How to create instance of subclass with constructor from super class

Question

I'd like to create a registry for classes which are subclasses of a super class. The classes are stored in a map which acts as registry. A class is picked from the registry depending on a key and an instance of that class will be created via reflection.

I'd like to instantiate a class depending on a constructor (with 1 parameter) of the super class. It works only if I declare the constructor in the subclasses as well.

Is there a way to instantiate the class using a constructor of a super class? Is there a way to make that code type-safe?

Example code:

public class ReflectionTest {

    /**
     * Base class with no-args constructor and another constructor with 1 parameter
     */
    public static class BaseClass {

        Object object;

        public BaseClass() {
            System.out.println("Constructor with no args");
        }

        public BaseClass( Object object) {
            this.object = object;
            System.out.println("Constructor with parameter= " + object);
        }

        public String toString() {
            return "Object = " + object;
        }
    }

    /**
     * Subclass with 1 parameter constructor
     */
    public static class SubClass1 extends BaseClass {
        public SubClass1( Object object) {
            super(object);
        }
    }

    /**
     * Subclass with no-args constructor
     */
    public static class SubClass2 extends BaseClass {

    }

    public static void main(String[] args) {

        // registry for classes
        Map<Integer,Class<?>> registry = new HashMap<>();
        registry.put(0, SubClass1.class);
        registry.put(1, SubClass2.class);

        // iterate through classes and create instances
        for( Integer key: registry.keySet()) {

            // get class from registry
            Class<?> clazz = registry.get(key);

            try {

                // get constructor with parameter
                Constructor constructor = clazz.getDeclaredConstructor( Object.class);

                // instantiate class
                BaseClass instance = (BaseClass) constructor.newInstance(key);

                // logging
                System.out.println("Instance for key " + key + ", " + instance);

            } catch (NoSuchMethodException | SecurityException | InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
                e.printStackTrace();
            }

        }

        System.exit(0);
    }
}

The example gives the following console output:

Constructor with parameter= 0
Instance for key 0, Object = 0
java.lang.NoSuchMethodException: swing.table.ReflectionTest$SubClass2.<init>(java.lang.Object)
    at java.lang.Class.getConstructor0(Class.java:3082)
    at java.lang.Class.getConstructor(Class.java:1825)
    at swing.table.ReflectionTest.main(ReflectionTest.java:63)

What would you want to do if a particular subclass had both zero and one-arg constructors? — Andy Turner
– Andy Turner, Commented Mar 9, 2016 at 6:40

Graham Russell · Accepted Answer · 2018-09-14 17:34:05Z

12

A superclass has no knowledge of its children.
Constructors are not inherited.

Therefore, without making assumptions about the subclass ctor, you cannot write the code that you want.

So what can you do? Use an Abstract Factory pattern.

We can create an interface Factory:

@FunctionalInterface
public interface SuperclassFactory {
    Superclass newInstance(Object o);
}

You could create more than one method on the Factory, but that would make it less neat for lambdas.

Now you have a Map<Integer, SuperclassFactory>, and populate it:

Map<Integer,SuperclassFactory> registry = new HashMap<>();
registry.put(0, SubClass1::new);
registry.put(1, SubClass2::new);

So, in order to use this Map you simply do:

for(final Map.Entry<Integer,SuperclassFactory> e: registry.entrySet()) {
    //...
    final BaseClass instance = e.getValue().newInstance(e.getKey());
    //...
}

If your subclass does not have the appropriate ctor, this code will not compile as there will be no ctor reference that can be used. This is Good Thing (TM). In order to compile with the current Subclass2 you would need to use:

registry.put(1, obj -> new SubClass2());

So now we have:

Lost the reflection
Acquired compile time type safety
Lost the ugly cast (although that was through misuse of reflection)

N.B. loop through a Map's entrySet() not its keySet().

edited Sep 14, 2018 at 17:34

Graham Russell

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answered Mar 9, 2016 at 6:45

Boris the Spider

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2 Comments

user4910279 Over a year ago

e.getValue().apply(e.getKey()) must be e.getValue().newInstace(e.getKey()).

Spotted Over a year ago

Great answer that definitely handles the problem in a clean and safe way (I wish there are more of these on SO). Reminds me of this question.

Ankit Bansal · Accepted Answer · 2016-03-09 06:37:49Z

1

Constructors need to be defined explicitly in the subclass. If a constructor is defined in superclass that doesn't mean that constructor can be used to create an instance of subclass whether you are using reflection or not.

Since your SubClass2 doesn't have constructor with one argument, so when you try to create an instance of it with one argument, it's throwing NoSuchMethodException.

answered Mar 9, 2016 at 6:37

Ankit Bansal

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Comments

Andy Turner · Accepted Answer · 2016-03-09 06:49:27Z

1

Use clazz.getDeclaredConstructors() to get all constructors of the class;
Iterate through them to find the best applicable constructor, e.g. pick the zero-args one if a single-Object-arg constructor is not available;
Invoke that constructor using the appropriate parameters.

There is no way for this to be entirely safe, since you can't know in advance whether any applicable public constructors exist for a given class, e.g. the ctor might be private, or the available constructors might not accept parameters of the type you want (e.g. needs a String, but you only have an Object).

edited Mar 9, 2016 at 6:49

answered Mar 9, 2016 at 6:43

Andy Turner

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1 Comment

Boris the Spider Over a year ago

One might argue that with the magic of reflection, private is not a hindrance. But that way madness lies...

mirmdasif · Accepted Answer · 2016-03-09 07:17:30Z

0

You are using this line to get the constructor

clazz.getDeclaredConstructor( Object.class);

But your Subclass2 does not have a single argument constructor hence the exception is thrown.

Use clazz.getDeclaredConstructors() method instead and invoke constructor based on parameters count.

            BaseClass instance;
            // get constructor with parameter
            Constructor constructor = clazz.getDeclaredConstructors()[0];
            if (constructor.getParameterCount() == 1) {
                instance = (BaseClass) constructor.newInstance(key);
            } else {
                instance = (BaseClass) constructor.newInstance();
            }

answered Mar 9, 2016 at 7:17

mirmdasif

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3 Comments

Boris the Spider Over a year ago

And what if I have a ctor that takes String and one that takes number? You need to check the types of those parameters.

mirmdasif Over a year ago

This was not the issue in his code right? BaseClass instance = (BaseClass) constructor.newInstance(key); He was wants to create an instance with explicitly passing the parameters. If he wanted a to pass a string he would not passed the KEY in the constructor.

Boris the Spider Over a year ago

Sure, not this exact instance. But the OP is looking for a general solution - I'm sure the OP is perfectly capable of solving this one problem in some manner or another. The question asks for how to do this in general - and this is not a complete answer for that.

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