Overview

Vavr is a functional component library that provides persistent data types and functional control structures. I started using it recently, and really loved it! Its simplicity, immutable data types, and the functional programming concept are really remarkable. In this article, I will introduce Vavr’s List, by doing a comparison with the built-in Java List and its implementations in Java 8.

After reading this article, you will understand:

  • List creation
  • Add element
  • Get element
  • Update element
  • Delete element
  • List streaming
  • From Vavr to Java
  • Thread safety

For this article, I’m using Vavr 0.9.3 io.vavr:vavr:0.9.3.

List Creation

In Java 8, you can create a list by calling the constructor of any implementation of java.util.List. Or using a factory method which returns a list.

// java.util.List
List<String> animals = new ArrayList<>();
List<String> another = new ArrayList<>(animals);
List<String> animals = new LinkedList<>();
List<String> another = new LinkedList<>(animals);
List<String> animals = Arrays.asList("🐱", "🐶");
List<String> animals = Collections.singletonList("🐱");
List<String> animals = Collections.unmodifiableList(...);

In Vavr, you can create a list using the factory methods of interface io.vavr.collection.List:

// io.vavr.collection.List
List<String> animals = List.of("🐱", "🐶");
List<String> another = List.ofAll(animals);
List<String> empty = List.empty();

There’re also other factory methods which allow you to create a list of primitives. But I won’t go into more detail here.

Add Element

In Java, interface java.util.List defines method add(E e) for adding new element of type E into the existing list. Therefore, all the implementations of List must override the method add. The new element will be added at the end of the list.

// java.util.List
List<String> animals = new ArrayList<>();
animals.add("🐱");
animals.add("🐶");
// "🐱", "🐶"
List<String> animals = new LinkedList<>();
animals.add("🐱");
animals.add("🐶");
// "🐱", "🐶"

In case of a read-only (immutable) list, an exception will throw when calling the add method, which is a side-effect. This is tricky because when using interface List, you don’t know if the underlying implementation is immutable.

// java.util.List
List<String> animals = Arrays.asList("🐱", "🐶");
animals.add("💥");
// java.lang.UnsupportedOperationException
List<String> animals = Collections.singletonList("🐱");
animals.add("💥");
// java.lang.UnsupportedOperationException
List<String> animals = Collections.unmodifiableList(Arrays.asList("🐱", "🐶"));
animals.add("💥");
// java.lang.UnsupportedOperationException

In Vavr, list does not have add() method. It has prepend() and append(), which adds a new element respectively before and after the list, and creates a new list. It means that the original list remains unchanged.

// io.vavr.collection.List
List<String> animals = List.of("🐱", "🐶");
List<String> another = animals.prepend("🙂");
// animals: "🐱", "🐶"
// another: "🙂", "🐱", "🐶"
List<String> animals = List.of("🐱", "🐶");
List<String> another = animals.append("😌");
// animals: "🐱", "🐶"
// another: "🐱", "🐶", "😌"

This is very similar to the addFirst() and addLast() methods of java.util.LinkedList.

Get Element

In Java, getting the element at the specified position in the list can be done using get(int).

// java.util.List
List<String> animals = Arrays.asList("🐱", "🐶");
animals.get(0)
// "🐱"

In Vavr, you can get the first element using get() without input params, or get the element at specific position using get(int). You can also get the first element using head() and get the last element using last().

// io.vavr.collection.List
List<String> animals = List.of("🐱", "🐶");
animals.get();
// "🐱"
animals.head();
// "🐱"
animals.get(1);
// "🐶"
animals.last();
// "🐶"

Performance. If you’re doing “get” operation on a list with high volume of elements, it’s important to consider the performance issue. The “get” operation with index in Vavr takes linear time to finish: O(N). While for Java lists, some implementations, like java.util.ArrayList takes constant time to do the same operation; and other implementations, like java.util.LinkedList takes linear time. If you need something faster in Vavr, you might want to consider io.vavr.collection.Array.

Remove Element

Firstly, let’s take a look on removing element.

In Java, removing an element can be done using List#remove(Object). Note that the input parameter is not parameterized T, but Object. So you can pass any object to try to remove it from the list. They don’t need to have the same type. The element will be removed if it is equal to the input object. For more detail, see Stack Overflow: Why aren’t Java Collections remove methods generic?.

