Creating new instance from Class with constructor parameter

Question

I have situation where my Java class needs to create a ton of certain kind of objects. I would like to give the name of the class of the objects that are created as a parameter. In addition, I need to give the created class a parameter in its constructor. I have something like

class Compressor {

    Class ccos;

    public Compressor(Class ccos) {
        this.ccos = ccos;
    }

    public int getCompressedSize(byte[] array) {
        OutputStream os = new ByteArrayOutputStream();
        // the following doesn't work because ccos would need os as its constructor's parameter
        OutputStream cos = (OutputStream) ccos.newInstance();
        // ..
    }
}

Do you have any ideas how I could remedy this?

Edit:

This is part of a research project where we need to evaluate the performance of multiple different compressors with multiple different inputs. Class ccos is a compressed OutputStream either from Java's standard library, Apache Compress Commons or lzma-java.

Currently I have the following which appears to work fine. Other ideas are welcome.

OutputStream os = new ByteArrayOutputStream();
OutputStream compressedOut = (OutputStream) ccos.getConstructor(OutputStream.class).newInstance(os);
final InputStream sourceIn = new ByteArrayInputStream(array);

Why do you need to tell Compressor what kind of objects to create? What kinds of classes will be used here? — Karl Knechtel
– Karl Knechtel, Commented Dec 8, 2010 at 11:29
Have you considered using an abstract factory for this rather than hacking it with reflections? — Tom Hawtin - tackline
– Tom Hawtin - tackline, Commented Dec 8, 2010 at 13:19
I guess the question is complete now. Any additional information is perhaps for another question. — Bozho
– Bozho, Commented Dec 8, 2010 at 15:16
It would be great if somebody could write a proper answer based on all the information above so I could accept it. — lhahne
– lhahne, Commented Dec 8, 2010 at 17:38
@Ihahne - the additional information does not seem directly related to this question. And there is no question there at all. — Bozho
– Bozho, Commented Dec 9, 2010 at 12:10

Bozho · Accepted Answer · 2010-12-08 11:29:16Z

18

You can use the Class.getConstructor(paramsTypes...) method and call newInstance(..) on the constructor. In your case:

Compressor.class.getConstructor(Class.class).newInstance(Some.class);

answered Dec 8, 2010 at 11:29

Bozho

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3 Comments

lhahne Over a year ago

I think you mean Compressor.class.getConstructor(Class.class).newInstance(someparameter);

Bozho Over a year ago

@lhahne well, since your parameter is of type Class, it can be Some.class

srinivas Over a year ago

Simple example: CreateObjects is a java class, Can invoke newInstance by the below way without any arguments for constructor. CreateObjects obj2 = CreateObjects.class.getConstructor().newInstance();

lisak · Accepted Answer · 2013-05-15 17:23:48Z

4

Using Spring ClassUtils and BeanUtils classes you can avoid dealing with those tedious exceptions that is Spring handling for you :

Constructor<Car> constructor = ClassUtils.getConstructorIfAvailable(Wheels.class, Etc.class);
Car car = BeanUtils.instantiateClass(constructor, new Wheels(), new Etc());

answered May 15, 2013 at 17:23

lisak

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Comments

NPE · Accepted Answer · 2010-12-08 11:29:09Z

1

You have to get to the relevant Constructor object (e.g. via Class.getConstructors or Class.getConstructor) and then call constructor.newInstance, giving it the arguments it requires.

answered Dec 8, 2010 at 11:29

NPE

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Comments

David Newcomb · Accepted Answer · 2018-11-25 13:53:27Z

0

An example you can use is as follows: lets say conn is a connection to the database.

Class[] btarray = { conn.getClass() };      
try {
   if (classname != null) {
      pmap = (Mapper) Class.forName(classname)
                           .getConstructor(btarray)
                           .newInstance(
                              new Object[] { conn }
                           );
   }
} catch (Throwable x) {
   x.printStackTrace(Log.out);
}

btarray allows you to pass in arguments to the constructor.

edited Nov 25, 2018 at 13:53

David Newcomb

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answered May 6, 2013 at 0:28

leroneb

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Comments

WBAR · Accepted Answer · 2010-12-08 11:48:27Z

-1

class Compresor<T> {
    private Class<? extends T> clazz;
    Compresor(final Class<? extends T> cls){
        this.clazz = cls
    }
}

answered Dec 8, 2010 at 11:48

WBAR

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