I'm rather fresh in programming and I try to make simple app. App will allow user to make a character sheet for popular RPG game. Right now I trying to model it (on paper) and have (maybe a stupid one) question.
Each character have set of skills - a lot (33). And each skill cane have value of 0-5. Now my question is: is it better to make 33 ints insied Character object or make Class Skill and make array of Skills in Character? My knowledge of Java is not good enough to decide :( If anyone find some useful article about it or can just simply explain it to me I would be grateful.
I just hope it is not completely stupid question :)
Have a nice day!
If you have a fixed number of skills you can use multiple attributes, will increase readability when used, but will make your class a bit ugly and less mantainable.
If values are always an int, I would reccomend to use a Map instead of array or Collection.
Map<String, Integer> skills;
So you can have a class with nice declaration and refer to skills by name and not by position in the array:
Integer strenghtLevel = skills.get("strenght");
I definitely recommend to create a specific Skill class. Today you are using ints from 0 to 5; but such things tend to become more complex pretty fast.
Therefore you should carefully design the "Skill" class upfront (you might also consider to have a Skill interface; as you might face the need to have different implementations (for different skill types) at some future point).
You character class could then be using a dynamic container; for example java.util.ArrayList to hold all the various skills.
It's probably better to make a class skill, because then you can ensure each skill has a name and a value. Then, you can also do things like creating a method to print out or display the name and value, and if you want to add another attribute to the skill, it is fairly easy.
You could use an array of int elements. It should be enough, unless you want some custom logic. If the values should have states and actions, then they should be objects.
I am currently working on a project in which I am retrieving data about names from the Social Security website. Basically I'm given a number x, and years y and z. I have to return the top x names from each of the years y through z.
So the data returned from the website is a name, a rank, and a year. I have to enter each name returned into either a TreeMap, HashMap, or LinkedHashMap, but I'm not sure how to store them, because no matter what I use as the key, there might be duplicates. The year cannot be the key as I will have the top x names from each year, so they would all be from the same year. If there are multiple years, there would be several names of rank 1, etc, as there is one for each year, so that could not be the key. And the name itself could not be the key as the same name might have been in the top several names for multiple years.
I've managed to understand most of the complicated parts of this project, yet this--one of the simplest parts, I can't seem to understand!
I've heard of ways that I can use something like the year as the key and make the value a list of names or something similar, but I'm not sure how I would add values in implementations like that. I would greatly appreciate any recommendations!
Thanks so much.
Edit: Please note that I was specifically told I MUST use TreeMap, HashMap, or LinkedHashMap. I've heard of MultiMap but that's not one of my options.
I think using a hashmap with an List is what you're specifically asking for. An example of how to instantiate such an object would be:
HashMap<Integer, List<String>> myHashMap = new HashMap<Integer, List<String>>();
Note that we have to use Integer because Hashmaps only work with objects. To add values to this, you could do:
myHashMap.get([whatever year you wanted]).add("[whatever name you want]");
However, a look at this question shows this would not be quite as easy as this, as you must instantiate each List for all your key's (that question deals specifically with multidimensional hashmaps, but the premise is the same). However, it is doable, as the answer to that question demonstrates. You should have a look at it, as I think it will you help you understand what's going on with all this, but the code that might work for you could look like (taken almost directly from the answer to the linked question):
if (!myHashMap.containsKey(myYear)) {
myHashMap.put(myYear, new List<String>());
}
Edit: If you can't use the List inside either, I suppose you could put another hashmap inside, but I don't see that having much real use for this unless it's just an arbitrary requirement.
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I understand that only one instance of any object according to .equals() is allowed in a Set and that you shouldn't "need to" get an object from the Set if you already have an equivalent object, but I would still like to have a .get() method that returns the actual instance of the object in the Set (or null) given an equivalent object as a parameter.
Any ideas/theories as to why it was designed like this?
I usually have to hack around this by using a Map and making the key and the value same, or something like that.
EDIT: I don't think people understand my question so far. I want the exact object instance that is already in the set, not a possibly different object instance where .equals() returns true.
As to why I would want this behavior, typically .equals() does not take into account all the properties of the object. I want to provide some dummy lookup object and get back the actual object instance in the Set.
While the purity argument does make the method get(Object) suspect, the underlying intent is not moot.
