orElse, orElseGet, orElseThrow of Optional class

orElseは、isEmpty() = trueの場合に別の値にすり替える
orElseGet()はSupplier
orElseThrow()は extends Throw> Supplier

class SampleException extends Exception {
}
public class Outer {
    public static void main(String[] args) {
        System.out.println(Optional.<String>of("A").orElse("X"));
        System.out.println(Optional.<String>empty().orElse("X"));
        System.out.println(Optional.<String>empty().orElseGet(() -> "Y"));
        try {
            System.out.println(Optional.<String>empty().orElseThrow(() -> new SampleException()));
        } catch (SampleException ex) {
            ex.printStackTrace();
        }
    }
}

A
X
Y
com.mycompany.mavenproject1.SampleException
	at com.mycompany.mavenproject1.Outer.lambda$main$1(Outer.java:18)
	at java.base/java.util.Optional.orElseThrow(Optional.java:408)
	at com.mycompany.mavenproject1.Outer.main(Outer.java:18)