I found this question from a book but it was hard to figure out the way behind the answer.
public class Drink implements Comparable{
public String name ;
public int compareTo(Object o){
return 0;
}
}
and:
Drink one=new Drink();
Drink two=new Drink();
one.name="Coffee";
two.name="Tea";
TreeSet set=new TreeSet();
set.add(one);
set.add(two);
If a programmer iterates over the TreeSet and prints the name of each Drink object, what is the result?
Could someone please help me to get the answer by describing the way behind it.
Only "Coffee" (one) is added to the set, since the TreeSet returns equality according to the compareTo(), and since the element is already in the set, noting will happen when you try to add two.
From the java docs:
TreeSet:
Note that the ordering maintained by a set (whether or not an explicit
comparator is provided) must be consistent with equals if it is to
correctly implement the Set interface. (See Comparable or Comparator
for a precise definition of consistent with equals.) This is so
because the Set interface is defined in terms of the equals operation,
but a TreeSet instance performs all element comparisons using its
compareTo (or compare) method, so two elements that are deemed equal
by this method are, from the standpoint of the set, equal. The
behavior of a set is well-defined even if its ordering is inconsistent
with equals; it just fails to obey the general contract of the Set
interface.
add():
...
If this set already contains the element, the call leaves the set
unchanged and returns false.
Also note, that this is unadvised behavior that do not follow the Set interface contract, since you did not override equals().
Related
I have a class that I'd like to put in a TreeSet, which implements Comparable to sort them by priority. Here's a small segment:
public abstract class PacketListener implements Comparable<PacketListener> {
public enum ListenerPriority {
LOWEST, LOW, NORMAL, HIGH, HIGHEST
}
private final ListenerPriority priority; // Initialized in constructor
// ... class body ...
#Override
public final int compareTo(PacketListener o) {
return priority.compareTo(o.priority);
}
}
The idea is obviously for the TreeSet to sort the objects by priority, allowing me to iterate through the listeners in order. However, I've discovered that, for some reason, I can't add a second PacketListener to the set object. After adding two different PacketListener objects, the size of the set remains at 1.
Should I not be using TreeSet?
The API docs for TreeSet contain this important information:
Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. [...] This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the set, equal.
In other words, a TreeSet can accommodate multiple instances of your PacketListener class, but only so long as each has a different priority than all the others, so that for every pair of elements A and B, exactly one of these holds: either A == B or A.compareTo(B) != 0.
If you must accommodate multiple PacketListener instances with the same priority in the same collection, then you need a different type of collection. HashSet is perfectly good with classes that use the equals() and hashCode() methods inherited from Object, provided that that is indeed the desired sense of instance equality. You could also consider a LinkedHashSet if you want some kind of guarantee about iteration order, or maybe a PriorityQueue if you want to order by priority but you are willing to use a different mechanism to avoid duplicates.
TreeSet treats two objects for which compareTo returns 0 to be equals. Which means you'll can never have two objects with the same priority in your tree set with your current implementation.
On way to fix your problem is to make your compareTo method consider all values that you care about (i.e. only objects that are actually equal return 0).
A Set is a collection of unique elements. As you are using the priority to compare PacketListener, I assume that there are at most five instances in you TreeSet, one for each priority.
If the structure allows you, you can find a secondary key to compare PacketListener, in case they have the same priority. If you can't, then TreeSet is the wrong way to go.
There is a java bean Car that might contains two values: model and price.
Now suppose I override equals() and hashcode() checking only for model in that way:
public boolean equals(Object o) {
return this.model.equals(o.model);
}
public int hashCode() {
return model.hashCode();
}
This permit me to check if an arraylist already contain an item Car of the same model (and doesn't matter the price), in that way:
List<Car> car = new ArrayList<Car>();
car.add(new Car("carA",100f));
car.add(new Car("carB",101f));
car.add(new Car("carC",110f));
System.out.println(a.contains(new Car("carB",111f)));
It returns TRUE. That's fine, because the car already exist!