There are various class and interface families that slightly redefine equals(Object). One need look no further than the collections interfaces. For example, an ArrayList and a LinkedList can be equal; their respective contents merely need to be the same and in the same order.
Consequently, there are very good reasons for finding the matching element in a set. Perhaps a clearer way of indicating intent is to have a method like
public interface Collection<E> extends ... {
...
public E findMatch(Object o) throws UnsupportedOperationException;
...
}
Note that this API has value broader that within Set.
As to the question itself, I don't have any theory as to why such an operation was omitted. I will say that the minimal spanning set argument does not hold, because many operations defined in the collections APIs are motivated by convenience and efficiency.
The problem is: Set is not for "getting" objects, is for adding and test for presence.
I understand what are you looking for, I had a similar situation and ended using a map of the same object in key and value.
EDIT: Just to clarify: http://en.wikipedia.org/wiki/Set_(abstract_data_type)
I had the same question in java forum years ago. They told me that the Set interface is defined. It cannot be changed because it will break the current implementations of Set interface. Then, they started to claim bullshit, like you see here: "Set does not need the get method" and started to drill me that Map must always be used to get elements from a set.
If you use the set only for mathematical operations, like intersection or union, then may be contains() is sufficient. However, Set is defined in collections to store data. I explained for need get() in Set using the relational data model.
In what follows, an SQL table is like a class. The columns define attributes (known as fields in Java) and records represent instances of the class. So that an object is a vector of fields. Some of the fields are primary keys. They define uniqueness of the object. This is what you do for contains() in Java:
class Element {
public int hashCode() {return sumOfKeyFields()}
public boolean equals(Object e) {keyField1.equals(e) && keyField2.equals(e) && ..}
I'm not aware of DB internals. But, you specify key fields only once, when define a table. You just annotate key fields with #primary. You do not specify the keys second time, when add a record to the table. You do not separate keys from data, as you do in mapping. SQL tables are sets. They are not maps. Yet, they provide get() in addition to maintaining uniqueness and contains() check.
In "Art of Computer Programming", introducing the search, D. Knuth says the same:
Most of this chapter is devoted to the study of a very simple search
problem: how to find the data that has been stored with a given
identification.
You see, data is store with identification. Not identification pointing to data but data with identification. He continues:
For example, in a numerical application we might want
to find f(x), given x and a table of the values of f; in a
nonnumerical application, we might want to find the English
translation of a given Russian word.
It looks like he starts to speak about mapping. However,
In general, we shall suppose that a set of N records has been stored,
and the problem is to locate the appropriate one. We generally require
the N keys to be distinct, so that each key uniquely identifies its
record. The collection of all records is called a table or file,
where the word "table" is usually used to indicate a small file, and
"file" is usually used to indicate a large table. A large file or a
group of files is frequently called a database.
Algorithms for searching are presented with a so-called argument, K,
and the problem is to find which record has K as its key. Although the
goal of searching is to find the information stored in the record
associated with K, the algorithms in this chapter generally ignore
everything but the keys themselves. In practice we can find the
associated data once we have located K; for example, if K appears in
location TABLE + i, the associated data (or a pointer to it) might be
in location TABLE + i + 1
That is, the search locates the key filed of the record and it should not "map" the key to the data. Both are located in the same record, as fileds of java object. That is, search algorithm examines the key fields of the record, as it does in the set, rather than some remote key, as it does in the map.
We are given N items to be sorted; we shall call them records, and
the entire collection of N records will be called a file. Each
record Rj has a key Kj, which governs the sorting process. Additional
data, besides the key, is usually also present; this extra "satellite
information" has no effect on sorting except that it must be carried
along as part of each record.
Neither, I see no need to duplicate the keys in an extra "key set" in his discussion of sorting.
... ["The Art of Computer Programming", Chapter 6, Introduction]
entity set is collection or set all entities of a particular entity type
[http://wiki.answers.com/Q/What_is_entity_and_entity_set_in_dbms]
The objects of single class share their class attributes. Similarly, do records in DB. They share column attributes.
A special case of a collection is a class extent, which is the
collection of all objects belonging to the class. Class extents allow
classes to be treated like relations
... ["Database System Concepts", 6th Edition]
Basically, class describes the attributes common to all its instances. A table in relational DB does the same. "The easiest mapping you will ever have is a property mapping of a single attribute to a single column." This is the case I'm talking about.
