To start off, I would like to know what exactly is a record in comparison to an array? From what I'm trying to understand from my textbook, "Gilles Dowek, Principles of Programming Languages", it's almost like an array but needs labels and values associated to these labels. The example given is:
labels: {latitude, longitude, altitude}
record: {latitude = 48.715, longitude = 2.208, altitude = 156}
What also confuses me is how one implements this 2-D matrix with a single record (which I assume is a single row vector like the example above).
I have only a year experience with programming in general so any sort of readings or sources with a descriptive answer would be greatly appreciated.
Array can only store values which you can access by indices, while record assigns one label to one value (you can access value using a key).
You can use a Map to store multiple records with different keys, example:
HashMap<String, Double> map = new HashMap<>();
map.put("latitude", 48.715);
map.put("longitude", 2.208);
map.put("altitude", 156.0);
Then, when you want to get a value:
double latitude = map.get("latitude");
To answer my own question, (with the resources provided in the comments to my question) a record is simply a Java object that has instance variables, but no instance methods. Using this knowledge, an array is very different from a record. The comparison would essentially be between an object class and an array.
Implementing a 2-D matrix, or more specifically, visually implementing the matrix so it looks like a square when printed is not the record's job, as this would require instance methods which isn't part of a record.
The solution to implementing an nxn matrix using a single record would simply be initializing "n times n" instance variables within a class. In general, the solution to making any record would be to initialize any required number of instance variables within a class.
Before we start, I should mention that I'm trying to do this in Android, though I believe it is not relevant to what I am trying to resolve in this question specifically.
I should also mention that I have seen similar things asked already, though they did not exactly help me in what I am trying to do. While some of them have helped me get a general idea of how it should be done, in the end, I've been left helpless by the too-specific problems the other users had.
I believe it would also be useful to clarify that I am relatively new to programming in Java, with very little experience or knowledge of complex data structures.
What I am trying to do is as follows:
Say I have a class named WordThing, which contains two strings, called name and color, and an int called picture (which is used for retrieving the ID for an imagebutton). I also have a separate char called uno, whose value is a. There is an instance of WordThing called a, with its values properly assigned. I want to ask if it'd be possible to call one of the values of WordThing a using the value of uno, like this:
getValue(uno).picture
While I am aware that the above example is, in many ways, incorrect, nevertheless am I asking whether such a thing would be possible.
To clarify, in reality there are more chars than just uno, namely four, all with different letters, who are subject to randomization. My intention is to use these random letters to call upon four distinct WordThing instances, whose values will then be used in altering some elements of a given View, like this:
upperleftButton.setImageResource(getValue(uno).picture);
Any kind of help is appreciated.
What you are looking for is a Map, also know as an Associative Array or dictionary. Maps are used to store items in key/value pairs, where the key can be used to look up the value within a map. In java (and android) you can use a map like this.
Map<String, WordThing> wordThingMap = new HashMap<String, WordThing>();
// Add some items to the map
wordThingMap.put("a", new WordThing());
wordThingMap.put("b", new WordThing());
// Do some other stuff
someSuperSpecialMethod();
// Retrieve values from the map
WordThing a = map.get("a");
As a side note, you should probably think about re-naming some of your variables. WordThing isn't really that descriptive, and variable names like "a" and "b" will become very difficult to work with very quickly. For some tips on naming variables, see this question for a start.
Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 4 years ago.
Improve this question
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.
This question already has answers here:
How to return multiple values? [duplicate]
(3 answers)
Closed 3 years ago.
This is a small issue, as I could easily whip up a pair class to do the job. I don't really want to do this though, and I feel like there should be some simple, built-in, java-like way of returning two values. What do you guys is the best, simplest way of doing this? Arrays? Some other data structure?
As far as I know, there is unfortunately no built-in representation of a pair in Java (and I certainly wish there was). Personally, when I code a project where I find that a pair class often would be useful, I create a generic Pair<T, U> class (which is probably what you were thinking of). Returning an array is a fast and simple way, but you might come to regret it later, because people who use your method will wonder whether the method might at some point return more than two values.
Whichever solution you choose: whenever you feel that you need a Pair, you should consider whether the time saved today by using e.g. a generic Pair class really is worth the loss of information to the next person who reads the code (and that person may well be you in six months). Writing a separate class for the return type takes more time now, but it would convey more information to those that use your method (namely, it tells the users what the return value represents, and contains useful member names for the two values). If it is a non-public method that is used only a few places, though, a Pair is more acceptable.
