在方法中实现 generics 的最佳方法是什么？

Question

I'm having issues with a programming exercise because the book I'm reading only showed how to use print generic methods. Which is easy. But the exercise asks to find the the largest and smallest value of an ArrayList. I figured out a way to do it, shown in my code, but I'm just thinking to myself what's the point of using generics in the first place?

Here are the exercise instructions:

Write a generic class named MyList, with a type parameter T. The type parameter T should be constrained to an upper bound: the Number class. The class should have as a field an ArrayList of T. Write a public method named add, which accepts a parameter of type T. When an argument is passed to the method, it is added to the ArrayList. Write two other methods, largest and smallest, which return the largest and smallest values in the ArrayList.

My code:

//Main

import java.io.*;
import java.util.*; 

public class Main {

    static PrintWriter writer = new PrintWriter(System.out, true);

    public static void main(String[] args) throws IOException {

        ArrayList<Integer> list = new ArrayList<>();

        MyList<Integer> myList = new MyList<>(list);

        myList.add(3);
        myList.add(5);
        myList.add(44);
        myList.add(654);

        int first = myList.getArrayList().get(0);
        int largest = 0;
        int smallest = 0;

        for (int i = 1; i < list.size(); i++) {
            largest = myList.getLargest(first, myList.getArrayList().get(i));
            smallest = myList.getSmallest(first, myList.getArrayList().get(i));
        }

        System.out.println("Largest: " + largest + "\nSmallest: " + smallest);

    }
}

//MyList Class

import java.util.ArrayList;

public class MyList <T extends Number> {
    private ArrayList<T> arrayList;

    public MyList(ArrayList<T> arrayList) {
        this.arrayList = arrayList;
    }

    public void add(T item) {
        arrayList.add(item);
    }

    public ArrayList<T> getArrayList() {
        return arrayList;
    }

    public int getLargest(int x, int y) {
        return Math.max(x, y);
    }

    public int getSmallest(int x, int y) {
        return Math.min(x, y);
    }
}

If I try writing the methods with T types I get errors when using operators like > or + and Math.max() also won't work. I understand why now, I just don't understand what type am I suppose to return and input for the methods. int, Integer, or T? And if I wanted to use a an ArrayList using doubles what then so the methods can work with both types? Any input as to what would be the best way to solve this problem using generics would be greatly appreciated. Thank you.

Answer 1

I won't do your homework for you, but I can put you on the right track...

1 - I would define my ArrayList and initialized it on the same line (and not require the user to supply one on the constructor).

public class MyList<T extends Number> {
  private ArrayList<T> arrayList = new ArrayList<T>();

2 - Define your two methods to return T

public T getLargest() {
}

public T getSmallest() {
}

3 - You'll want to convert everything to Double's (the largest possible Number implementation)

T largest = (T) Double.valueOf(0f);

T smallest = (T) Double.valueOf(Double.MAX_VALUE);

4 - Loop through all the numbers in your arrayList property and do the appropriate comparisons

for (T number : numbers) {
  // because T is an extension of Number, we can invoke the doubleValue method
  // offered on the Number class
  // convert your T variables to doubles
  double v = number.doubleValue();

  // you can then compare the doubleValue to keep the largest/smallest values.
}

Hope this was helpful without doing your homework for you. :-)