I'm so verbose on proving the analogy (isomorphism) between objects and DB records because there are stupid people who do not accept it (to prove that their Set must not have the get method)
You see in replays how people, who do not understand this, say that Set with get would be redundant? It is because their abused map, which they impose to use in place of set, introduces the redundancy. Their call to put(obj.getKey(), obj) stores two keys: the original key as part of the object and a copy of it in the key set of the map. The duplication is the redundancy. It also involves more bloat in the code and wastes memory consumed at Runtime. I do not know about DB internals, but principles of good design and database normalization say that such duplication is bad idea - there must be only one source of truth. Redundancy means that inconsistency may happen: the key maps to an object that has a different key. Inconsistency is a manifestation of redundancy. Edgar F. Codd proposed DB normalization just to get rid of redundancies and their inferred inconsistencies. The teachers are explicit on the normalization: Normalization will never generate two tables with a one-to-one relationship between them. There is no theoretical reason to separate a single entity like this with some fields in a single record of one table and others in a single record of another table
So, we have 4 arguments, why using a map for implementing get in set is bad:
the map is unnecessary when we have a set of unique objects
map introduces redundancy in Runtime storage
map introduces code bloat in the DB (in the Collections)
using map contradicts the data storage normalization
Even if you are not aware of the record set idea and data normalization, playing with collections, you may discover this data structure and algorithm yourself, as we, org.eclipse.KeyedHashSet and C++ STL designers did.
I was banned from Sun forum for pointing out these ideas. The bigotry is the only argument against the reason and this world is dominated by bigots. They do not want to see concepts and how things can be different/improved. They see only actual world and cannot imagine that design of Java Collections may have deficiencies and could be improved. It is dangerous to remind rationale things to such people. They teach you their blindness and punish if you do not obey.
Added Dec 2013: SICP also says that DB is a set with keyed records rather than a map:
A typical data-management system spends a large amount of time
accessing or modifying the data in the records and therefore requires
an efficient method for accessing records. This is done by identifying
a part of each record to serve as an identifying key. Now we represent
the data base as a set of records.
Well, if you've already "got" the thing from the set, you don't need to get() it, do you? ;-)
Your approach of using a Map is The Right Thing, I think. It sounds like you're trying to "canonicalize" objects via their equals() method, which I've always accomplished using a Map as you suggest.
I'm not sure if you're looking for an explanation of why Sets behave this way, or for a simple solution to the problem it poses. Other answers dealt with the former, so here's a suggestion for the latter.
You can iterate over the Set's elements and test each one of them for equality using the equals() method. It's easy to implement and hardly error-prone. Obviously if you're not sure if the element is in the set or not, check with the contains() method beforehand.
This isn't efficient compared to, for example, HashSet's contains() method, which does "find" the stored element, but won't return it. If your sets may contain many elements it might even be a reason to use a "heavier" workaround like the map implementation you mentioned. However, if it's that important for you (and I do see the benefit of having this ability), it's probably worth it.
So I understand that you may have two equal objects but they are not the same instance.
Such as
Integer a = new Integer(3);
Integer b = new Integer(3);
In which case a.equals(b) because they refer to the same intrinsic value but a != b because they are two different objects.
There are other implementations of Set, such as IdentitySet, which do a different comparison between items.
However, I think that you are trying to apply a different philosophy to Java. If your objects are equal (a.equals(b)) although a and b have a different state or meaning, there is something wrong here. You may want to split that class into two or more semantic classes which implement a common interface - or maybe reconsider .equals and .hashCode.
If you have Joshua Bloch's Effective Java, have a look at the chapters called "Obey the general contract when overriding equals" and "Minimize mutability".
Just use the Map solution... a TreeSet and a HashSet also do it since they are backed up by a TreeMap and a HashMap, so there is no penalty in doing so (actualy it should be a minimal gain).
You may also extend your favorite Set to add the get() method.
[]]
I think your only solution, given some Set implementation, is to iterate over its elements to find one that is equals() -- then you have the actual object in the Set that matched.
K target = ...;
Set<K> set = ...;
for (K element : set) {
if (target.equals(element)) {
return element;
}
}
If you think about it as a mathematical set, you can derive a way to find the object.
