Creating an instance using the class name and calling constructor

Question

Is there a way to create an instance of a particular class given the class name (dynamic) and pass parameters to its constructor.

Something like:

Obj         


        

Answer 1

Very Simple way to create an object in Java using Class with constructor argument(s) passing:

Case 1:- Here, is a small code in this Main class:

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Main {

    public static void main(String args[]) throws ClassNotFoundException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {

        // Get class name as string.
        String myClassName = Base.class.getName();
        // Create class of type Base.
        Class myClass = Class.forName(myClassName);
        // Create constructor call with argument types.
        Constructor ctr = myClass.getConstructor(String.class);
        // Finally create object of type Base and pass data to constructor.
        String arg1 = "My User Data";
        Object object = ctr.newInstance(new Object[] { arg1 });
        // Type-cast and access the data from class Base.
        Base base = (Base)object;
        System.out.println(base.data);
    }

}

And, here is the Base class structure:

public class Base {

    public String data = null;

    public Base() 
    {
        data = "default";
        System.out.println("Base()");
    }

    public Base(String arg1) {
        data = arg1;
        System.out.println("Base("+arg1+")");
    }

}

Case 2:- You, can code similarly for constructor with multiple argument and copy constructor. For example, passing 3 arguments as parameter to the Base constructor will need the constructor to be created in class and a code change in above as:

Constructor ctr = myClass.getConstructor(String.class, String.class, String.class);
Object object = ctr.newInstance(new Object[] { "Arg1", "Arg2", "Arg3" }); 

And here the Base class should somehow look like:

public class Base {

    public Base(String a, String b, String c){
        // This constructor need to be created in this case.
    }   
}

Note:- Don't forget to handle the various exceptions which need to be handled in the code.