Trying to understand hashCode( )

Question

Hello again, I'm trying to understand the hashCode( ) method. I have a textbook which describes it this way:
"A hashcode is a value that uniquely identifies an individual object. For example, you could use the memory address of an object as it's hashcode if you wanted. It's used as a key in the standard java.util.Hashtable[i] class."
Please forgive me if this emplies that I may be a bit slow with this, but this explanation is too vague for me to grasph what's going on here. Can someone thoroughly explain (in a non scientific way), how and why this method is used?
Thanks in advance.

Answer 1

I thought the examples I provided might give you an idea why the hashCode method is useful. I'll try one more brief one, and if that doesn't connect that spare wire to the light bulb over your head, then I'll have to agree with schapel and suggest that you study up on your CS theory.
This will be a really crocked up example, and you could skirt some of the issues with better design, but it's a valid example nonetheless.
You run a greenhouse or an arboretum or something. You've got 25,000 Tree objects (forget their subclasses for now). Trees have properties like species, name (I don't know why you'd name a tree--just bear with me), location, age, medical history. You're going to put your Trees into a java.util.Set, so you've got a collection with one reference to each tree. However, you're getting overlapping subsets of your trees from various sources--that is getAllElms() returns some trees that are also in getAllOlderThan(10). You need to combine all those results into a 1-each collection of Trees.
When you go to add an Object to the Set, the Set has to first check if that Object is already present. The Set implementation could iterate over all its elements and call equals() to see if that Object is already there. Or, it could first get the hashCode (let's say you've implemented hashCode to return the xor of the species and the age). So, maybe 100 out of your 25,000 trees are 50 yeard old Elms. Internally, if the Set implementation keeps a mapping from hashCode to an array of Trees with that hashcode, then it only has to copmare to those 100 trees that match, rather than all 25,000.
As I said, this is a cheezy example. One problem is that if trees are mutable, this Set implementation won't work, since the Set won't know that a Tree's internal values (and hence, possibly, its hashCode) have changed.
Is is making any sense yet?
Okay so please help me understand this rudimentary,
logical reason for wanting to use a hashCode( )? Say
I create a top level class called "Plant.java". Now
from that class I create a subclass called Tree.java
from which several other subclasses and objects are
created. Help me gain a basic understanding of why
and how the hashCode( ) method might be beneficial.
(lol... I feel sort of like I've opened up a cool new
electronic toy, I'm standing here with a "spare wire
in my hand wondering what the heck this part is
for?")
Thanks again for your help. I really apprecitate it.