Intersect the set with a collection of object containing only the object you want to find. If the intersection is not empty, the only item left in the set is the one you were looking for.
public <T> T findInSet(T findMe, Set<T> inHere){
inHere.retainAll(Arrays.asList(findMe));
if(!inHere.isEmpty){
return inHere.iterator().next();
}
return null;
}
Its not the most efficient use of memory, but its functionally and mathematically correct.
"I want the exact object instance that is already in the set, not a possibly different object instance where .equals() returns true."
This doesn't make sense. Say you do:
Set<Foo> s = new Set<Foo>();
s.Add(new Foo(...));
...
Foo newFoo = ...;
You now do:
s.contains(newFoo)
If you want that to only be true if an object in the set is == newFoo, implement Foo's equals and hashCode with object identity. Or, if you're trying to map multiple equal objects to a canonical original, then a Map may be the right choice.
I think the expectation is that equals truely represent some equality, not simply that the two objects have the same primary key, for example. And if equals represented two really equal objects, then a get would be redundant. The use case you want suggests a Map, and perhaps a different value for the key, something that represents a primary key, rather than the whole object, and then properly implement equals and hashcode accordingly.
Functional Java has an implementation of a persistent Set (backed by a red/black tree) that incidentally includes a split method that seems to do kind of what you want. It returns a triplet of:
The set of all elements that appear before the found object.
An object of type Option that is either empty or contains the found object if it exists in the set.
The set of all elements that appear after the found object.
You would do something like this:
MyElementType found = hayStack.split(needle)._2().orSome(hay);
Object fromSet = set.tailSet(obj).first();
if (! obj.equals(fromSet)) fromSet = null;
does what you are looking for. I don't know why java hides it.
Say, I have a User POJO with ID and name.
ID keeps the contract between equals and hashcode.
name is not part of object equality.
I want to update the name of the user based on the input from somewhere say, UI.
As java set doesn't provide get method, I need to iterate over the set in my code and update the name when I find the equal object (i.e. when ID matches).
If you had get method, this code could have been shortened.
Java now comes with all kind of stupid things like javadb and enhanced for loop, I don't understand why in this particular case they are being purist.
I had the same problem. I fixed it by converting my set to a Map, and then getting them from the map. I used this method:
public Map<MyObject, MyObject> convertSetToMap(Set<MyObject> set)
{
Map<MyObject, MyObject> myObjectMap = new HashMap<MyObject, MyObject>();
for(MyObject myObject: set){
myObjectMap.put(myObject, myObject);
}
return myObjectMap
}
Now you can get items from your set by calling this method like this:
convertSetToMap(myset).get(myobject);
You can override the equals in your class to let it check on only a certain properties like Id or name.
if you have made a request for this in Java bug parade list it here and we can vote it up. I think at least the convenience class java.util.Collections that just takes a set and an object
and is implemented something like
searchSet(Set ss, Object searchFor){
Iterator it = ss.iterator();
while(it.hasNext()){
Object s = it.next();
if(s != null && s.equals(searchFor)){
return s;
}
}
This is obviously a shortcoming of the Set API.
Simply, I want to lookup an object in my Set and update its property.
And I HAVE TO loop through my (Hash)Set to get to my object... Sigh...
I agree that I'd like to see Set implementations provide a get() method.
As one option, in the case where your Objects implement (or can implement) java.lang.Comparable, you can use a TreeSet. Then the get() type function can be obtained by calling ceiling() or floor(), followed by a check for the result being non-null and equal to the comparison Object, such as:
TreeSet myTreeSet<MyObject> = new TreeSet();
:
:
// Equivalent of a get() and a null-check, except for the incorrect value sitting in
// returnedMyObject in the not-equal case.
MyObject returnedMyObject = myTreeSet.ceiling(comparisonMyObject);
if ((null != returnedMyObject) && returnedMyObject.equals(comparisonMyObject)) {
:
:
}
The reason why there is no get is simple:
If you need to get the object X from the set is because you need something from X and you dont have the object.
If you do not have the object then you need some means (key) to locate it. ..its name, a number what ever. Thats what maps are for right.
map.get( "key" ) -> X!
Sets do not have keys, you need yo traverse them to get the objects.
So, why not add a handy get( X ) -> X
That makes no sense right, because you have X already, purist will say.
