Dynamically casting to primitives in Java

Edit: This has since been solved. Thanks to everyone who helped. Invoking the method after casting the object as the correct wrapper class worked. But String.valueOf() is much, much simpler to achieve the same effect.

Hello--

What I'm trying to do may not even be possible. I've spent a few hours now researching and experimenting with various things, so I figured I'd finally ask around to see if anyone knows if this is even possible.

Is it possible, using reflection, to dynamically cast a wrapper for a primitive of an unknown type as a primitive?

I'm basically trying to create a generic toString function which can handle the conversion of any type of primitive to a string. Such a seemingly simple thing is frustratingly difficult (and I am aware I could just test each type to see if it is of type Wrapper.class and cast it specifically, but at this point I'm just pursuing this out of stubbornness).

The following throws a ClassCastException. The primClass class appears 开发者_Go百科to be the right one (gives "int" when printing primClass.getName()).

    private String toString(Number obj){
    String result = "";
    try{
        Class objClass = obj.getClass();
        Field field = objClass.getDeclaredField("TYPE");
        Class primClass = (Class)field.get(obj);
        Method method = objClass.getMethod("toString", new Class[]{primClass});
        Object args = new Object[]{primClass.cast(obj)};
        result = (String)method.invoke(null, args);
    }catch(Exception ex){
        //Unknown exception. Send to handler.
        handleException(ex);
    }
    return result;
}

So I'm a bit at a loss, really. Anyone have any ideas? Any help would be greatly appreciated.

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Perhaps I'm missing something, but obj.toString() would do.

If you look at the implementations, it is calling String.valueOf(value) which in turn calls Double.toString(..) or Long.toString(..) or whatever. So, calling toString() automatically calls the required method. Without any reflection from your part.

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You might want to have a look at Apache Commons Lang, Especially ToStringBuilder.reflectionToString(). Even if you don't want to introduce a dependency just for a toString(), it's open source so you can have a look at the implementation.

method.invoke accept the Wrapper types instead of the primivtes types.

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Perhaps there is something I don't understand in your question but for primitive, you can do ""+primitive to cast it to a String.

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What you are trying to do doesn't really make sense.... when your function is called with a primitive argument (e.g. an int) then it will automatically get boxed into an Integer. So you might as well just call obj.toString() on it.....

However if you really want to do something special with primitives, you might want to do something like the following using method overloading:

private String toString(Object obj){
  return obj.toString();
}

private String toString(int intValue) {
  // code to return string for the primitive int case, assuming it is different
}

// more overloads for other primitive argument types as needed.....

This can be a very useful technique for dealing with primitives in some cases.

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The immediate problem in your code is that obj is an Object, and therefore cannot be an instance of a primitive type. (It must be an instance of the corresponding wrapper type). The primClass.cast(obj) call must fail for any primitive type class.

But if you simply want to turn a primitive wrapper instance to a String, just call the instance's toString() method.

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String.valueOf(arg) will do it nicely too.