We Keep Coding

What is the benefit of using a custom class over a map? [duplicate]

This question already has answers here:
Class Object vs Hashmap
(3 answers)
Closed 3 years ago.
I have some piece of code that returns a min and max values from some input that it takes. I need to know what are the benefits of using a custom class that has a minimum and maximum field over using a map that has these two values?
//this is the class that holds the min and max values
public class MaxAndMinValues {
private double minimum;
private double maximum;
//rest of the class code omitted
}
//this is the map that holds the min and max values
Map<String, Double> minAndMaxValuesMap

The most apparent answer would be Object Oriented Programming aspects like the possibility to data with functionality, and the possibility to derive that class.
But let's for the moment assume, that is not a major factor, and your example is so simplistic, that I wouldn't use a Map either. What I would use is the Pair class from Apache Commons: https://commons.apache.org/proper/commons-lang/javadocs/api-3.1/org/apache/commons/lang3/tuple/Pair.html
(ImmutablePair):
https://commons.apache.org/proper/commons-lang/javadocs/api-3.1/org/apache/commons/lang3/tuple/ImmutablePair.html
The Pair class is generic, and has two generic types, one for each field. You can basically define a Pair of something, and get type safety, IDE support, autocompletion, and the big benefit of knowing what is inside. Also a Pair features stuff that a Map can not. For example, a Pair is potentially Comparable. See also ImmutablePair, if you want to use it as key in another Map.
public Pair<Double, Double> foo(...) {
// ...
Pair<Double, Double> range = Pair.of(minimum, maximum);
return range;
}
The big advantage of this class is, that the type you return exposes the contained types. So if you need to, you could return different types from a single method execution (without using a map or complicated inner class).
e.g. Pair<String, Double> or Pair<String, List<Double>>...

In simple situation, you just need to store min and max value from user input, your custom class will be ok than using Map, the reason is: in Java, a Map object can be a HashMap, LinkedHashMap or and TreeMap. it get you a short time to bring your data into its structure and also when you get value from the object. So in simple case, as you just described, just need to use your custom class, morever, you can write some method in your class to process user input, what the Map could not process for you.

I would say to look from perspective of the usage of a programming language. Let it be any language, there will be multiple ways to achieve the result (easy/bad/complicated/performing ...). Considering an Object oriented language like java, this question points more on to the design side of your solution.
Think of accessibility.
The values in a Map is kind of public that , you can modify the contents as you like from any part of the code. If you had a condition that the min and max should be in the range [-100 ,100] & if some part of your code inserts a 200 into map - you have a bug. Ok we can cover it up with a validation , but how many instances of validations would you write? But an Object ? there is always the encapsulation possibilities.
Think of re-use
. If you had the same requirement in another place of code, you have to rewrite the map logic again(probably with all validations?) Doesn't look good right?
Think of extensibility
. If you wanted one more data like median or average -either you have to dirty the map with bad keys or create a new map. But a object is always easy to extend.
So it all relates to the design. If you think its a one time usage probably a map will do ( not a standard design any way. A map must contain one kind of data technically and functionally)
Last but not least, think of the code readability and cognitive complexity. it will be always better with objects with relevant responsibilities than unclear generic storage.
Hope I made some sense!

The benefit is simple : make your code clearer and more robust.
The MaxAndMinValues name and its class definition (two fields) conveys a min and a max value but overall it makes sure that will accept only these two things and its class API is self explanatory to know how to store/get values from it.
While Map<String, Double> minAndMaxValuesMap conveys also the idea that a min and a max value are stored in but it has also multiple drawbacks in terms of design :
we don't know how to retrieve values without looking how these were added.
About it, how to name the keys we we add entries in the map ? String type for key is too broad. For example "MIN", "min", "Minimum" will be accepted. An enum would solve this issue but not all.
we cannot ensure that the two values (min and max) were added in (while an arg constructor can do that)
we can add any other value in the map since that is a Map and not a fixed structure in terms of data.
Beyond the idea of a clearer code in general, I would add that if MaxAndMinValues was used only as a implementation detail inside a specific method or in a lambda, using a Map or even an array {15F, 20F} would be acceptable. But if these data are manipulated through methods, you have to do their meaning the clearest possible.

