Why does the java.util.Set interface not provide a get(Object o) method? [closed]

Answer 1

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.

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Answer 2

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;
  }
}

Answer 3

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".

Answer 4

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.

Answer 5

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:

1. The set of all elements that appear before the found object.
2. An object of type Option that is either empty or contains the found object if it exists in the set.
3. 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);

Answer 6

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...