But now look at it as non purist, and see if you really want this:
Say I make object Y, wich matches the equals of X, so that set.get(Y)->X. Volia, then I can access the data of X that I didn have. Say for example X has a method called get flag() and I want the result of that.
Now look at this code.
Y
X = map.get( Y );
So Y.equals( x ) true!
but..
Y.flag() == X.flag() = false. ( Were not they equals ?)
So, you see, if set allowed you to get the objects like that It surely is to break the basic semantic of the equals. Later you are going to live with little clones of X all claming that they are the same when they are not.
You need a map, to store stuff and use a key to retrieve it.
I understand that only one instance of any object according to .equals() is allowed in a Set and that you shouldn't "need to" get an object from the Set if you already have an equivalent object, but I would still like to have a .get() method that returns the actual instance of the object in the Set (or null) given an equivalent object as a parameter.
The simple interface/API gives more freedom during implementation. For example if Set interface would be reduced just to single contains() method we get a set definition typical for functional programming - it is just a predicate, no objects are actually stored. It is also true for java.util.EnumSet - it contains only a bitmap for each possible value.
It's just an opinion. I believe we need to understand that we have several java class without fields/properties, i.e. only methods. In that case equals cannot be measured by comparing function, one such example is requestHandlers. See the below example of a JAX-RS application. In this context SET makes more sense then any data structure.
#ApplicationPath("/")
public class GlobalEventCollectorApplication extends Application {
#Override
public Set<Class<?>> getClasses() {
Set<Class<?>> classes = new HashSet<Class<?>>();
classes.add(EventReceiverService.class);
classes.add(VirtualNetworkEventSerializer.class);
return classes;
}
}
To answer your question, if you have an shallow-employee object ( i.e. only EMPID, which is used in equals method to determine uniqueness ) , and if you want to get a deep-object by doing a lookup in set, SET is not the data-structure , as its purpose is different.
List is ordered data structure. So it follows the insertion order. Hence the data you put will be available at exact position the time you inserted.
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
list.get(0); // will return value 1
Remember this as simple array.
Set is un ordered data structure. So it follows no order. The data you insert at certain position will be available any position.
Set<Integer> set = new HashSet<>();
set.add(1);
set.add(2);
set.add(3);
//assume it has get method
set.get(0); // what are you expecting this to return. 1?..
But it will return something else. Hence it does not make any sense to create get method in Set.
**Note****For explanation I used int type, this same is applicable for Object type also.
I think you've answered your own question: it is redundant.
Set provides Set#contains (Object o) which provides the equivalent identity test of your desired Set#get(Object o) and returns a boolean, as would be expected.
I have a program that I am trying to make a Markov text generator for. I plan on splitting some text up at a set interval and then storing that into a class. The problem that I don't know how to solve is how to handle naming the instances of the class I am going to make. I was planning on generating the instances in a for loop. The user will pass the method some amount of text (the length of which is not known beforehand). Pseudo-code below:
create vector for sets and tail letter;
for (int c = 0; c < text.length; c++) {
Check to make sure overflow doesnt happen;
Create instance of set named c;
store set and tailLetter into vector;
}
public class set {
String characters;
char tailLetter;
}
I'm sorry if that's not clear enough. I'm teaching myself Java and this is my first post here.
If you are learning Java, I'd suggest that you first focus on how to model the problem with Java's classes and methods.
A Markov Chain is a model or statistical elaboration of the seed text, right? Using it to model a text, it normally describes how often each word is followed by each other word. (normally you'd split the text on word boundaries). That feels like it needs a class; it might be called MarkovChain.
Within the MarkovChain class, you need something to hold each word that occurs in the text, and maps that word to the other words in the text, and the count of frequency of those other words.
Suppose the word is 'and'. In the text, 'and' is followed by "the" four times, and "then" 3 times. So you'd need some data structure to hold something like this:
and -->
the (4)
then (3)
One way to do this is to use an ArrayList to hold all words, then a Map<T1,T2> that holds the relationship between words and the frequency of following words.
In this case T1 is probably a string, and the T2 is probably an ArrayList of pairs - a string and the (integer) count for that string.
But wait, now you don't need the base ArrayList<> to store the words, because they are just the keys in the map.
...and so on. The next step would be to figure out how to populate that data structure. That's probably an internal (private) method that gets called when a caller instantiates the MarkovChain class with a seed text.
