java access Integer constructor through reflection

Question

I'm having this code. Why doesn't it works?(Working meaning that it display 3) How can I fix it?

public class Main {
    public static<V> V copy(V var){
        try{ 
            return (V) var.getClass().getConstructor(var.getClass()).newInstance(var);
        }
        catch(Exception e){
            System.out.println("Copy faield " + e.getMessage() + " ");
            e.printStackTrace();
        }
        return null;
    }

    public static void main(String[] args) {
        Integer a = new Integer(3);
        Integer b = copy(a);

        System.out.println(a);
        System.out.println(b);


    }
}

This is the output:

 Copy faield java.lang.Integer.<init>(java.lang.Integer) 
    java.lang.NoSuchMethodException: java.lang.Integer.<init>(java.lang.Integer)
        at java.lang.Class.getConstructor0(Class.java:2818)
        at java.lang.Class.getConstructor(Class.java:1723)
        at Main.copy(Main.java:7)
        at Main.main(Main.java:19)
    3
    null

Thanks!

Answer 1

The problem here is the distinction between:

Integer.class
int.class

The constructor for Integer takes an int parameter, not an Integer.

To get your magic method to work, you would need to do a special check of the type, and if it's a wrapper class, actually look for a constructor whose parameter is the corresponding primative type.

AFAIK there's no built in way to get the primatove class from the wrapper class - you could use a map and populate it with the mappings:

private static final Map<Class<?>, Class<?>> MAP = new HashMap<>() {{
    put(Integer.class, int.class);
    put(Long.class, long.class);
    // etc
}};

Then in your method:

Class<?> type = MAP.containsKey(var.getClass()) ? MAP.get(var.getClass()) : var.getClass();
return (V) var.getClass().getConstructor(type).newInstance(var);

It's OK to pass the int as an Integer in the parameter value - that at least gets auto unboxed.