Can I use Class.newInstance() with constructor arguments?

I would like to use Class.newInstance() but the class I am instantiating does not have a nullary constructor. Therefore I need to be able to pass in constructor arguments. Is there a way to do this?

Asked by: Michelle568 | Posted: 28-01-2022

Answer 1

MyClass.class.getDeclaredConstructor(String.class).newInstance("HERESMYARG");

obj.getClass().getDeclaredConstructor(String.class).newInstance("HERESMYARG");

Answered by: Kellan974 | Posted: 01-03-2022

Answer 2

myObject.getClass().getDeclaredConstructors(types list).newInstance(args list);

Edit: according to the comments seems like pointing class and method names is not enough for some users. For more info take a look at the documentation for getting constuctor and invoking it.

Answered by: Adrian669 | Posted: 01-03-2022

Answer 3

Assuming you have the following constructor

class MyClass {
    public MyClass(Long l, String s, int i) {

    }
}

You will need to show you intend to use this constructor like so:

Class classToLoad = MyClass.class;

Class[] cArg = new Class[3]; //Our constructor has 3 arguments
cArg[0] = Long.class; //First argument is of *object* type Long
cArg[1] = String.class; //Second argument is of *object* type String
cArg[2] = int.class; //Third argument is of *primitive* type int

Long l = new Long(88);
String s = "text";
int i = 5;

classToLoad.getDeclaredConstructor(cArg).newInstance(l, s, i);

Answered by: Kellan302 | Posted: 01-03-2022

Answer 4

Do not use Class.newInstance(); see this thread: Why is Class.newInstance() evil?

Like other answers say, use Constructor.newInstance() instead.

Answered by: John479 | Posted: 01-03-2022

Answer 5

You can get other constructors with getConstructor(...).

Answered by: Elise657 | Posted: 01-03-2022

Answer 6

Follow below steps to call parameterized consturctor.

Get Constructor with parameter types by passing types in Class[] for getDeclaredConstructor method of Class
Create constructor instance by passing values in Object[] for
newInstance method of Constructor

Example code:

import java.lang.reflect.*;

class NewInstanceWithReflection{
    public NewInstanceWithReflection(){
        System.out.println("Default constructor");
    }
    public NewInstanceWithReflection( String a){
        System.out.println("Constructor :String => "+a);
    }
    public static void main(String args[]) throws Exception {

        NewInstanceWithReflection object = (NewInstanceWithReflection)Class.forName("NewInstanceWithReflection").newInstance();
        Constructor constructor = NewInstanceWithReflection.class.getDeclaredConstructor( new Class[] {String.class});
        NewInstanceWithReflection object1 = (NewInstanceWithReflection)constructor.newInstance(new Object[]{"StackOverFlow"});

    }
}

output:

java NewInstanceWithReflection
Default constructor
Constructor :String => StackOverFlow

Answered by: Kimberly468 | Posted: 01-03-2022

Answer 7

You can use the getDeclaredConstructor method of Class. It expects an array of classes. Here is a tested and working example:

public static JFrame createJFrame(Class c, String name, Component parentComponent)
{
    try
    {
        JFrame frame = (JFrame)c.getDeclaredConstructor(new Class[] {String.class}).newInstance("name");
        if (parentComponent != null)
        {
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }
        else
        {
            frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        }
        frame.setLocationRelativeTo(parentComponent);
        frame.pack();
        frame.setVisible(true);
    }
    catch (InstantiationException instantiationException)
    {
        ExceptionHandler.handleException(instantiationException, parentComponent, Language.messages.get(Language.InstantiationExceptionKey), c.getName());
    }
    catch(NoSuchMethodException noSuchMethodException)
    {
        //ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.NoSuchMethodExceptionKey, "NamedConstructor");
        ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.messages.get(Language.NoSuchMethodExceptionKey), "(Constructor or a JFrame method)");
    }
    catch (IllegalAccessException illegalAccessException)
    {
        ExceptionHandler.handleException(illegalAccessException, parentComponent, Language.messages.get(Language.IllegalAccessExceptionKey));
    }
    catch (InvocationTargetException invocationTargetException)
    {
        ExceptionHandler.handleException(invocationTargetException, parentComponent, Language.messages.get(Language.InvocationTargetExceptionKey));
    }
    finally
    {
        return null;
    }
}

Answered by: Ada145 | Posted: 01-03-2022

Answer 8

I think this is exactly what you want http://da2i.univ-lille1.fr/doc/tutorial-java/reflect/object/arg.html

Although it seems a dead thread, someone might find it useful

Answered by: Charlie194 | Posted: 01-03-2022

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