Probably you also want that MarkovChain class to expose another method, a public one, that callers invoke when they want to generate some random sequence from the chain, relying on probabilities based on the frequency counts.
...
This is just one way to think about the modelling of the problem.
Anyway I would focus on that modelling/design exercise, before writing code.
Can't you use a Map<String, Set> where the key is the generated name?
You can use an ArrayList to manage the instances. I like the Map idea better so you can dynamically set the names instead of trying to access instances by an index number.
I don't see the point of the names:
If they are just so that 'set' objects will have some distinct String for debugging, the default toString() implementation will give you that.
If you specifically need to do lookup of these 'set' objects, then a numeric identifier or a sequence number will work better.
If you explained the purpose of the names, and how you intend to use them, maybe we could give you better advice.
I want to store some objects and then be able to retrieve them later as efficiently as possible. I will also remove some of them under certain conditions. It seems a hash map would be the right choice.
But, from what I've seen, hash maps always associate a value with another? For example, "john" and "555-5555", his phone number.
Now, my situation. Suppose I have a bunch of people, and each person is connected to other people. So, I need each person to store its contacts.
What I'm doing is have each person have a hashmap, and then I'd add to the hash otherPerson, otherPerson. Basically, the key is the value. Am I doing it wrong?
EDIT I don't think the HashSet would solve my problem because I have to retrieve the value to update it and there is no get method. Remove returns a boolean, so I can't even remove it to put it back again, which would probably be a bad idea anyway.
If all you need is checking if A is one of B's contacts, then Set is choice. It has contains() for that purpose.
Otherwise, the most suitable might be Map, as you need efficient retrieval operation. You said currently you use same object as key and value, but I'm not sure how you get the the key in the first place. Say you'd like to get contact A from B's contacts, and you use something like 'B.contacts.get(A)', where do you get A from? If you already have A, what's for to get it from the map again? (maybe there are multiple instances of the same person?)
Unless there are multiple instances of the same person, I'd say for each Person, define a ID like unique attribute, and use that as the key for the contacts map. Also, do you define equal()/hashCode() for person class? Map/Set uses hashCode() and equal() for finding the match. Depending on your usage, you might need to consider rewrite them for efficiency.
I don't think the HashSet would solve my problem because I have to retrieve the value to update it and there is no get method.
This is a puzzling statement. Why would you want to retrieve a value using a get method to update it? Surely, if you know which object you need to retrieve from the set/map, you don't need to retrieve it.
For example:
HashSet<Person> relations = ...
Person p = ...
if (relations.remove(p)) {
// we removed an object such that p.equals(obj) is true.
}
Now if you are worried that the object that was removed was equal to, but not identical to p, it seems to me that something is wrong with your design. Either:
you should not be creating multiple Person instances that are equal, or
you should not be caring that Person instances are not identical, or
you should not have overridden equals(Object).
In short, the problem is that you are not managing object identity properly.
Well, the data structure you'd be looking for here, would be a HashSet (or some other kind of set), I think (if your framework/library offers it). A set just says "I have the following items" instead of "I have the following items mapped to the following values". Which would be what you're modeling here.
As for HashSet vs. other implementations (if present): That all depends on what you're doing. If you need fast lookup, i. e. "is this element in the set?" questions, then hashing is a good thing. Other underlying data structures are perhaps better optimized for other set operations, such as union, intersection, etc.
A hash table/map simply requires that you have a way to get the values you're interested in looking up later; that's what the key is for.
However, in your specific case, it sounds like you're looking for a way to store relationships between people, and what you're keeping track of is whether or not person A has a relationship with person B. A better representation for that sort of thing is an adjacency list.
Am I missing something or don't you simply need an ArrayList<Person>?
I would just store the contacts in a List<Person>. E.g.
public class Person {
private List<Person> contacts;
}
With regard to editing the individual contact, it is really not the parent Person's responsibility to do that. It should at highest add/remove contacts. You can perfectly do that by contacts.add(otherPerson) or contacts.remove(otherPerson).
When you want to edit an individual Person, which may be one of the contacts, just get a handle to it independently, e.g. personDAO.find(personId) and then update it accordingly. It's actually also the Person's own responsibility to edit own details. With a good ORM under the hood, the changes will be reflected in the contact list of other Persons.
If you need to iterate through the people, or require them to have ordering, consider TreeMap or TreeSet instead of hashing.