Inferred type is not a valid substitute for a Comparable generic type

Question

Consider the code:

public abstract class Item implements Comparable
{
    protected T item;

    public int compareTo(T o)
    {
        re         


        

Answer 1

Actually more detailed explanation of this error gives your javac itself:

java: no suitable method found for sort(java.util.ArrayList>)

method java.util.Collections.sort(java.util.List,java.util.Comparator) is not applicable (cannot instantiate from arguments because actual and formal argument lists differ in length)

method java.util.Collections.sort(java.util.List) is not applicable (inferred type does not conform to declared bound(s) inferred: MyItem bound(s): java.lang.Comparable>)

So, the main question is:
why is method Collections.sort(java.util.List)) not applicable?

The answer is:
because in Collections.sort(java.util.List) method declaration there are bounds on parameter T: >.

In another words, T must implement Comparable interface on it self. For example String class implements such interface: ...implements ... Comparable.

In your case Item class doesn't implement such interface:

Item implements Comparable is not same thing as Item implements Comparable>.

So, for solving this problem, your should change your Item class to this one:

public abstract class Item implements Comparable>
{
    protected T item;

    public int compareTo(Item o)
    {
        return 0; // this doesn't matter for the time being
    }
}