Select positives from a list with Generics

user_17595294 0
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I am pretty new here, so excuse me if you do not understand my question.(Be nice :D) So basically I have to make a method which select positives "numbers" from a list using Generics.

The base of the method should look like this :

public static <T> List<T> selectPositives(List<? extends Number> list) {

        T positiveNumber = null;
        for (int i = 0; i < list.size(); i++) {
            if (list.get(i) > 0) {
                return (List<T>) positiveNumber;
            }
        }
        return (List<T>) positiveNumber;
    }

Well obviously this is not working, so I would greatly appreciate any help. Thanks, Alma

numbers list generics java
3 Answers

Using Stream API, the list of positives may be filtered out of the input:

public static <T extends Number> List<T> selectPositives(List<T> list) {
    return list.stream()
        .filter(x -> x.doubleValue() > 0.0)
        .collect(Collectors.toList());
}

Also, List::removeIf with inverted condition may be applied to the given input/mutable copy of the input:

public static <T extends Number> List<T> selectPositives(List<T> list) {

    List<T> result = new ArrayList<>(list);
    
    result.removeIf(x -> x.doubleValue() <= 0);
    
    return result;
}
user_132798310

I modified your code so that it would return a list of positive numbers.

    //public static <T> List<T> selectPositives(List<? extends Number> list) {
    public static <T extends Number> List<T> selectPositives(List<T> list) {
        //T positiveNumber = null;
        List<T> positiveNumbers = new ArrayList<>();
        for (int i = 0; i < list.size(); i++) {
            if (list.get(i).doubleValue() > 0) {
                //return (List<T>) positiveNumber;
                positiveNumbers.add(list.get(i));
            }
        }
        return positiveNumbers;
    }

    public static void main(String[] args) {
        
        List<Number> list = List.of(-1,-2,1.5, 3,-4,-5, 6);
        System.out.println(selectPositives(list));
    }

Output:

[1.5, 3, 6]
user_176946260

check for absolute value of the number, then compare it to the number without abs. if they match, then it's positive, and if not its negative.

add the elements that match to another list to have all the positive numbers in the list

user_177912220
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