for that only know .

[Campbell Ritchie]

Ravi Kiran V wrote: , thanks david . . . .

His name's Dieter

[Jaikiran Pai]

Ravi Kiran V wrote:

every project is developed for that only know .

Not really. I have many projects (or rather just some collection of code) which I use as quick examples for trying out various things. i.e. they are not meant for "serious" use.