How to make the following method common?

Question

I am currently studying object-oriented programming, and now I am studying the Generics section. I have the following code from a Java class, and I ask you to do this in order to make the last calculateSquares method general. I am really puzzled by this, I can tell you.

Here is the class:

public class OefeningWildcards {

public static void main(String[] args){
    new OefeningWildcards().run();
}

public OefeningWildcards(){

}

public void run(){
    Number[] numbers = {2, 3.1, 5, 12, 2.3};
    List<Number> numbersList = new ArrayList<>();
    for (Number n : numbers){
        numbersList.add(n);


    List<Number> dest = new ArrayList<>();
    calculateSquares(dest, numbersList);

    Double[] doubles = {3.2, 5.6, 4.4, 6.5, 12.2};
    List<Double> doublesList = new ArrayList<>();

    List<Number> dest2 = new ArrayList<>();
    calculateSquares(dest2, doublesList);

    List<Double> dest3 = new ArrayList<>();
    calculateSquares(dest3, doublesList);

    }
}


//This method needs to become generic
public void calculateSquares(List<Number> dest, List<Number> src){
    dest.clear();
    for (Number n : src) {
        dest.add(n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

What I learned is that the List Number should become a List? extends Number , for example:

public void calculateSquares(List<? extends Number> dest, List<? extends Number> src){
    dest.clear();
    for (Number n : src) {
        dest.add(n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

The thing is, I don't know what to do with the for loop:

for (Number n : src) {
        dest.add(n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

My logical guess is:

for (? extends Number n : src) {
        dest.add(n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

But that seems wrong. What should I do for the for loop?

Answer 1

, , . , ? extends Number. , . List<Integer>, Double. , , .

,

public void <T extends Number> calculateSquares(List<Number> dest, List<T> src){
    dest.clear();
    for (T n : src) {
        dest.add(n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

, , . - :

public void calculateSquares(List<Number> dest, List<? extends Number> src){
    dest.clear();
    for (Number n : src) {
        dest.add(n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

, List<Double> List<Number> . , :

public void calculateSquares(List<? super Double> dest, List<? extends Number> src){
    dest.clear();
    for (Number n : src) {
        dest.add((double)n * n.intValue());
        dest.add(n.doubleValue() * n.doubleValue());
    }
}

Answer 2

-, dest. . , , .

-, , , - . , . . SO .

-, . . :

public List<Number> calculateSquares(List<? extends Number> src){
        List<Number> squares = new ArrayList<>();
        for (Number n : src) {
            squares.add(n.doubleValue() * n.doubleValue());
        }
        return squares;
    }