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An Aperitif: Optional<T>

This is the Schrödinger's Cat of objects. Let us learn about it. It will appear a lot when we study the Streams API and it is also useful in a variety of other contexts.

jshell> Optional<String> s = Optional.empty();
s ==> Optional.empty

jshell> s
s ==> Optional.empty

jshell> s.isEmpty()
$3 ==> true

jshell> //m
No such command

jshell> //make an Optional<String> with "cat" in it.

jshell> s.isPresent()
$4 ==> false

jshell> s = new Optional<>();
|  Error:
|  cannot infer type arguments for java.util.Optional<>
|    reason: cannot infer type-variable(s) T
|      (actual and formal argument lists differ in length)
|  s = new Optional<>();
|      ^--------------^

jshell> s.getClass()
$5 ==> class java.util.Optional

jshell> s = Optional.of("cat")
s ==> Optional[cat]

jshell> s.orElse("cow")
$7 ==> "cat"

jshell> Optional t = Optional.empty();
t ==> Optional.empty

jshell> t.orElse("cow")
|  Warning:
|  unchecked call to orElse(T) as a member of the raw type java.util.Optional
|  t.orElse("cow")
|  ^-------------^
$9 ==> "cow"

jshell> s
s ==> Optional[cat]

jshell> String pet = s.get()
pet ==> "cat"

jshell> s.toString()
$12 ==> "Optional[cat]"V

The Streams API

There are three components.

• A source
• Zero or more transformations
• A terminal operation

Streams are lazy; they do nothing until a terminal operaton tells them to.

Once a stream is consumed, you have to make a new one.

A string will give you an IntStream of its characters
A Collection will give you a stream of its elements
An Array will give you a stream of its elements using
the static method Arrays.stream().
A buffered reader will give you a stream of strings using lines()

How do you get a Stream from an ArrayList?

How do you get a Stream from a lowly array?

How do you get a Stream from a file?