How to Write a generic method to find the maximal element and invoke that method?

Question

While I was trying to solve exercise from generics tutorial Q&A My answers were slightly different

My Answers

public static <T extends Comparable<? super T>>
    T max(List<? extends T> list, int begin, int end) //Option1

public static <T extends Comparable<T>>
    T max(List<? extends T> list, int begin, int end) //Option2

from quoted answer below

So My question is

• Option1 :Would it make any difference if T extends Object & Comparable<? super T> is replaced with T extends Comparable<? super T>. Isn't extends Object implicit ?

• Option2 :Would it make any difference if Comparable<? super T> is replaced with Comparable<T> ? if so How ?

• Eclipse code completion creates local variable List<? extends Comparable<? super Comparable<? super T>>> list; on Ctrl+1 max(list, 1, 10); which is bit lengthy. How to Define a classes (hierarchy) that extends Comparable<? super T> , create list and add instances to the list and invoke below method ? Basically I want to know how to invoke max() after adding class instances A or B into a list where class B extends A

---

Write a generic method to find the maximal element in the range [begin, end) of a list.

Answer:

import java.util.*;

public final class Algorithm {
    public static <T extends Object & Comparable<? super T>>
        T max(List<? extends T> list, int begin, int end) {

        T maxElem = list.get(begin);

        for (++begin; begin < end; ++begin)
            if (maxElem.compareTo(list.get(begin)) < 0)
                maxElem = list.get(begin);
        return maxElem;
    }
}

Answer 1

Would it make any difference if Comparable<? super T> is replaced with Comparable<T> ? if so How ?

Remember that Comparables are always consumers, i.e., a Comparable<T> consumes T instances, so it should always be preferrable to use Comparable<? super T> instead of Comparable<T> (Quoting - PECS). It would make difference in case you are comparing a type whose super class implements a Comparable<SuperType>. Consider the following code:

class Parent implements Comparable<Parent> {
    protected String name;

    @Override
    public int compareTo(Parent o) {
        return this.name.compareTo(o.name);
    }
}

class Child extends Parent {
    public Child(String name) {
        this.name = name;
    }
}

Now if you give your type parameter as T extends Comparable<T>, you won't be able to call that method for List<Child>, as Child does not implement Comparable<Child> but Comparable<Parent>:

public static <T extends Comparable<T>> T max(List<? extends T> list, int begin, int end) {
    ...
}

public static void main(String[] args) {
    List<Child> list = new ArrayList<Child>();
    max(list, 0, 2);  // Error with current method. Child does not implement Comparable<Child>
}

Hence the type parameter bounds should be T extends Comparable<? super T>.

Note that, you can't change your Child class to:

class Child extends Parent implements Comparable<Child>

because in that case, Child class would extend from different instantiation of same generic type, which is not allowed.

---

Would it make any difference if T extends Object & Comparable<? super T> is replaced with T extends Comparable<? super T>. Isn't extends Object implicit ?

Well, there is a difference between the two bounds. In the 1^st bound, the erasure of the type parameter is Object, whereas in the 2^nd bound, the erasure is Comparable.

So, without Object bound, your code will compile to:

public static Comparable max(List list, int begin, int end)

The issue might come when you are generifying the legacy non-generic code. It's neccessary to give Object also as upper bound to avoid breaking the Byte Code compatibility. You can read more about it on this link: Angelika Langer - Programming Idioms