But now I decide that the ArrayList is not good, because I want to maintain the items ordered, so I substitute it with a TreeSet in this way:
Set<Car> car = new TreeSet<Car>(new Comparator<Car>() {
#Override
public int compare(Car car1, Car car2) {
int compPrice = - Float.compare(car1.getPrice(), car2.getPrice());
if (compPrice > 0 || compPrice < 0)
return compPrice;
else
return car1.getModel().compareTo(car2.getModel());
}});
car.add(new Car("carA",100f));
car.add(new Car("carB",101f));
car.add(new Car("carC",110f));
System.out.println(a.contains(new Car("carB",111f)));
But now there is a problem, it return FALSE... why?
It seems that when I invoke contains() using an arrayList the method equals() is invoked.
But it seems that when I invoke contains() using a TreeSet with a comparator, the comparator is used instead.
Why does that happen?
TreeSet forms a binary tree keeping elements according to natural (or not) orders, so in order to search quickly one specific element is the collection, TreeSet uses Comparable or Comparator instead of equals().
As TreeSet JavaDoc precises:
Note that the ordering maintained by a set (whether or not an explicit
comparator is provided) must be consistent with equals if it is to
correctly implement the Set interface. (See Comparable or Comparator
for a precise definition of consistent with equals.) This is so
because the Set interface is defined in terms of the equals operation,
but a TreeSet instance performs all element comparisons using its
compareTo (or compare) method, so two elements that are deemed equal
by this method are, from the standpoint of the set, equal. The
behavior of a set is well-defined even if its ordering is inconsistent
with equals; it just fails to obey the general contract of the Set
interface.
We can find a similarity with the HashCode/Equals contract:
If equals() returns true, hashcode() has to return true too in order to be found during search.
Likewise with TreeSet:
If contains() (using Comparator or Comparable) returns true, equals() has to return true too in order to be consistent with equals().
THEREFORE: Fields used within TreeSet.equals() method have to be exactly the same (no more, no less) than within your Comparator implementation.
A TreeSet is implicitly sorted, and it uses a Comparator for this sorting. The equals() method can only tell you if two objects are the same or different, not how they should be ordered for sorting. Only a Comparator can do that.
More to the point, a TreeSet also uses comparisons for searching. This is sort of the whole point of tree-based map/set. When the contains() method is called, a binary search is performed and the target is either found or not found, based on how the comparator is defined. The comparator defines not only logical order but also logical identity. If you are relying on logical identity defined by an inconsistent equals() implementation, then confusion will probably ensue.
The reason for the different behaviour is, that you consider the price member in the compare method, but ignore it in equals.
new Car("carB",101f) // what you add to the list
new Car("carB",111f) // what you are looking for
Both instances are "equals" (sorry...) since their model members are equal (and the implementation stops after that test). They "compare" as different, though, because that implementation also checks the price member.
TreeSet has a constructor that takes a comparator, meaning even if the objects you store aren't Comparable objects by themselves, you can provide a custom comparator.
Is there an analogous implementation of a nonordered set? (e.g. an alternative to HashSet<T> that takes a "hasher" object that calculates equals() and hashCode() for objects T that may be different from the objects' own implementations?)
C++ std::hash_set gives you this, just wondering if there's something for Java.
Edit: #Max brings up a good technical point about equals() -- fair enough; and it's true for TreeMap and HashMap keys via Map.containsKey(). But are there other well-known data structures out there that allow organization by custom hashers?
No, having a "hasher" object is not supported by the Collections specifications. You can certainly implement your own collection that supports this but another way to do this is to consider the Hasher to be a wrapping object that you store in your HashSet instead.
Set<HasherWrapper<Foo>> set = new HashSet<HasherWrapper<Foo>>();
set.add(new HasherWrapper(foo));
...
The wrapper class would then look something like:
private class HasherWrapper<T> {
T wrappedObject;
public HasherWrapper(T wrappedObject) {
this.wrappedObject = wrappedObject;
}
#Override
public int hashCode() {
// special hash code calculations go here
}
#Override
public boolean equals(Object obj) {
// special equals code calculations go here
}
}
There is no such implementation in the standard library, but it doesn't prevent you from rolling your own. This is something i've often wanted to have myself.
