泛型类的特定于类型的构造函数

Question

I have the following generic class:

public class DropdownItem<V, D> {

    private V value;
    private D display;

    public DropdownItem(V value, D display) {
        this.value = value;
        this.display = display;
    }

    public V getValue() {
        return value;
    }

    public void setValue(V value) {
        this.value = value;
    }

    public D getDisplay() {
        return display;
    }

    public void setDisplay(D display) {
        this.display = display;
    }
}

How do I create a constructor for specific types?

For example,

public DropdownItem(CustomClass custom) {
    this(custom.getFoo(), custom.getBar());
}

or

public DropdownItem(CustomClass custom) {
    this.value = custom.getFoo();
    this.display = custom.getBar();
}

Neither of those solutions work. It does work to do this when implementing the generic class:

DropdownItem<Integer, String> myItem = new DropdownItem<Integer, String>(custom.getFoo(), custom.getBar());

However, I would like to include a constructor in the generic class to accomplish this. Any ideas?

Answer 1

It looks like a factory method, in addition to the existing constructor, can help you:

public static DropdownItem<Integer, String> getCustomClassInstance(CustomClass custom)
{
    return new DropdownItem<Integer, String>(custom.getFoo(), custom.getBar());
}

It can't be another constructor. Your class is generic, so any constructor must deal with generic types V and D to assign them to value and display . it can't be specific types in the constructor(s) for this generic class.