CODEWITHSUNDEEP
javaguava
fishtoprecords
fishtoprecords

Reputation: 2404

Can't use Guava Optional for default constructor?

I can't figure out the proper way to have a default constructor argument that is an Guava Optional. The following code will not compile with Java 1.6

public class ShoppingCart implements Serializable {
private final Optional<DiscountCoupon> discountCoupon;

public ShoppingCart() {
    this(Optional.absent());
}
public ShoppingCart(Optional<DiscountCoupon> dc) {
    Preconditions.checkNotNull(dc);
    if (dc.isPresent()) {
        Preconditions.checkArgument(dc.get().getPennyDiscount() != 0 || 
                        dc.get().getPercentDiscount() != 0);
    }
    discountCoupon = dc;
}

Error reported is is:

/Users/pfarrell/sandbox//com/wayfinderdigital/struct/ShoppingCart.java:29: cannot find symbol
symbol  : constructor ShoppingCart(com.google.common.base.Optional<java.lang.Object>)
location: class com.wayfinderdigital.struct.ShoppingCart
    this(Optional.absent());

Upvotes: 1

Views: 1364

Answers (2)

Mike Samuel
Mike Samuel

Reputation: 120546

Try changing

this(Optional.absent());

to

this(Optional.<DiscountCoupon>absent());

The second uses an explicit type parameter specification, so doesn't require type parameter inference. Type parameter inference can be done based on input parameters or based on the type to which an expression is being assigned, but many other site-of-use inferences don't work automatically so the type parameter assumes its lower bound.

Upvotes: 3

Louis Wasserman
Louis Wasserman

Reputation: 198321

Java can't automatically infer the generics from the code you've written. Instead, use

public ShoppingCart() {
  this(Optional.<DiscountCoupon>absent());
}

(Though I have to ask -- are you sure that Optional is really appropriate here?)

Upvotes: 5

Related Questions