So imagine I have two instances of a class:
public class MyClass {
public void sayHello() {
System.out.println("Hello");
}
}
a = new MyClass();
b = new MyClass();
Now I add those to another object, such as:
public class OtherClass {
private ArrayList<MyClass> myClsList = new ArrayList<>();
public void add(MyClass obj) {
myClsList.add(obj);
}
public void remove(MyClass obj) {
// ????
}
}
c = new OtherClass();
c.add(a);
c.add(b);
Now I want to remove one specific instance e.g
c.remove(a);
Could I just iterate over them and test for equality, I mean this should theoretically work, since the two instances have distinct "internal pointers"?
I guess using a HashMap based approach would be more efficient, but what can I use as an key there (suppose I can't add unique instance ids or something).
EDIT: There is some confusion as to what exactly I'd like to know.
The key here is that I'd like to know if there is any way of removing that specific instance from c's ArrayList or whatever Aggregator Object I might use, just by providing the respective object reference.
I imagine this could be done by just keeping the ArrayList and testing for equality (although I'm not a 100% sure) but it would be cleaner if it was possible without iterating through the whole list.
I'd just like to know if anything of the like is possible in Java. (I know how to workaround it by using additional information but the clue is to just have the respective object reference for filtering/ retrieving purposes.
You can use a.toString(), according to the Java doc,
The toString method for class Object returns a string consisting of
the name of the class of which the object is an instance, the at-sign
character `#', and the unsigned hexadecimal representation of the hash
code of the object.
This should give you an unique identifier for your class instance, hence you can use this as a hash key without storing / creating any extra identifiers.
NB: Be careful with this practice, don't rely on the the value returned by `Object.toString(), as being related to the actual object addres, see detailed explanation here.
While your question is one that many beginners have (including myself), I believe that your concern is not justified in this case. The features you are asking for are already built into the Java language at the specification level.
First of all, let's look at Object.equals(). On the one hand, the Language Specification states that
The method equals defines a notion of object equality, which is based on value, not reference, comparison.
However, the documentation for Object.equals() clearly states that
The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).
This means that you can safely redirect OtherClass.remove to ArrayList.remove(). Whatever Object.equals is comparing works exactly like a unique ID. In fact, in many (but not all) implementations, it compares the memory addresses to the objects, which are a form of unique ID.
Quite understandably, you do not wish to use linear iteration every time. As it happens, the machinery of Object is perfectly suited for use with something like a HashSet, which, by the way is the solution I recommend you use in this case.
If you are not dealing with some huge data set, we do not need to discuss the optimization of Object.hashCode(). You just need to know that it will implement whatever contract is necessary to work correctly with Object.equals to make HashSet.remove work correctly.
The spec itself only states that
The method hashCode is very useful, together with the method equals, in hashtables such as java.util.Hashmap.
This does not really say much, so we turn to the API reference. The two relevant point are:
If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.
Simply put, the hashCode of equal objects must be the same, but an equal hashCode does not necessarily mean equal objects. Object implements this contract, so you can use it with a HashSet, which is backed by a HashMap.
The one piece of information that is missing to make this a formal argument in favor of not doing any additional work, is why I keep citing the API reference as if it was the language specification. As it happens:
As noted above, this specification often refers to classes of the Java SE platform API. In particular, some classes have a special relationship with the Java programming language. Examples include classes such as Object, Class, ClassLoader, String, Thread, and the classes and interfaces in package java.lang.reflect, among others. This specification constrains the behavior of such classes and interfaces, but does not provide a complete specification for them. The reader is referred to the Java SE platform API documentation.
[emphasis mine], but you get the idea. The Java SE API reference is the language spec as far as the behavior of the methods of Object is concerned.
As an aside, you will probably want to stay away from something like TreeSet, because that will require you to add a bunch of machinery to your implementation. As a minimum, MyClass instances will have to be orderable, either by implementing Comparable, or by assigning a custom Comparator to the Set.
TL;DR
The language specification states that you have at least the following two options available to you with no additional effort on your part:
Make myClsList an ArrayList and use the appropriate add()/remove() methods as you see fit.
Make myClsList a HashSet and use the appropriate add()/remove() methods.
I recommend the second option. In fact, instead of containment, you may consider extending HashSet so you don't have to bother implementing your own add/remove methods.
Final Note
All this works as long as MyClass overrides neither Object.equals nor Object.hashCode. The moment you do that, you put the burden of satisfying contractual requirements entirely on yourself.
If I have a Name object and have an ArrayList of type Name (names), and I want to ascertain whether my list of names contains a given Name object (n), I could do it two ways:
boolean exists = names.contains(n);
or
boolean exists = names.stream().anyMatch(x -> x.equals(n));
I was considering if these two would behave the same and then thought about what happens if n was assigned null?
