4.3. Use the Optional class

4.3. Use the `Optional` class
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4.3. Use the `Optional` class


	[CORE_JAVA9_IMPATIENT] Chapter 8. Section 7. The Optional Type.

A java.util.Optional<T> object is either a wrapper for an Object of type T or a wrapper for no object. It is intended as a safer alternative than a reference of type T that refers to an Object or null.

Following is the declaration for java.util.Optional<T> class:


public final class Optional<T> extends Object {
    ...
}

Creating Optional objects

The first step before working with Optional is to learn how to create optional objects. There are several ways:

Empty Optional
You can get hold of 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(...):
```
String str = "ABC";
Optional<String> optStr = Optional.of(str);

								
```
If str were null, a NullPointerException would be immediately thrown!
Optional from nullable value
Finally, by using the static factory method Optional.ofNullable(...), you can create an Optional object that may hold a null value:
```
String str = null;
Optional<String> optStr = Optional.ofNullable(str);

								
```
In the example above, since the str is null, the resulting Optional object would be empty.
The ofNullable(...) method is intended as a bridge from the use of null values to optional values. The Optional.ofNullable(obj) returns Optional.of(obj) if obj is not null, and Optional.empty() otherwise.

Unwrapping an Optional

The Optional class provides several instance methods to read the value contained by an Optional instance.

Optional.get() - is the simplest but also the least safe of these methods. It returns the wrapped value if present but throws a NoSuchElementException otherwise. For this reason, using this method is almost always a bad idea unless you are really sure the optional contains a value.
Optional.orElse(T other) it allows you to provide a default value for when the optional does not contain a value.
NOTE: the other value may be null.
Optional.orElseGet(Supplier<? extends T> other) is the lazy counterpart of the orElse method, because the supplier is invoked only if the optional contains no value. You should use this method either when the default value is time-consuming to create or you want to be sure this is done only if the optional is empty.
<X extends Throwable> Optional.orElseThrow(Supplier<? extends X> exceptionSupplier) is similar to the get() method in that it throws an exception when the optional is empty, but in this case it allows you to choose the type of exception that you want to throw.
Optional.ifPresent(Consumer<? super T> consumer) lets you execute the action given as argument if a value is present; otherwise no action is taken.
Optional.isPresent() returns true if the Optional contains a value, false otherwise.
Optional.isEmpty() returns true if the Optional is empty, false otherwise.

Converting an Optional into a Stream

The Optional.stream() method allows you to convert an Optional to a Stream.

If the Optional contains a value, it will return a Stream containing only that value, otherwise, it will return an empty Stream:


Optional<Integer> in = Optional.of(1);
in.stream()
    .filter(i -> i > 0)
    .forEach(System.out::println);

The Optional.stream() method can be helpful when you are dealing with a stream of Optional elements. It will help to filter out empty optionals and keep the ones with values.

Assume we have a list of optionals received from a method, and some of the optionals are empty:


Stream<Optional<String>> stream = Stream.of(
    Optional.of("Java"),
    Optional.empty(), 
    Optional.of("is"),
    Optional.empty(), 
    Optional.of("good"));

In Java 8.0 you could remove empty optionals as follows:


List<String> list = stream
    .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
    .collect(Collectors.toList());

In Java 9.0 and later you can remove empty optionals with shorter code using flatMap():


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

The map() and flatMap() in Optional

Both Optional.map() and Optional.flatMap() return an Optional.

Use Optional.map() if the function returns the object you need; or Optional.flatMap() if the function returns an Optional.

If the function returns the exact type we need:


Optional<String> s = Optional.of("Java");
s = s.map(String::toUpperCase); // function returns String
s.ifPresent(System.out::print);

If we have a function that returns an Optional then using map() would lead to a nested structure of optionals, as the map() does an additional wrapping. Use flatMap() to keep a flat structure:


Optional<String> s = Optional.of("Java");
s = s.flatMap(val -> Optional.of(val.toUpperCase())); // function returns Optional<String>
s.ifPresent(System.out::print);


	If `str` were `null`, a `NullPointerException` would be immediately thrown!

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