We used custom class over Hashmap to sort Map based on values part

Java: Wrapping objects in some type of collection to store duplicates in a set

I want to make a set of some type of collection (not sure which one yet) as a way of "storing duplicates" in a set. For example if I wanted to add the integer 5 with 39 additional copies I could put it into an arraylist at index 39. Thus if I were to get the size of the arraylist, I would know how many copies of 5 existed within the set.
There are a few other ways I could implement this but I have yet to decide on one. The main issue I'm having with implementing this is that I'm not sure how I can "dynamically" make arraylists (or whatever collection I may end up using) so that whenever someone were to call mySet.add(object), the object is first inserted into a unique arraylist then into the set itself.
Can anyone give me some ideas on how I could approach this?
EDIT:
Sorry I should have been more clear in my question. The point of the code that I'm writing is that we have a set-like collection that allows duplicates. And yes some of the associated methods will be re-written/will have to be re-written. Also my code should be written under the assumption that we do not know what type of object is being inserted(only one data type per set though) nor how many instances of the same object will be added nor how many different unique objects will be added.

I would rather go for using a Map like
HashMap list <Object, Integer>
where Object is the Object that you want to count and Integer is the count

You could try guava's MultiSet, I think it's what you want.
It can store the count of each object. What you need to do is just
multiSet.put(object);
And if it is put for the first time, like you said, a new list will be created, or its count will added by one.

can a Map also be a Collection?

I'd like to have a Map that is also a Collection. Or more specifically, I'd like to be able to iterate over the entries in a Map, including the case where there are multiple entries for a particular key.
The specific problem I'm trying to solve is providing an object that can be used in jstl both to iterate over using c:forEach and in an expression like ${a.b.c}. In this example, I'd want ${a.b.c} to evaluate to the the first value of c (or null if there are none), but also be able to iterate over all cs with <c:forEach items="${a.b.c}"> and have the loop body see each individual value of c in turn, although they have the same key in the Map.
Looking at things from a method point of view, this should be straightforward, just provide a Map implementation whose entrySet() method returns a set with multiple Entries with the same key. But since this seems to violate the contract of a Map, will things break in subtle yet disastrous ways? Has anyone else done this sort of thing?
(If you guessed I'm trying to present xml, you'd be correct)
EDIT
Please note that this is for use in jstl, so whatever interface I present must meet 2 conditions:
for use with the [] and . operators, it must be a Map, List, array or JavaBeans object (and of those it can't be a List or array because the indexes will not be numbers)
for use with forEach it must be an array, Collection, Iterator, Enumeration, Map, or String.
So I guess the real question is, can I count on jstl only calling .containsKey(), .get(), and .entrySet() and not caring about invariants being violated, and not internally making a copy of the Map which would not preserve the special iteration behavior.

What you are looking for is a Multimap. Guava provides an implementation of it and specifically you are looking for ArrayListMultimap.

I barely remember jstl, but what you're saying sounds a kind of controversial:
In foreach:
here ${a.b.c} should point to some container of values and then we iterate over it.
On the other hand you say, ${a.b.c} "should evaluate to the the first value of c" (or null...)
Its an ambiguous definition.
If you feel like Multimap is not what you want, you can provide your own collection implementation (probably internally based on Multimap)
Just as an idea you can always look at a single element as a list (that accidentally
is comprised of one element). This way you would resolve your ambiguity, I guess.
I hope this helps

Having a Map with multiple entries for the same key irreparably breaks the Map contract. If Multimap doesn't work for you, then there's no way to do this without breaking a lot of things.
Specifically, if you pass your monstrosity to something that's specified to take a Map, it'll almost certainly break...and it sounds like that's what you want to do with it, so yeah.

