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HashMap Java Tutorial



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hashmaps were one of those topics that

were pretty confusing to me even just

the name itself hashmap like how am I

supposed to know what that does well it

turns out that hash maps are nothing

more than a list of keys and values and

different keys have different values

this is my hash map Java tutorial hey

what is up it's Alex back again helping

you learn Java on this channel I make a

new Java tutorial that's fresh and new

and exciting that you can check out

every single week so if you're new here

then consider subscribing so let's kick

it off by going into our program editor

I'll just go to file new Java project

hash map cuts tutorial and then open it

up right click on source new class call

it hash map awesome this awesomeness

public static void they finished

Java is really great at storing data

like we can store integer variables like

this into a equals 10 we can store

string variables like this string b

equals hello and we could store a really

long list of data too so we can have an

array of numbers c and then set that

equal to some list of numbers 1 2 3 and

this solves the problem of storing one

value into one variable named a is 10 b

is hello c is a list of 1 2 3 but what

would we do if we had 1 million of these

variables say we had a bunch of integers

like we have another integer B that's 3

we have another integer see that's like

88 how could we store a is 10 B is 3 C

is 88 into 1 easily accessible place

this is where hashmaps come in the word

hash map itself was a little confusing

to me and I honestly don't know why it's

called that but I'm going to show you

what it is and how you can use it so to

store our a b and c into a hash map we

would just type hash map with a capital

H and M we'll name it

- like happy because it makes us very

happy that we're finally getting to

learn when hashmaps are and set that

equal to new hashmap with parenthesis

and a semicolon hover over the name

hashmap and click this import hashmat

Java util this will auto generate this

import statement at the very top and

just tells Java to bring in the code for

a hatch map so we can use it to put a b

and c and their corresponding values

into happy we just type the name of the

hashmap we're just happy type a dot to

bring up everything that this hash map

can do and this can do quite a bit of

things for us but we're gonna say put

and hit enter it auto generated some

stuff for us but i'm just gonna delete

that where ki was I'm gonna put the

string inside of double quotes and I'm

going to put the integer amount 10 into

the second one after the comma and then

I'm gonna put a semicolon now inside of

happy there's an an e that's equal to 10

I'm gonna do the other ones and then I'm

gonna show you more of how it works

so we'll do happy dot put B is value

three and happy dot put C is value 88

you'll notice that there are a lot of

yellow underlines here these first three

are just saying that a B and C aren't

being used but that's okay these ones if

you hover over it it says hash map is a

raw type references to generic type hash

map Cavey should be parameterised this

just means it wants to know the type of

the key and the value what's on the left

side of the parenthesis is the key or

the name and on the right is what is

stored into it so to tell it what type

our values are we put some less than and

greater than sign and then our first one

is a string since it's in double code so

we'll say string and a comma and the

second one is

and integer so we say capital integer it

has to be capital integer and not

lowercase integer because this is a

primitive type it turns purple it's a

key word and to say the actual type we

have to uppercase it it's kind of

confusing but this will get it to work

for you finally we could just copy this

and put it over on this side so now it

knows that every key is a string and

every value is an integer let's see that

this is working by printing out our hash

map go to system dot out dot print line

and just type the name of our hash map

save it and run it and we will see a is

equal to 10 B is equal to 3 and C is 88

which is pretty awesome everything that

we did happy put as the key on the left

so a and the value on the right 10 so if

we want to get the value of C I'm just

going to go in here we're happy is I'm

going to have a dot and we can get the

value of C by typing get and replace key

with C save it and run it you'll see

that it returns 88 which is what we have

here now let's do another example where

we do some more advanced things to our

hash map let's make it by just typing

hash map well name it fun and say equals

new hash map with our parentheses and

semicolon now let's do like a user and a

password which are two strings user is

usually a string password usually a

string and these yellow underlines are

saying it wants to know exactly what

type we know that so we'll just type

string comma string now I'll do the same

over here I'll just make this a little

wider so you can see more to add our

users and passwords is super easy we

just type the new or

SMAP a dot and then put will say our

first user is um Bobby Jo 1996 and the

password name is fluffy ponies like that

now we'll add another user func put

Hello Kitty fan 21 value and put one

more pass phrase like password once you

three we print out everything just

typing turned out fun just down with an

E save it running and we get it just

like before except now it's with two

strings you can remove elements from a

hashmap by typing the name of the

hashmap

dot remove and then type the name of in

this case the user so let's say we want

to remove Hello Kitty fan 21 just copy

type that into there save it and run it

and now you see when we print out

everything that's no longer in fun the

user Hello Kitty fan 21 and the password

are no longer in the hashmap

what you can also do is see if a hashmap

contains a certain value so you can say

fun dot contains value and we'll see if

it has the value password 1 2 3 in there

let's paste that in there save it and it

returns true if it is in there and false

if it's not in there so in this case it

is in there but if I said password 1 2 4

then it would be false notice how if I

do cool guy swag into here and save it

and run it it still brings back false

because this is saying if fun contains

the value

and the values are always on the right

but cool guy swag is on the left that's

actually a key but we can change this to

contains key save it and run it in that

war returned true this is really useful

inside an if statement so you can say if

fun dot contains key cool guy swag then

we know the cool guy swag users in there

and we can do different things you can

also get the size and other things like

that so we'll do fun dot size and we get

three you can even replace so say a user

changes a password we could do fun that

replace will say Bobby Joe 1996 will

place his password with better password

like that if we run it and print out fun

you'll see that this first statement fun

that replace returns the old value

fluffy ponies right here but when we

print out everything you'll see that

Bobby Jo 1996 has his new password

notice how the order is different this

is a big difference between hash maps

and ArrayList hashmaps don't really have

an order can't rely on the specific

order of a hash map only through the

keys and values but an ArrayList you can

get the certain index based on its

position well you can also do is get all

of the values of being fun dot Val use

let's delete that last one we get all

the values and you can do the same with

keys but doing key set computer science

classes really like hash maps because it

sort of tests your brain knowledge of

how these data structures are working

they're really confusing to me just just

from the name hash map alone honestly

but they're really not as bad as you

think it's just a long list of things on

the left

that are set to the thing on the right

and you can access all of them by doing

hashmat dot what I showed you before so

if you enjoyed this hashmap java

tutorial let me know by leaving a like

and i'll see you all in the next video

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