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!

Integer only has int or String argument constructors, no Integer copy constructor. With reflection, you cannot unbox the parameters automatically. — Gábor Bakos
– Gábor Bakos, Commented Jan 7, 2015 at 13:43
Then why does this works Integer c = new Integer(new Integer(5)); — yonutix
– yonutix, Commented Jan 7, 2015 at 13:45
You are making a lot of assumptions in a generic method here. Not all classes are going to have a constructor that takes a value of the same class. — mikea
– mikea, Commented Jan 7, 2015 at 13:45
@CosminMihai That's because auto unboxing happens from Integer to int automatically there.. — SMA
– SMA, Commented Jan 7, 2015 at 13:46
More importantly: Why do you want to copy an instance of an immutable class? What do you gain if, you have another boxed Integer with the same int value? — Balázs Édes
– Balázs Édes, Commented Jan 7, 2015 at 14:02

Bohemian · Accepted Answer · 2015-01-07 14:05:16Z

5

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.

edited Jan 7, 2015 at 14:05

answered Jan 7, 2015 at 13:54

Bohemian♦

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4 Comments

yonutix Over a year ago

Ok, I understand, thanks. If you have any idea how I could dodge that keeping the purpose of the method generic please share.

Bohemian Over a year ago

@CosminMihai See code for how to handle elegantly. Merry Xmas :)

yonutix Over a year ago

Thank you very much, hope that the corresponding class for char primitive type is Character

Bohemian Over a year ago

@CosminMihai yes, it is. All wrappers except Integer and Character are the same name as the primative but capitalised.

Nagappan · Accepted Answer · 2015-01-07 14:22:26Z

2

Generic approach of copying any object to another object. This util class is available in the package - org.apache.commons.lang3.

 Integer c = (Integer) SerializationUtils.clone(a);

answered Jan 7, 2015 at 14:22

Nagappan

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2 Comments

yonutix Over a year ago

SerializationUtils is not in the standard library, I was looking for something nice.

Balázs Édes Over a year ago

Using a mature library, that is already used by many users and big projects is much nicer, than putting something together in a few hours by yourself. Just my opinion.

BarrySW19 · Accepted Answer · 2015-01-07 13:59:52Z

To really understand why "new Integer(new Integer(5))" works while reflections doesn't it's useful to look at the generated byte code for the first case:

ICONST_5
INVOKESPECIAL java/lang/Integer.<init> (I)V
INVOKEVIRTUAL java/lang/Integer.intValue ()I
INVOKESPECIAL java/lang/Integer.<init> (I)V

... as you can see, a call is being made to the intValue() method of Integer "under the hood". So, the java compiler is actually translating your "new Integer(new Integer(5))" to "new Integer(new Integer(5).intValue())". This means it can use the constructor which takes an int.

Because the java compiler cannot know the actual runtime type of the variable for the reflections call it cannot do anything similar and can only look for a constructor with the actual runtime type argument.

I think i will put some if statements or a switch for Integer, Float, Double and Long classes to resolve those particular cases

Wael Dahman · Accepted Answer · 2015-01-07 14:43:10Z

-1

int is a type but Integer is a wrapper (class) that use int as an object with many fonction

answered Jan 7, 2015 at 14:43

Wael Dahman

1

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