how about you use a Map with Collections as values? then you can have different values for the same key and you can iterate over them by a nested foreach-loop
you can also easily write a wrapper for an existing map-implementation, which gives you a single iterator over all values, if you need it that way

Java - remove last known item from HASHMAP on MAP!s

OK so this is a BIT different. I have a new HashMap
private Map<String, Player> players = new HashMap<String, Player>();
How do I remove last known item from that? Maybe somethign like this?
hey = Player.get(players.size() - 1);
Player.remove(hey);

The problem is, a HashMap is not sorted like a list. The internal order depends on the hashCode() value of the key (e.g. String). You can use a LinkedHashMap which preserves the insert order. To remove the last entry on this you can use an iterator in combination with a counter which compares to the size and remove the last entry.
It's so easy. Try this:
Map<String, Player> players = new LinkedHashMap<String, Players>();
List<String> list = new ArrayList<String>(players.keySet());
map.remove(list.get(list.size()-1));

I'm a little bit confused. First of all, you're saying that you've got a new ArrayList and you're illustrating this with a line that creates a new HashMap. Secondly, does the Player class really have static methods like get(int) and remove(Object)?
HashMap doesn't have a particular order, ArrayList (as any other List) does.
Removing from an ArrayList
If you've got a list of players, then you can do the following:
private List<Player> players = new ArrayList<Player>();
// Populate the list of players
players.remove(players.size() - 1);
Here, I've used the remove(int) method of List, which allows to remove an item at an arbitrary index.
Removing from a HashMap
If you've got a map of players, there's no such thing as "the last item". Sure, you can iterate over the map and one of the items will pop out last, but that doesn't mean anything. Therefore, first you have to find out what you want to remove. Then you can do the following:
private Map<String, Player> players = new HashMap<String, Player>();
// Populate the map of players
// Find the key of the player to remove
players.remove(toRemove);
Here, I've used the remove(Object) method of Map. Note that in order to remove some key-value pair, you have to show the key, not the value.

There's no "first" and "last" in a HashMap. It's unordered. Everything is accessible by its key, not by index.

You cannot delete from HashMap like that. You need to use LinkedHashMap.

Simple, just do something of this effect.
1) Get a keyset iterator;
2) Create a Key somelastKey = null
3) Iterate through the iterator and assigning somelastKey until iterator finishes.
4) finally, do players.remove(somelastKey);
Bear in mind that HashMap is unordered, it depends on Object's hashCode to determine insertion order.
Instead of using HashMap, try using LinkedHashMap which keeps a predictable iteration order.
Hope this helps....

You'll probably have to extend HashMap, override put so that it caches the key, and then create a new method that just removes the key that was cached.
Unfortunately, this will only let you remove the most recently added. If you need to remove the most recently added multiple times (without inserting in-between the removes), you're out of luck.
In that case, I'd probably do the same overrides, just write the keys to a List. So you'd have both a list and a Map.

When adding:
String key; Player value;
lastKey = key;
map.put(key, value);
//...later...
Player lastAdded = map.remove(lastKey);
Other than that there's really no way without using a LinkedHashMap or in some way creating your own wrapper map or extending HashMap.

