HashMaps or ArrayList?

Brett Basinger · Jun 7, 2016

Hey I have a Question, for my plugin (kits), I was wondering if I should use a hashmap instead of an arraylist because I am trying to make it so that one a player who performs a command can get certain things... -Example- I want it so that only the kit vipers(Poison sword) diamond sword, is the only diamond that does that. Well I am running into a problem where every kit can do that now. I am using the arraylist (thought it would seperate it) but would a hashmap do that??

Sorry I am learning java, my only source of help is from youtube. And I learning java from bukkit.

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

JBoss925 · Mar 30, 2014

If u want, use a hash map and store the player's name and then the string of the class. For example:

The player's name is JBoss925 he chose viper kit

So store the key as: "JBoss925"

and store the value as: "vipers"

now when JBoss925 goes to do /kit tank

get the key "JBoss925" and get the value "vipers".

check if the output "vipers" is equal to "tank"

since it is not don't allow him the kit.

now he does /kit viper

get the key "JBoss925" and get the value "vipers".

check if the output "vipers" is equal to "vipers"

since it is allow him the kit.

That simple really.

jthort · Mar 30, 2014

Hashmap:

A hashmap works like this (this is a little bit simplified, but it illustrates the basic mechanism):
It has a number of "buckets" which it uses to store key-value pairs in. Each bucket has a unique number - that's what identifies the bucket. When you put a key-value pair into the map, the hashmap will look at the hash code of the key, and store the pair in the bucket of which the identifier is the hash code of the key. For example: The hash code of the key is 235 -> the pair is stored in bucket number 235. (Note that one bucket can store more then one key-value pair).
When you lookup a value in the hashmap, by giving it a key, it will first look at the hash code of the key that you gave. The hashmap will then look into the corresponding bucket, and then it will compare the key that you gave with the keys of all pairs in the bucket, by comparing them with equals().
Now you can see how this is very efficient for looking up key-value pairs in a map: by the hash code of the key the hashmap immediately knows in which bucket to look, so that it only has to test against what's in that bucket.
Looking at the above mechanism, you can also see what requirements are necessary on the hashCode() and equals() methods of keys:

If two keys are the same (equals() returns true when you compare them), their hashCode()method must return the same number. If keys violate this, then keys that are equal might be stored in different buckets, and the hashmap would not be able to find key-value pairs (because it's going to look in the same bucket).

If two keys are different, then it doesn't matter if their hash codes are the same or not. They will be stored in the same bucket if their hash codes are the same, and in this case, the hashmap will use equals() to tell them apart.

Click to expand...

ArrayList:

Java ArrayList represents an automatic re-sizable array and used in place of array. Since we can not modify size of an array after creating it, we prefer to use ArrayList in Java which re-size itself automatically once it gets full. ArrayList in Java implements List interface and allow null. Java ArrayList also maintains insertion order of elements and allows duplicates opposite to any Set implementation which doesn't allow duplicates. ArrayList supports both Iterator and ListIterator for iteration but it’s recommended to use ListIterator as it allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the Iterator's current position in the list. But while using ListIterator you need to be little careful because ListIterator has no current element; its cursor position always lies between the element that would be returned by a call to previous () and the element that would be returned by a call to next (). In this Java ArrayList tutorial we will see how to create Java ArrayList and perform various operations on Java ArrayList. This collection class is also favorites on many core Java interviews with questions like Difference between ArrayList and Vector or LinkedList vs ArrayList.

Read more:http://javarevisited.blogspot.com/2011/05/example-of-arraylist-in-java-tutorial.html#ixzz2xUtLnYV4
Click to expand...

Take a look at http://javarevisited.blogspot.com/2011/05/example-of-arraylist-in-java-tutorial.html and http://stackoverflow.com/questions/6493605/how-does-a-hashmap-work-in-java for more information or a more detailed explanation.

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HashMaps or ArrayList?

Brett Basinger

JBoss925

jthort

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HashMaps or ArrayList?

Brett Basinger

JBoss925

jthort

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