CODEWITHSUNDEEP
javaguavacomparatorcomparetosortedset
legrass
legrass

Reputation: 417

SortedSet transform

I have the following case:

SortedSet<MyClass> sortedSet = ...;
SortedSet<HeavyToCompare> newSet = ...;
for (MyClass m: sortedSet ){
newSet.add(m.getHeavyToCompare())    
}

I want to avoid reordering of the newSet because HeavyToCompare it's very expensive to compare. Rather i want to keep the insertion order (that is the same of the original set). I understand that I can create a simple comparator that always return -1, but this breaks the contract of compareTo. What's the best practice here? I would need something like 

SortedSet<K> result = Sets.transformAndKeepOrder(SortedSet<T> from, Function<T, K> function)

UPDATE: I cannot change the SortedSet, it is a requirement

Upvotes: 0

Views: 593

Answers (3)

Chris Povirk
Chris Povirk

Reputation: 4002

You can hardcode a comparison order with Guava's Ordering.explicit. However, this produces a limited SortedSet. Specifically, its comparison methods (headSet, etc.) will work only when their arguments are members of the set. After all, Ordering.explicit only knows how to compare the values that you provide it. More concretely:

// In the real code, this values list will be computed in your loop:
List<Integer> values = Arrays.asList(2, 4, 6, 8);

Comparator<Integer> comparator = Ordering.explicit(values);
SortedSet<Integer> set = Sets.newTreeSet(comparator);
set.addAll(values);
set.headSet(4); // OK: [2]
set.headSet(5); // exception: "Cannot compare value: 5"

Upvotes: 0

Bohemian
Bohemian

Reputation: 425428

You could use a LinkedHashSet, which is a Set that iterates in insertion order:

Set<HeavyToCompare> newSet = new LinkedHashSet<HeavyToCompare>();

However, it is not a SortedSet, but hopefully that is not a requirement.

Upvotes: 1

OldCurmudgeon
OldCurmudgeon

Reputation: 65889

I would attempt to record the position in the sortedSet the object appears in. You could then compare the positions before calling the heavy compare.

How you achieve that will depend on a lot more of your code.

Upvotes: 0

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