See http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4771660 for the reason:
We wanted to avoid the complexity. We seriously entertained this notion
at the time the collections framework was designed, but rejected it. The
power-to-weight ration seemed to low. We felt that equals was what you
wanted 95% of the time; ==, 4%; and something else 1%. Writing sensible
contracts for bulk operations when is very tricky when equality predicates
differ.
No, there is not and there can not be by specification. Moreover, you misunderstood the way TreeSet uses it's Comparator.
From TreeSet Javadoc:
Note that the ordering maintained by a set (whether or not an explicit
comparator is provided) must be consistent with equals if it is to
correctly implement the Set interface. (See Comparable or Comparator
for a precise definition of consistent with equals.) This is so
because the Set interface is defined in terms of the equals operation,
but a TreeSet instance performs all element comparisons using its
compareTo (or compare) method, so two elements that are deemed equal
by this method are, from the standpoint of the set, equal. The
behavior of a set is well-defined even if its ordering is inconsistent
with equals; it just fails to obey the general contract of the Set
interface.
From Comparable javadoc:
The natural ordering for a class C is said to be consistent with
equals if and only if e1.compareTo(e2) == 0 has the same boolean value
as e1.equals(e2) for every e1 and e2 of class C. Note that null is not
an instance of any class, and e.compareTo(null) should throw a
NullPointerException even though e.equals(null) returns false.
From Collection javadoc:
boolean contains(Object o)
Returns true if this collection contains
the specified element. More formally, returns true if and only if this
collection contains at least one element e such that (o==null ?
e==null : o.equals(e)).
Therefore, by specification there can not be any kind of class that implements Collection<E> interface and fully depend on some external Comparator-style object to insert objects. All Collections should use equals method of an Object class to verify if the object is already inserted.
There is definitely nothing like that, hashcode() and equals() are defining attributes of an object and should not be changed. They define what makes an object equal to each other and this shouldn't be different from one set to another. The only way to do what you are talking about is to subclass the object and write a new hashcode() and equals() and this would only really make sense to do if the subclass had a defining variable that should be added in addition to the super class' hashcode() and equals(). I know this may not be what you are aiming for but I hope this helps. If you explain your reasoning more for wanting this then it might help to find a better solution if there exists one.
There is a Java bean object which has implemented equals function based on certain criteria (Criteria A). I have a requirement to identify unique objects based on another criteria (Criteria B). Since the equals function uses criteria A, I can not use HashSet. So I thought of using TreeSet with my custom Comparator which is based on criteria B. My question is, Is it allowed to do like this? Any issues with this approach?
Thank you.
Here is some guide from Oracle Java:
Note that the ordering maintained by a
set (whether or not an explicit
comparator is provided) must be
consistent with equals if it is to
correctly implement the Set interface.
(See Comparable or Comparator for a
precise definition of consistent with
equals.) This is so because the Set
interface is defined in terms of the
equals operation, but a TreeSet
instance performs all key comparisons
using its compareTo (or compare)
method, so two keys that are deemed
equal by this method are, from the
standpoint of the set, equal. The
behavior of a set is well-defined even
if its ordering is inconsistent with
equals; it just fails to obey the
general contract of the Set interface.
I think in terms of technical, no, you don't have any problems. But, in terms of coding, readability and maintainability, you have to be careful, because other people can misuse or misunderstand what you are doing
If you perform search often and add elements rarely, consider keeping them in a List sorted by Criteria B and using Collections.binarySearch.
You can wrap them:
class BeanWrapper {
...
public boolean equals(Object other) {
return myBean.critB.equals(((Bean)other).critB);
}
}
And put them in the set like that.
I'd like to have a SortedSet of Collections (Sets themselves, in this case, but not necessarily in general), that sorts by Collection size. This seems to violate the proscription to have the Comparator be consistent with equals() - i.e., two collections may be unequal (by having different elements), but compare to the same value (because they have the same number of elements).
Notionally, I could also put into the Comparator ways to sort sets of equal sizes, but the use of the sort wouldn't take advantage of that, and there's not really a useful + intuitive way to compare the Collections of equal size (at least, in my particular case), so that seems like a waste.
