Java Optional

Java Optional

In the HashMap chapter, you used get() to find a value by its key. But what if the key does not exist? Then get() returns null, which means "no value".

If you forget to check for null and use the value anyway, the program crashes:

HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put("England", "London");
capitalCities.put("Norway", "Oslo");

String city = capitalCities.get("Spain");
System.out.println(city.toUpperCase());  // Error

"Spain" is not in the map, so city is null. Calling toUpperCase() on null stops the program with a NullPointerException.

An Optional solves this. It is like a box that either holds a value or is empty. Before you can use the value, you have to open the box, so you always decide what should happen when it is empty.

Create an Optional

Import java.util.Optional, and create an Optional with one of these methods:

• Optional.of(value) creates a box with a value
• Optional.empty() creates an empty box
• Optional.ofNullable(value) creates a box with the value, or an empty box if the value is null

The type inside <> tells Java what the box can hold, just like with ArrayList<String>. Use isPresent() to check if the box has a value:

import java.util.Optional;

public class Main {
  public static void main(String[] args) {
    Optional<String> name = Optional.of("Liam");
    Optional<String> noName = Optional.empty();

    System.out.println(name.isPresent());    // true
    System.out.println(noName.isPresent());  // false
  }
}

Get the Value with orElse()

The orElse() method returns the value if there is one. If the box is empty, it returns the default value that you give it:

Optional<String> name = Optional.of("Liam");
Optional<String> noName = Optional.empty();

System.out.println(name.orElse("Guest"));    // Liam
System.out.println(noName.orElse("Guest"));  // Guest

Run Code If There Is a Value

The ifPresent() method runs code only when the box has a value. You give it the code as a lambda expression:

Optional<String> name = Optional.of("Liam");
name.ifPresent(n -> System.out.println("Hello " + n));  // Hello Liam

If name was empty, nothing would be printed.

Real-Life Example

Optional is most useful as the return value of a method that might not find anything. Here is the capital cities example from the start of this page, now with Optional:

import java.util.HashMap;
import java.util.Optional;

public class Main {
  static Optional<String> findCapital(String country) {
    HashMap<String, String> capitalCities = new HashMap<String, String>();
    capitalCities.put("England", "London");
    capitalCities.put("Norway", "Oslo");
    return Optional.ofNullable(capitalCities.get(country));
  }

  public static void main(String[] args) {
    System.out.println(findCapital("Norway").orElse("Not found"));  // Oslo
    System.out.println(findCapital("Spain").orElse("Not found"));   // Not found
  }
}

The method returns an Optional<String> instead of a String. This shows everyone who uses the method that the result can be missing, and orElse() handles that case without a crash.