Question

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());

Answer 1

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?)