Question

I am trying to construct an object using the following constructor in Java:

public UnaryNumberContract(Comparable<Number> expectedValue, Operations operator, Invariant inv) {
  super(inv, operator);
  this.expectedValue = expectedValue;
}

I am passing on the following parameters:

Comparable<Number> zero = new Integer(0); 
UnaryNumberContract contract = new UnaryNumberContract(zero, Operations.NOT_EQUALS, (Invariant) inv);

And I keep getting a Type mismatch: cannot convert from Integer to Comparable<Number>

I’ve checked and Number is a super class of Integer, so I don’t quite understand what is goinh wrong here.

Answer 1

If you replace references to Comparable<Number> with Comparable<? extends Number>, that will solve the issue.

Also, see this When do Java generics require <? extends T> instead of <T> and is there any downside of switching?