For contains, as I understand, if the argument is null, then it returns true if the list contains null. How would I achieve this anyMatch - would it be by using Objects.equals(x, n)?
If that is how it works, then which approach is more efficient - is it anyMatch as it can take advantage of laziness and parallelism?
The problem with the stream-based version is that if the collection (and thus its stream) contains null elements, then the predicate will throw a NullPointerException when it tries to call equals on this null object.
This could be avoided with
boolean exists = names.stream().anyMatch(x -> Objects.equals(x, n));
But there is no practical advantage to be expected for the stream-based solution in this case. Parallelism might bring an advantage for really large lists, but one should not casually throw in some parallel() here and there assuming that it may make things faster. First, you should clearly identify the actual bottlenecks.
And in terms of readability, I'd prefer the first, classical solution here. If you want to check whether the list of names.contains(aParticularValue), you should do this - it just reads like prose and makes the intent clear.
EDIT
Another advantage of the contains approach was mentioned in the comments and in the other answer, and that may be worth mentioning here: If the type of the names collection is later changed, for example, to be a HashSet, then you'll get the faster contains-check (with O(1) instead of O(n)) for free - without changing any other part of the code. The stream-based solution would then still have to iterate over all elements, and this could have a significantly lower performance.
They should provide the same result if hashCode() and equals() are written in reasonable way.
But the performance may be completely different. For Lists it wouldn't matter that much but for HashSet contains() will use hashCode() to locate the element and it will be done (most probably) in constant time. While with the second solution it will loop over all items and call a function so will be done in linear time.
If n is null, actually doesn't matter as usually equals() methods are aware of null arguments.
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.
JavaDoc of ImmutableSet says:
Unlike Collections.unmodifiableSet, which is a view of a separate collection that can still change, an instance of this class contains its own private data and will never change. This class is convenient for public static final sets ("constant sets") and also lets you easily make a "defensive copy" of a set provided to your class by a caller.
But the ImmutableSet still stores reference of elements, I couldn't figure out the difference to Collections.unmodifiableSet(). Sample:
StringBuffer s=new StringBuffer("a");
ImmutableSet<StringBuffer> set= ImmutableSet.of(s);
s.append("b");//s is "ab", s is still changed here!
Could anyone explain it?
Consider this:
Set<String> x = new HashSet<String>();
x.add("foo");
ImmutableSet<String> guava = ImmutableSet.copyOf(x);
Set<String> builtIn = Collections.unmodifiableSet(x);
x.add("bar");
System.out.println(guava.size()); // Prints 1
System.out.println(builtIn.size()); // Prints 2
In other words, ImmutableSet is immutable despite whatever collection it's built from potentially changing - because it creates a copy. Collections.unmodifiableSet prevents the returned collection from being directly changed, but it's still a view on a potentially-changing backing set.
Note that if you start changing the contents of the objects referred to by any set, all bets are off anyway. Don't do that. Indeed, it's rarely a good idea to create a set using a mutable element type in the first place. (Ditto maps using a mutable key type.)
Besides the behavioral difference that Jon mentions, an important difference between ImmutableSet and the Set created by Collections.unmodifiableSet is that ImmutableSet is a type. You can pass one around and have it remain clear that the set is immutable by using ImmutableSet rather than Set throughout the code. With Collections.unmodifiableSet, the returned type is just Set... so it's only clear that the set is unmodifiable at the point where it is created unless you add Javadoc everywhere you pass that Set saying "this set is unmodifiable".
Kevin Bourrillion (Guava lead developer) compares immutable / unmodifiable collections in this presentation. While the presentation is two years old, and focuses on "Google Collections" (which is now a subpart of Guava), this is a very interesting presentation. The API may have changed here and there (the Google Collections API was in Beta at the time), but the concepts behind Google Collections / Guava are still valid.
You might also be interested in this other SO question ( What is the difference between google's ImmutableList and Collections.unmodifiableList() ).
A difference between the two not stated in other answers is that ImmutableSet does not permit null values, as described in the Javadoc
A high-performance, immutable Set with reliable, user-specified iteration order. Does not permit null elements.
(The same restriction applies to values in all Guava immutable collections.)
For example:
ImmutableSet.of(null);
ImmutableSet.builder().add("Hi").add(null); // Fails in the Builder.
ImmutableSet.copyOf(Arrays.asList("Hi", null));
All of these fail at runtime. In contrast:
Collections.unmodifiableSet(new HashSet<>(Arrays.asList("Hi", null)));
This is fine.