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Look at what is the amount we are going to demonstrate that you need to be very careful when using multiple

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keys that is the objects that you use for keys when they are immutable.

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You need to be very careful and that's what we're going to demonstrate.

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And this particular Demel so far is amount a new method called immutable Keys has been added to the

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map demo class.

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So let's just go ahead and file this.

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So the key has to be a mutable object.

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So let's just go ahead and create an analysis for that.

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So it's not a list off and Degesch.

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So we have added this element 1.

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Now let's create a hash map.

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The key would be list of indigenous unvalued it's just habit as indigo it doesn't matter.

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Now let's just our just do this.

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So we have added the list that we created here.

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The mutable list.

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So now let's go ahead and I add a new element to the list.

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So we already added it to to the other to the map after adding it we are changing the state we are in

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or we are mutating it and we are adding more elements.

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So let's just add one more element to.

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Now let's say map Daut get.

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OK let's do a search now and search by list.

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Now if you do this you would expect that it will return this value 1.

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But as you can see through it and send null value.

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And that's because so then we you know when we when it inserted the list here.

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It involved the hash map has worked the hash code method for inserting this mapping.

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So I applied the hash code method on the key.

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We know that it applies that and when it applied you know the hash code met third returned a particular

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Hoshko.

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OK but after that was added.

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So we are adding a new element here to the list and then when we're in working this search method and

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when you're run we are passing the key.

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The hash map will once again invoke the hash code matter.

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And when it is working here for the second time the hash code that is returned is different from the

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hash code that is written.

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Initially when we were inserting the image that is because it is dependent on the state of this object

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.

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OK so this other list actually or writes the hash code method in in the Object class.

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OK.

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So let's actually go ahead and look at it.

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So let's just let me just do an EF 3 here.

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So here as part of this glass and the Attila's class actually extends obstructionist.

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So that's where the Hoshko matter is is all and here it is.

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So here is the hash code matter.

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So this is a list hardest and the hash code the way it is computing is.

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So it is iterating through each element and it is computing the hash code.

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OK.

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So initially when it added up it is when it inserted the list which had only one element.

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So it had you know you did it at once.

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And so there was a certain Hoshko that got generated which was written and based on that the hash map

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has inserted the mapping.

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Now after that we inserted the second element and then when we were searching this method is called

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Once again but this time it is going to do it twice.

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So the hash code would be different would be different.

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OK.

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So it is using the elements off the list to compute the hash code.

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OK so that's the reason that it gets a different dashboard and the hash function well hash to a different

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bucket cancer treatment and null value here.

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So if the hash card method if we are overloading the hash code method and if our hash card method is

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dependent on the state of the the state then we need to be very careful.

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If you if we have mutated state after inserting it after inserting it into the map.

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Now this also applies to the equals method also because the equals method is used when two elements

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you know read more than one element gets added into the same bucket.

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So the class matter is used in the linked list.

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The search for a particular mapping.

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So if they close by their results are dependent on the objects date.

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Then we need to be very careful.

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OK.

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So we need to be very careful when using mutated objects.

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But if we do not write any hash code or equals matter if you are not overwriting the methods then we

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may be fine.

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OK.

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So let's also look at that for instance here.

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I created a custom class called student which is ID or name on the corresponding getters and setters

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.

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And here it is the student also has the constructor for initializing the state.

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So let's go ahead and just add some more code here.

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It's just a student

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search the ID and name for name that's passed null.

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And let me create a map of a student.

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So student object is the key and it's just how integer doesn't matter.

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It's called a map to

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some random one.

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So we added the student object you're not there to change the state.

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It's just a set name

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.

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So here the object has been notated.

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And here would be Map do not get s

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neither does it are turning this round you run.

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OK.

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So it's not good none.

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In this case that's because you know the student matter the Student class did not override either hash

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code or the equals method.

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So it is using just the just the default method from the Object class.

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Both equals as well as hash.

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And they just use the odd object identity which is the memory address.

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OK so typically that is what it does is what those methods use.

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And the object memory address remains the same.

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So those methods of doing nothing with with the object state.

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And so it would return the same value over here and also over here the hash code method would read on

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the same value at both these places.

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OK.

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So that's the reason why it was able to extract the right value even though the object got muted.

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So this is just to demonstrate that you don't need to be very careful when using mutated objects as

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keys.

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So typically you would use on the immutable objects.

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So that's about it.

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Thank you and you can find this court in the resources section.

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Happy coding.