Create new instance of object's class

Question

Let's say I have an object called myCar that is an instance of Car.

myCar = new Car();

How would I do to create a new instance of that class based on the object? Let's say that I don't know which class myCar was created from.

otherObject = new myCar.getClass()(); // Just do demonstrate what I mean (I know this doesn't work)

UPDATE

public class MyClass {
    public MyClass(int x, int y, Team team) { }
    public MyClass() { }
}

Object arg = new Object[] {2, 2, Game.team[0]};

try {
    Constructor ctor = assignedObject.getClass().getDeclaredConstructor(int.class, int.class, Team.class);
    ctor.setAccessible(true);
    GameObject obj = (GameObject) ctor.newInstance(arg);

} catch (InstantiationException x) {
    x.printStackTrace();
} catch (IllegalAccessException x) {
    x.printStackTrace();
} catch (InvocationTargetException x) {
    x.printStackTrace();
} catch (NoSuchMethodException x) {
    x.printStackTrace();
}

I get the following error:

java.lang.IllegalArgumentException: wrong number of arguments

getDeclaredConstructor() works and finds my constructor with three args, but newInstance(arg) won't work for some reason, it says "wrong number of arguments". Any idea why?

This should help, also: http://stackoverflow.com/questions/7698237/whats-the-proper-way-to-use-reflection-to-instantiate-objects-of-unknown-classe — laminatefish, Aug 14 '13 at 20:10

Sotirios Delimanolis · Accepted Answer · 2016-07-07T19:41:32.180

16

With reflection

otherObject = myCar.getClass().newInstance();

Assuming your class has a default constructor. You can do more advanced operations with non default (empty) constructors

Constructor[] constructors = myCar.getClass().getConstructors();

And choose the one you want.

Read through this for more details about Java's Reflection capabilities.

edited Jul 07 '16 at 19:41

answered Aug 14 '13 at 20:09

Sotirios Delimanolis

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Thank you, is there any way I can pass in parameters to the constructor? – lawls Aug 14 '13 at 20:13
@lawls That is a more advanced topic, [here's](http://docs.oracle.com/javase/tutorial/reflect/member/ctorInstance.html) how. You need to pass arguments to the [`newInstance()`](http://docs.oracle.com/javase/7/docs/api/java/lang/reflect/Constructor.html#newInstance(java.lang.Object...)) method of the `Constructor` class, if and only if that constructor accepts those arguments. – Sotirios Delimanolis Aug 14 '13 at 20:14
It says I have to pass an array with the arguments. Let's say I have two arguments, a string and an int. How would I create that array with different types? Thanks. – lawls Aug 14 '13 at 20:31
Please take a look at my updated question. I have provided my code plus the error I'm getting. Do you know what seems to be the problem? Thank you. – lawls Aug 14 '13 at 20:54
@lawls I was wrong about how to call it. Use `(MyClass) ctor.newInstance(2, 2, Game.team[0]);` The method expects a vararg parameter, not an `Object[]`. – Sotirios Delimanolis Aug 14 '13 at 21:19
@lawls No problem, sorry for confusion. I couldn't understand why for a moment lol. Good luck! – Sotirios Delimanolis Aug 14 '13 at 21:22
@dantuch From the method side it is, but if you pass an `Object[]` to a method expecting a vararg, it'll be considered as one element in the first index position of that array. – Sotirios Delimanolis Aug 14 '13 at 21:28
@SotiriosDelimanolis show some example proving it. I heard something similar before, but right now I've written simple code that works just fine: `public static void main(String[] args) throws Exception { Object[] objects = new Object[] { "foo", "bar" }; DupaReflect.foo(new Object()); }` & `private static void foo(Object... objects) { for (Object object : objects) { System.out.println(object); } }` it prints elements of `Object[]` as `foo bar`, not as something like `java.lang.Object@1bab50a`... – dantuch Aug 14 '13 at 21:53
1

@SotiriosDelimanolis Ah, now I remeber - array of primitives will not work, as it cannot be autoboxed to array of wrappers http://stackoverflow.com/a/2926653/575659 #3. But when it comes to Objects it does not matter if it is explicit vararg or array of them. – dantuch Aug 14 '13 at 21:58
@dantuch You can edit my question with that info if you want. Or we just leave it in here. Good find. – Sotirios Delimanolis Aug 14 '13 at 21:59

score 0 · Answer 2 · answered Aug 14 '13 at 20:11

0

Through reflection. Use this:

myCar.getClass().newInstance();

answered Aug 14 '13 at 20:11

Kuldeep Jain

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score 0 · Answer 3 · answered Aug 14 '13 at 21:20

Regarding your UPDATE/error - look here:

public class DupaReflect {

    private String name;
    private int id;

    private DupaReflect(String name, int id) {
        super();
        this.name = name;
        this.id = id;
    }

    @Override
    public String toString() {
        return "DupaReflect [name=" + name + ", id=" + id + "]";
    }

    public static void main(String[] args) throws Exception {
        Object[] objects = new Object[] { "asd", 1 };
        DupaReflect dupa = DupaReflect.class.getDeclaredConstructor(String.class, int.class).newInstance(objects);
        System.out.println(dupa);
    }

}

This works and produces desired output. DupaReflect [name=asd, id=1]

Object[] objects = new Object[]{"asd", 1}; - that's the reason why my code works and yours does not.

In your code you have:

Object arg = new Object[] {2, 2, Game.team[0]};

Object[] is indeed type of Object in java, but this changes 3-element array of some objects into single object being that array. So you try to create a new object not with 3 arguments, but with 1, thus the exception is thrown

SOLUTION: Declare your Object arg as Object[] args and all should work.

Create new instance of object's class

3 Answers3