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use_the_optional_class.md

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Use the Optional class

An Optional<T> object is a wrapper for either an object of type T or no object. In the former case, we say that the value is present. The Optional<T> type is intended as a safer alternative for a reference of type T that either refers to an object or is null.

Creating Optional objects

  • Empty Optional

You can get an empty optional object using the static factory method Optional.empty()

Optional<String> str = Optional.empty();

  • Optional from a non-null value

You can also create an optional from a non-null value with the static factory method Optional.of(...)

Optional<String> opStr = Optional.of("some string");

When using Optional.of(...), the object passed in must not be null or a NPE will be thrown

  • Optional from nullable value

You can create an Optional object that may hold a null by using the static factory method Optional.ofNullable(...)

Optional<String> opStr = OPtional.ofNullable(null);

In the example above, the resulting Optional object would be empty

Unwrapping an Optional

  • Optional.get()
    It returns the wrapped value if present but throws a NoSuchElementException otherwise

  • Optional.orElse(T other) Allows you to provide a default value for when the optional does not contain a value

  • Optional.orElseGet(Supplier<? extends T> other) The Supplier is invoked only if the optional contains no value.

  • Optional.orElseThrow(Supplier<? extends X> exceptionSupplier) Similar to get() in that it throws an exception when empty, but in this case you can choose the type of exception

  • Optional.ifPresent(Consumer<? super T> consumer) Lets you execute the action given as an argument if a value if present; otherwise no action is taken

  • Optional.isPresent() Returns true is the Optional contains a value, false otherwise

  • Optional.isEmpty() Returns true if the Optional id empty, otherwise it returns false

Converting an Optional into a Stream

You can convert an Optional into a stream using the Optional.stream() method. If the Optional contains a value, it will return a Stream containing only that value, otherwise it will return an empty stream

It can be helpful to remove empty optionals and keep only the ones with values. In Java 9 you can use the flatMap() to remove empty optionals

List<String> list - stream.flatMap(Optional::stream).collect(Collectors.toList());

In the example above where the variable stream is a stream of optionals, the flatMap calls Optional.stream() on each optional. As described above, this will either return an empty stream or a stream containing only the value of the optional before finally flattening the stream of streams to produce a stream which contains only optionals with a value

Code Samples

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