Using a container class is the easiest way.
public class Pair<T, U> {
public final T t;
public final U u;
public Pair(T t, U u) {
this.t= t;
this.u= u;
}
}
The closest thing I've seen to a "pair" in the standard libraries are the Map.Entry interface and the AbstractMap.SimpleEntry and AbstractMap.SimpleImmutableEntry classes that implement it.
If both objects are the same class an array is easier to use.
Apache Commons Lang3 provides an abstract Pair class with a couple implementations including ImmutablePair and MutablePair.
Three approaches, all not so great:
Roll your own Pair<A, B>. You said you didn't want to do that.
Return a Object[]. This is not type safe.
Mimic out variables or pointers by supplying single element arrays as parameters.
An example of #3:
public boolean getUserDetails(String userId, String[] lastName, String[] firstName, Date[] dob) {
assert lastName != null && lastName.length == 1;
assert firstName != null && firstName.length == 1;
assert dob != null && dob.length == 1;
...
}
The third option makes life painful for the caller.
So like I said, no nice solution.
As an aside, Scala uses various Tuple classes (up to 21-tuple, from what I remember) to help you with this.
There is a pair class in JavaFX, but you shouldn't use it. What you SHOULD use is something like this:
// We've skipped imports and package declarations
public final class YourClass {
/* Assume there is a bunch of stuff here */
// I don't know what method you're using, so forgive the silly example
public YourClass.Pair sillyExampleOfPairs(String someString) {
return new YourClass.Pair(someString, someString.length() * 13);
}
#Value // Lombok annotation.
public static class Pair {
String text;
int integer;
}
// this is an even more succinct possibility
#Value public static final class ShorterPair {String text; int integer}
}
While the name Pair here is obviously not that well chosen, and you should choose a more descriptive name, the obvious ways this will work (the fields are final private and you have a getter on each, because of the annotation), should not be lost on you. And while yes, this is slightly more wordy than using Pair, it's much more robust. What if you do need to add an extra parameter to the return value? You "only" need to change this class then. And you can update all the relevant JavaDocs immediately, which is also nice. If you have to change types, they would both entail similar amounts of work.
As long as you're only adding stuff, the old getText() and getInteger() methods would keep working as they did before. You also avoid having to add Yet Another Dependency to your projects. It's not a big win. Having Pair available is nice for prototyping, but it's not nice for later.
My final theoretical CS-y argument is that Pair is the same type as Pair. But if you have a Phonebook.Entry (with String and int) and say, Inventory.Item (with a name and a number of items we currently have inventoried), these two are very distinct types, which do very distinct things. You can't put one into the other. This is a Good Thing.
It's also much clearer for us poor bastards that have to go and debug your systems to see something like "com.name.project.something.something.Phonebook.Entry" in a stack trace than "org.apache.commons.lang3.tuple.Pair". One of these tells me WHAT I'm supposed to be looking at, and gives me some info on WHY I'm seeing a pair. The other says... nothing.
Now you might not care that you have to type for 3 extra seconds to save me 3 minutes. But I choose to believe in the goodness of your heart, and the nobility of your soul. Therefore, do the right thing.
Write a small static class instead.
I have been told by experts that when faced with the question of pairs, one of two things is true:
You need to rethink your structure (this blunt answer doesn't help anyone)
You need to build your own class to hold the pair
I would suggest that the second case is not all that abnormal. However, if what you are doing seems too trivial for introducing a new class, then using a Map could work, as others have suggested. If you are simply sending a single response back, then a Map seems like a bit much.
If a list of pairs sounds like it would work, and you need to maintain order, you could try a LinkedHashMap so that order is maintained.
if both are integers then I would advise a java.awt.Point but otherwise just create a container class with two objects x and y
Some observation of mine:
Array is bulit-in, fast and easy to use, although imposible to expand its capacity. What if you want 3 values to be returned after 3 months?
ArrayList/other colletions can be good, allows you to increment the capacity(initially 10).
Note that Vector can be overkill in comparison to ArrayList when you only want to store 2 values to be fetched later. Map also can be good because it's always sorted and ordered.
Some user-defined class: maybe an option if is meaningful(means that the data returned is important-ish to be a Java Bean), and you want to store more than just 2 integers into it. Readibility is better in case you add more notes in its Javadoc. Can be expanded as you like, just add fields in this class. Slower, but safer.