List<String> animals = Arrays.asList("🐱", "🐶");
List<String> animals = new ArrayList<>();
animals.add("🐱");
animals.add("🐶");
animals.remove(true); // remove(Object)
// "🐱", "🐶"
animals.remove("🐱");
// "🐶"

In Vavr, removing an element can be done using List#remove(T). This method is defined by io.vavr.collection.Seq, which removes the first occurrence of the given element. Different from Java, it requires the input object has the same type T as the type of elements in the list. Note that list is immutable, and a new list is returned when doing remove operation.

// io.vavr.collection.List
List<String> animals = List.of("🐱", "🐶");
List<String> another = animals.remove("🐱");
// animals: "🐱", "🐶"
// another: "🐶"

Now, let’s take a look at removing by index.

In Java, removing an element by index can be done using List#remove(int). Note that this operation is very tricky when having a list of integer List<Integer> which auto-boxes the primitives.

List<Integer> numbers = new ArrayList<>();
numbers.add(2);
numbers.add(3);
// numbers: 2, 3
numbers.remove(Ingeter.valueOf(1)); // remove(Object)
// numbers: 2, 3
numbers.remove(1); // remove(int)
// numbers: 2

In Vavr, removing an element by index is done via another method, called removeAt(int). It makes the operation more explicit, and avoids error-prone.

List<Integer> numbers = List.of(2, 3);
List<Integer> another = numbers.removeAt(1);
// numbers: 2, 3
// another: 2

Streaming API

In Java, the streaming API is very explicit. From a collection x, you can start a stream using stream() method, followed by the operation wished, then ends with the desired collections using collect(...). There’s no shortcut / default options to make it simpler.

x.stream().$OPERATION.collect(...);

In Vavr, the stream-like operations are more implicit. You can simply call the operation and Vavr will transform it to a collection with the same type. Then, if you need something else, you can convert it using a collector method.

x.$OPERATION;

For example, in Java:

Arrays.asList("🐱", "🐶")
      .stream()
      .map(s -> s + s)
      .collect(Collectors.toList());
// "🐱🐱", "🐶🐶"
Arrays.asList("🐱", "🐶")
      .stream()
      .filter("🐱"::equals)
      .collect(Collectors.toList());
// "🐱"
List<String> cats = Arrays.asList("🐱", "🐈");
List<String> dogs = Arrays.asList("🐶", "🐕");
List<List<String>> lists = Arrays.asList(cats, dogs);
List<String> animals = lists.stream().flatMap(Collection::stream).collect(Collectors.toList());
// "🐱", "🐈", "🐶", "🐕"

In Vavr:

List.of("🐱", "🐶").map(s -> s + s);
// "🐱🐱", "🐶🐶"
List.of("🐱", "🐶").filter("🐱"::equals)
// "🐱"
List<String> cats = List.of("🐱", "🐈");
List<String> dogs = List.of("🐶", "🐕");
List<List<String>> lists = List.of(cats, dogs);
List<String> list = lists.flatMap(Function.identity());
// "🐱", "🐈", "🐶", "🐕"

From Vavr to Java

Vavr provides a lot of methods to convert a Vavr collection to Java collection. This is done by using syntax toJava*:

toJavaSet()
toJavaList()
toJavaMap()
...

Thread Safety

When developing concurrent Java application, let’s important to choose a thread-safe collections. In Java, you might consider the synchronized collections classes, concurrent collections, blocking queues etc. In Vavr, I believe everything is thread safe since they are immutable. However, I never had chance to develop complex concurrent application, so I will not go further in this topic.

Conclusion

In this article, we compare the differences of list between Java and Vavr: we discussed the CRUD operations, immutability, transformation (streaming), and thread safety. Hope you enjoy this article, see you the next time!

References

  • Daniel Dietrich and Robert Winkler, “Vavr User Guide (version 0.9.3) - §3.4.3. Performance Characteristics,” www.vavr.io, Jan. 19, 2019. [Online]. Available: http://www.vavr.io/vavr-docs/#_performance_characteristics
  • Chris Mazzola, “Why aren’t Java Collections remove methods generic?,” stackoverflow.com, Sep. 19, 2008. [Online]. Available: https://stackoverflow.com/q/104799
  • Yuval Adam, “Properly removing an Integer from a List<Integer>,” stackoverflow.com, Dec. 26 2010. [Online]. Available: https://stackoverflow.com/q/4534146
  • Brian Goetz with Tim Peierls et al., Java Concurrency in Practice. Addison Wesley: 2006.