Does this case of inconsistency seem like a problem?
SortedSet interface extends core Set and thus should conform to the contract outlined in Set specification.
The only possible way to achieve that is to have your element's equal() method behavior be consistent with your Comparator - the reason for that is that core Set operates based on equality whereas SortedSet operates based on comparison.
For example, add() method defined in core Set interface specifies that you can't add an element to the set if there already is an element whose equal() method would return true with this new element as argument. Well, SortedSet doesn't use equal(), it uses compareTo(). So if your compareTo() returns false your element WILL be added even if equals() were to return true, thus breaking the Set contract.
None of this is a practical problem per se, however. SortedSet behavior is always consistent, even if compare() vs equals() are not.
As ChssPly76 wrote in a comment, you can use hashCode to decide the compareTo call in the case where two Collections have the same size but are not equal. This works fine, except in the rare case where you have two Collections with the same size, are not equal, but have the same hashCode. Admittedly, the chances of that happening are pretty small, but it is conceivable. If you want to be really careful, instead of hashCode, use System.identityHashCode instead. This should give you a unique number for each Collection, and you shouldn't get collisions.
At the end of the day, this gives you the functionality of having the Collections in the Set sorted by size, with arbitrary ordering in the case of two Collections with matching size. If this is all you need, it's not much slower than the usual comparison would be. If you need the ordering to be consistent between different JVM instances, this won't work and you'll have to do it some other way.
pseudocode:
if (a.equals(b)) {
return 0;
} else if (a.size() > b.size()) {
return 1;
} else if (b.size() > a.size()) {
return -1;
} else {
return System.identityHashCode(a) > System.identityHashCode(b) ? 1 : -1;
}
This seems to violate the proscription
to have the Comparator be consistent
with equals() - i.e., two collections
may be unequal (by having different
elements), but compare to the same
value (because they have the same
number of elements).
There is no requirement, either stated (in the Javadoc) or implied, that a Comparator be consistent with an object's implementation of boolean equals(Object).
Note that Comparable and Comparator are distinct interfaces with different purposes. Comparable is used to define a 'natural' order for a class. In that context, it would be a bad idea for equals and compateTo to be inconsistent. By contrast, a Comparator is used when you want to use a different order to the natural order of a class.
EDIT: Here's the complete paragraph from the Javadoc for SortedSet.
Note that the ordering maintained by a
sorted set (whether or not an explicit
comparator is provided) must be
consistent with equals if the sorted
set is to correctly implement the Set
interface. (See the Comparable
interface or Comparator interface for
a precise definition of consistent
with equals.) This is so because the
Set interface is defined in terms of
the equals operation, but a sorted
set performs all element comparisons
using its compareTo (or compare)
method, so two elements that are
deemed equal by this method are, from
the standpoint of the sorted set,
equal. The behavior of a sorted set is
well-defined even if its ordering is
inconsistent with equals; it just
fails to obey the general contract of
the Set interface.
I have highlighted the final sentence. The point is that such a SortedSet will work as you would most likely expect, but the behavior of some operations won't exactly match the Set specification ... because the specification defines their behavior in terms of the equals method.
So in fact, there is a stated requirement for consistency (my mistake), but the consequences of ignoring it are not as bad as you might think. Of course, it is up to decide if you should do that. In my estimation, it should be OK, provided that you comment the code thoroughly and make sure that the SortedSet does not 'leak'.
However, it is not clear to me that a Comparator for collections that only looks at an collections "size" is going to work ... from a semantic perspective. I mean, do you really want to say that all collections with (say) 2 elements are equal? This will mean that your set can only ever contain one collection of any given size ...
There is no reason why a Comparator should return the same results as equals(). In fact, the Comparator API was introduced because equals() just isn't enough: If you want to sort a collection, you must know whether two elements are lesser or greater.
It's a little bit odd that SortedSet as a part of the standard API breaks the contract defined in the Set interface and uses the Comparator to define equality instead of the equals method, but that's how it is.
If your actual problem is to sort a collection of collections according to the containted collections' size, you are better of with a List, which you can sort using Collections.sort(List, Comparator>);