Question

I am reading the book “Core Java” by Hortsmann and Cornell (Vol.1).
In the Generics chapter, they suggest an input that is purposely wrong, in order to have a look at what the compiler says. My problem is: not only the compiler doesn’t give me the same error, but it will keep on giving me errors also when I correct the input as stated in the book. Here’s the deal:

public class PairTest1 {
    public static void main(String[] args) {
        double middle = ArrayAlg.getMiddle(3.14, 1729, 0);
    }
}

class ArrayAlg {
    public static <T> T getMiddle(T[] a) {
        return a[a.length / 2];
    }
}

The book says I should get the error:

found: java.lang.Number&java.lang.Comparable>, required: double.

Instead, Eclipse won’t let me run the program, saying

The method getMiddle(T[]) in the type ArrayAlg is not applicable for
the arguments (double, int, int)

The book then says

the remedy is to write all parameters as double values.

I therefore changed 1729 to 1729.1 and 0 to 0.1. Now Eclipse says:

The method getMiddle(T[]) in the type ArrayAlg is not applicable for
the arguments (double, double, double)

Mmmhh… I didn’t get what is going on. Suggestions?

Thank you & regards

EDIT: I tried to create the array double[] doubles = { 3.14, 1729.1, 0.1 }; and to give it as parameter, but now I get the error The method getMiddle(T[]) in the type ArrayAlg is not applicable for the arguments (double[])

Answer 1

You should change your call to

double middle = ArrayAlg.getMiddle(new Double[]{ 3.14, 1729.0, 0.0 });

If you want to pass them the way you are doing, then you should change the declaration to

public static <T> T getMiddle(T... a)
{
    return a[a.length / 2];
}

using the var-arg syntax of Java. Then you can call the method as

double middle = ArrayAlg.getMiddle(3.14, 1729.0, 0.0);