Java Generics - Write a generic method to find the maximal element in the range [begin, end] of a list

So, here's the thing, what you have is not a generic method. What you have is a generic class.

Now, it's a clearly defined generic class, but, it is not what the question asked for. (as a generic class it has a number of issues too, but let's get the method/class issue resolved first).

Generic methods

A generic method is just that, a method, except the parameters (or return value) are of a generic type. A generic method always has a <...> structure before the return-type declaration. A normal method is:

public RType methodName(P1Type p1, P2Type p2, ....) {...}

(where RType is the return type, and P1Type is parameter1 type, etc.).

A generic method has the <...> before the return type, and that construct is used somewhere in the method signature... for example:

public <T> RType methodName(T t, SomeType sparm) {....}

The above is a generic method that has the generic type T as a parameter.

Your Class

So, having stated that your solution is a generic class, not a generic method, let's assume the exercise goal was to produce a generic class. What then?

1. Unless you have exceptional reasons, don't extend ArrayList, "compose" it instead. Have a class that does not extend ArrayList, and have a class field instead, like:

   class MaximalElementList<E extends Comparable<E>> {
   
       List<E> data = ......
   

2. Your generics here are OK, no problem with <E extends Comparable<E>>

3. Your code should use a guard-condition instead of an else... let me explain. Your code has an if/else:

   public E getMaxInRange(int begin, int end) {
       E max;
       if (isEmpty())
           throw new IllegalStateException("Can't get a max element from an empty list.");
       else {
           max = this.get(begin);
           for (E elem : this.subList(begin, end)) {
               if (elem.compareTo(max) > 0) {
                   max = elem;
               }
           }
       }
       return max;
   }
   

   That should instead be:

   public E getMaxInRange(int begin, int end) {
       if (isEmpty()) {
           throw new IllegalStateException("Can't get a max element from an empty list.");
       }
   
       E max;
       max = this.get(begin);
       for (E elem : this.subList(begin, end)) {
           if (elem.compareTo(max) > 0) {
               max = elem;
           }
       }
       return max;
   }
   

4. Note now, that the max is a messy variable, it can just be:

       E max = this.get(begin);
       for (E elem : this.subList(begin, end)) {
           if (elem.compareTo(max) > 0) {
               max = elem;
           }
       }
       return max;
   

5. Your use of a sublist is smart, but I would consider it to be overkill in this case. How about a simpler implementation:

       E max = this.get(begin);
       for (int i = begin + 1; i < end; i++) {
           if (get(i).compareTo(max) > 0) {
               max = get(i);
           }
       }
       return max;
   

That is now some logic which I think would work well.

Making it a method

Putting that logic in a generic method would "simply" mean:

public <E extends Comparable<E>> E max(List<E> data, int begin, int end) {
    E max = data.get(begin);
    for (int i = begin + 1; i < end; i++) {
        if (data.get(i).compareTo(max) > 0) {
            max = data.get(i);
        }
    }
    return max;

}

With that method, for example, you could do:

List<String> data = Files.readAllLines(Paths.get("some file.txt"));
String maxLine = max(data, 0, 10);