Java - map a list of objects to a list with values of their property attributes

Question

I have the ViewValue class defined as follows:

class ViewValue {

private Long id;
private Integer value;
private String description;
private View view;
priv         


        

Answer 1

You can populate a map from the properties of a list of objects (say id as key and some property as value) as below

Map<String, Integer> mapCount = list.stream().collect(Collectors.toMap(Object::get_id, Object::proprty));

Answer 2

You could ude a wrapper:

public class IdList impements List<Long>
{
    private List<ViewValue> underlying;

    pubic IdList(List<ViewValue> underying)
    {
        this.underlying = underying;
    }

    public Long get(int index)
    {
        return underlying.get(index).getId()
    }

    // other List methods
}

though thats even more tedious work, it could improve performance.

You could also implement your and my solution genericaly using reflection, but that woud be very bad for perforance.

Theres no short and easy generic solution in Java, Im afraid. In Groovy, you would simply use collect(), but I believe that involves reflection as well.