在java中添加类的值

Question

There was a question in my book that said this (there are no answers):

Suppose we have a class Beta declared with the header:

 class Beta extends Alpha 

so Beta is a subclass of Alpha, and in class Alpha there is a method with header:

 public int value(Gamma gam) throws ValueException 

Write a static method called addValues which takes an object which could be of type ArrayList<Alpha> or ArrayList<Beta> , and also an object of type Gamma . A call to the method addValues must return the sum obtained by adding the result of a call of value on each of the objects in the ArrayList argument, with the Gamma argument as the argument to each call of value. Any call of value which causes an exception of type ValueException to be thrown should be ignored in calculating the sum.

My Attempt:

public static int addValues(ArrayList<? extends Alpha> arr, Gamma gam) {
    int sum = 0;
    for (int i = 0; i < arr.size(); i++) {
        try {
            sum += arr.get(i) + gam;
        } catch (Exception e) {
            i++;
        }
    }
    return sum;
}

Although I know for starters that the line sum += arr.get(i) + gam is going to give me an error, because they are not straight forward int s that can be added. The book provides no more information on the question so what I have written here is everything required for the question.

Answer 1

You are supposed to call the value method in order to get the values you are supposed to be adding.

Beside that, you can use the enhanced for loop to make your code cleaner.

public static int addValues(ArrayList<? extends Alpha> arr, Gamma gam) 
{
    int sum = 0;
    for (Alpha alpha : arr) {
        try {
            sum += alpha.value(gam);
        } catch (ValueException e) {
        }
    }
    return sum;
}

Answer 2

Use wild card with bounded type as Alpha as you want to access the value then use arr.

public static int addValues(ArrayList<? extends Alpha> arr, Gamma gam) 
{
    int sum = 0;
    for (Alpha alpha : arr) {
        try {
            sum += alpha.value(gam);
        } catch (Exception e) {
        }
    }
    return sum;
}

Regarding the second question of overriding the equal function

public boolean equals(Gamma g) {
        if(g!= null && g instanceof Alpha && value(g) == value(this.myGam)){
            return true;
        }else{
            return false;
        }
    }