Getting an Element from a Set

Getting an element from a Set

There would be no point of getting the element if it is equal. A Map is better suited for this usecase.

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If you still want to find the element you have no other option but to use the iterator:

public static void main(String[] args) {

    Set<Foo> set = new HashSet<Foo>();
    set.add(new Foo("Hello"));

    for (Iterator<Foo> it = set.iterator(); it.hasNext(); ) {
        Foo f = it.next();
        if (f.equals(new Foo("Hello")))
            System.out.println("foo found");
    }
}

static class Foo {
    String string;
    Foo(String string) {
        this.string = string;
    }
    @Override
    public int hashCode() { 
        return string.hashCode(); 
    }
    @Override
    public boolean equals(Object obj) {
        return string.equals(((Foo) obj).string);
    }
}

How to get first item from a java.util.Set?

From the Oracle docs:

As implied by its name, this interface models the mathematical set abstraction.

In Set Theory, "a "set" is a collection of distinct objects, considered as an object in its own right." - [Wikipedia - Set].

Mathematically, elements in sets are not individualised. Their only identity is derived from their presence in the set. Therefore, there is no point in getting the "first" element in a set, as conceptually such a task is illogical.

There may be no point to getting the "first" element from a set, but if all you need is to get one single object from a set (with no guarantees as to which object that is) you can do the following:

for(String aSiteId: siteIdSet) {
    siteId = aSiteId;
    break;
}

This is a slightly shorter way (than the method you posted) to get the "first" object of a Set, however since an Iterator is still being created (under the hood) it does not grant any performance benefit.

How to retrieve an element from a set without removing it?

Two options that don't require copying the whole set:

for e in s:
    break
# e is now an element from s

Or...

e = next(iter(s))

But in general, sets don't support indexing or slicing.

Good way to get *any* value from a Java Set?

A Set<T> is an Iterable<T>, so iterating to the first element works:

Set<T> things = ...;
return things.iterator().next();

Guava has a method to do this, though the above snippet is likely better.

Get the other element from a set (Python)

This answer addresses the particular question of how to access one element of a (two-element) set, given the other element.

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If you're using a set, you can use the set difference operator, then your method of choice to get the sole element of a set (the first bit is identical to the answer you linked):

options = {'a', 'b'}
other = options - {'a'} # b is {'b'}

# A few ways to get the sole element of a set
b = next(iter(other))
b = min(other) # or max
b, = other # or "[b] = other"

The last method for extracting the sole element of a set also raises an exception if the set has multiple elements.

If you don't mind destroying the list of possible values (i.e. if you already are using a copy), you can remove or discard¹ the value you don't want, then pop the remaining value.

options = {'a', 'b'}
options.remove('a')
value = options.pop()
# options is now empty, and value is 'b'

In general, to understand sets, I'd start by reading the documentation.

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¹ remove raises an exception if the element is missing; discard does not

Java: Get first item from a collection

Iterables.get(yourC, indexYouWant)

Because really, if you're using Collections, you should be using Google Collections.

The most efficient method to find an element in Set

Because TreeSet is a NavigableSet, you can use TreeSet.subSet, which leverages knowledge about the order of the elements to extract a range of elements as close as possible to the element you are interested in:

Person pattern = new Person(id);

return
    // Get the Persons between pattern (inclusive) and pattern (inclusive).
    // In other words: all the Persons with id equal to the input,
    // of which there are zero or one.
    idTreeSet.subSet(pattern, true, pattern, true).stream()
        .findFirst()
        .orElse(null);

Get the only element in a Set

Your question asks to get something from the Set. But your example just needs a check.

If you know what to expect in the Set, this works fine.

    if (enabled != null && enabled.size() == 1 && enabled.contains("true")) {
        ...
    }

Otherwise, if you just want to get the element but don't know what it is, the iterator you suggested works fine.

    String getOnlyElement(Set<String> enabled, String default) {
        return (enabled == null || enabled.size() != 1) ? default : enabled.iterator().next();
    }

I like having null checks but it depends on what methodToGetValues returns.

How to write to an Element in a Set?

You are getting the error because columns might be a set of struct. So columns.first will give you an immutable value. If you were to use a class, you will get a mutable result from columns.first and your code will work as expected.
Otherwise, you will have to do as explained by @Sweeper in his answer.

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