You shouldn't be using a raw hashmap anywhere because things like this happen.
Get in the habit of wrapping your collections in business logic classes.
See, in your case right now you need to associate these two related variables--your hashmap and a "Last entered" item so you can remove it.
If you need to remove the last item from some other class, you need to pass both items.
Any time you find yourself passing 2 or more items together into more than one API, you are probably missing a class.
Create a new class that contains the hashmap and a "lastAdded" variable. Have put and remove methods that are just forwarded to the hashmap, but the put method would also set the lastAdded variable.
Also be sure to add a removeLast() method.
NEVER allow access to your hashmap outside this class, it needs to be completely private (this is what I mean by wrapped). In this way you can ensure it doesn't get out of sync with the lastAdded variable (also completely private).
Just to reiterate getters and setters for these variables would be a terrible idea (as they are with nearly all actual OO code).
You will quickly find a bunch of other methods that NEED to be in this class in order to access data inside your hashmap--methods that never felt right in their current location. You will probably also notice that those methods always have an additional parameter or two passed in--those parameters should probably be members of your new class.
Once you get in the habit of doing actual OO design (via refactoring in this case), you'll find your code MUCH more manageable. To illustrate this point, if you find later that you need multiple levels of "delete last", it will be TRIVIAL to add to your class because it will be extremely clear exactly what methods can modify your hashtable and where your new "stack" of lastItems should be located--in fact it's probably a 2 line code change.
If you do not make this wrapper class, various locations will each have code to set "lastAdded" when they add code to the hashtable. Each of those locations will have to be modified, some may be in other classes requiring you to pass your new stack around with the hashtable. It will be easier to get them out of synch if you forget to change one location.

Extract elements from list based on object property type

Often, I have a list of objects. Each object has properties. I want to extract a subset of the list where a specific property has a predefined value.
Example:
I have a list of User objects. A User has a homeTown. I want to extract all users from my list with "Springfield" as their homeTown.
I normally see this accomplished as follows:
List users = getTheUsers();
List returnList = new ArrayList();
for (User user: users) {
if ("springfield".equalsIgnoreCase(user.getHomeTown())
returnList.add(user);
}
I am not particularly satisfied with this solution. Yes, it works, but it seems so slow. There must be a non-linear solution.
Suggestions?

Well, this operation is linear in nature unless you do something extreme like index the collection based on properties you expect to examine in this way. Short of that, you're just going to have to look at each object in the collection.
But there may be some things you can do to improve readability. For example, Groovy provides an each() method for collections. It would allow you to do something like this...
def returnList = new ArrayList();
users.each() {
if ("springfield".equalsIgnoreCase(it.getHomeTown())
returnList.add(user);
};

You will need a custom solution for this. Create a custom collection such that it implements List interface and add all elements from original list into this list.
Internally in this custom List class you need to maintain some collections of Map of all attributes which can help you lookup values as you need. To populate this Map you will have to use introspection to find list of all fields and their values.
This custom object will have to implement some methods as List findAllBy(String propertyName, String propertyValue); that will use above hash map to look up those values.
This is not an easy straightforward solution. Further more you will need to consider nested attributes like "user.address.city". Making this custom List immutable will help a lot.
However even if you are iterating list of 1000's of objects in List, still it will be faster so you are better off iterating List for what you need.

As I have found out, if you are using a list, you have to iterate. Whether its a for-each, lambda, or a FindAll - it is still being iterated. No matter how you dress up a duck, it's still a duck. As far as I know there are HashTables, Dictionaries, and DataTables that do not require iteration to find a value. I am not sure what the Java equivalent implementations are, but maybe this will give you some other ideas.

If you are really interested in performance here, I would also suggest a custom solution. My suggestion would be to create a Tree of Lists in which you can sort the elements.
If you are not interested about the ordering of the elements inside your list (and most people are usually not), you could also use a TreeMap (or HashMap) and use the homeTown as key and a List of all entries as value. If you add new elements, just look up the belonging list in the Map and append it (if it is the first element of course you need to create the list first). If you want to delete an element simply do the same.
In the case you want a list of all users with a given homeTown you just need to look up that list in the Map and return it (no copying of elements needed), I am not 100% sure about the Map implementations in Java, but the complete method should be in constant time (worst case logarithmic, depending on the Map implementation).

I ended up using Predicates. Its readability looks similar to Drew's suggestion.
As far as performance is concerned, I found negligible speed improvements for small (< 100 items) lists. For larger lists (5k-10k), I found 20-30% improvements. Medium lists had benefits but not quite as large as bigger lists. I did not test super large lists, but my testing made it seem the large the list the better the results in comparison to the foreach process.