1. Introduction
In this quick article, we’ll focus on how to use the Apache’s Bag collection.
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2. Maven Dependency
Before we start, we need to import the latest dependencies from Maven Central:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-collections4</artifactId>
<version>4.1</version>
</dependency>
3. Bags vs Collections
Simply put, Bag is a collection that allows storing multiple items along with their repetition count:
public void whenAdded_thenCountIsKept() {
Bag<Integer> bag = new HashBag<>(
Arrays.asList(1, 2, 3, 3, 3, 1, 4));
assertThat(2, equalTo(bag.getCount(1)));
}
3.1. Violations of the Collection Contract
While reading Bag‘s API documentation, we may notice that some methods are marked as violating the standard Java’s Collection contract.
For example, when we use an add() API from a Java collection, we receive true even if the item is already in the collection:
Collection<Integer> collection = new ArrayList<>();
collection.add(1);
assertThat(collection.add(1), is(true));
The same API from a Bag implementation will return a false when we add an element which is already available in the collection:
Bag<Integer> bag = new HashBag<>();
bag.add(1);
assertThat(bag.add(1), is(not(true)));
To resolve these issues, Apache Collections’ library provides a decorator called the CollectionBag. We can use this to make our bag collections compliant with the Java Collection contract:
public void whenBagAddAPILikeCollectionAPI_thenTrue() {
Bag<Integer> bag = CollectionBag.collectionBag(new HashBag<>());
bag.add(1);
assertThat(bag.add(1), is((true)));
}
4. Bag Implementations
Let’s now explore the various implementations of the Bag interface – within Apache’s collections library.
4.1. HashBag
We can add an element and instruct the API on the number of copies this element should have in our bag collection:
public void givenAdd_whenCountOfElementsDefined_thenCountAreAdded() {
Bag<Integer> bag = new HashBag<>();
bag.add(1, 5); // adding 1 five times
assertThat(5, equalTo(bag.getCount(1)));
}
We can also delete a specific number of copies or every instance of an element from our bag:
public void givenMultipleCopies_whenRemove_allAreRemoved() {
Bag<Integer> bag = new HashBag<>(
Arrays.asList(1, 2, 3, 3, 3, 1, 4));
bag.remove(3, 1); // remove one element, two still remain
assertThat(2, equalTo(bag.getCount(3)));
bag.remove(1); // remove all
assertThat(0, equalTo(bag.getCount(1)));
}
4.2. TreeBag
The TreeBag implementation works like any other tree, additionally maintaining Bag semantics.
We can naturally sort an array of integers with a TreeBag and then query the number of instances each individual element has within the collection:
public void givenTree_whenDuplicateElementsAdded_thenSort() {
TreeBag<Integer> bag = new TreeBag<>(Arrays.asList(7, 5,
1, 7, 2, 3, 3, 3, 1, 4, 7));
assertThat(bag.first(), equalTo(1));
assertThat(bag.getCount(bag.first()), equalTo(2));
assertThat(bag.last(), equalTo(7));
assertThat(bag.getCount(bag.last()), equalTo(3));
}
The TreeBag implements a SortedBag interface, all implementations of this interface can use the decorator CollectionSortedBag to comply with the Java Collections contract:
public void whenTreeAddAPILikeCollectionAPI_thenTrue() {
SortedBag<Integer> bag
= CollectionSortedBag.collectionSortedBag(new TreeBag<>());
bag.add(1);
assertThat(bag.add(1), is((true)));
}
4.3. SynchronizedSortedBag
Another widely used implementation of Bag is the SynchronizedSortedBag. Precisely, this is a synchronized decorator of a SortedBag implementation.
We can use this decorator with our TreeBag (an implementation of SortedBag) from the previous section to synchronize access to our bag:
public void givenSortedBag_whenDuplicateElementsAdded_thenSort() {
SynchronizedSortedBag<Integer> bag = SynchronizedSortedBag
.synchronizedSortedBag(new TreeBag<>(
Arrays.asList(7, 5, 1, 7, 2, 3, 3, 3, 1, 4, 7)));
assertThat(bag.first(), equalTo(1));
assertThat(bag.getCount(bag.first()), equalTo(2));
assertThat(bag.last(), equalTo(7));
assertThat(bag.getCount(bag.last()), equalTo(3));
}
We can use a combination of APIs – Collections.synchronizedSortedMap() and TreeMap – to simulate what we did here with SynchronizedSortedBag.
5. Conclusion
In this short tutorial, we’ve learned about the Bag interface and its various implementations.
As always, the code for this article can be found over on GitHub.
