InstantiationException when using a generic java class

Question

I would like to create a simulation containing an entity. There can be two kinds of entities, complex and simple ones. When I instantiate a simple simulation, I want the simple entity to be instantiated, and when I instantiate a complex simulation I want the complex entity to be instantiated.

Entities:

class ComplexEntity extends Entity {
    public ComplexEntity(){}
}

class SimpleEntity extends Entity  {
    public SimpleEntity(){}
}

class Entity {
    public Entity(){}
}

Simulations:

class ComplexSimulation extends Simulation<ComplexEntity>
{

    public ComplexSimulation() throws InstantiationException, IllegalAccessException {
        super(ComplexEntity.class);
    }

}

class SimpleSimulation extends Simulation<SimpleEntity>
{
    public SimpleSimulation() throws InstantiationException, IllegalAccessException
    {
        super(SimpleEntity.class);
    }
}

class Simulation<E extends Entity> {
    protected final E entity;

    public Simulation(Class<E> class1) throws InstantiationException, IllegalAccessException 
    {
        entity = class1.newInstance();
    }
}

The problem is that when I try to construct a ComplexSimulation:

ComplexSimulation c = new ComplexSimulation();

I get the following InstantiationException:

java.lang.InstantiationException: test.Test$ComplexEntity
at java.lang.Class.newInstance(Unknown Source)
at test.Test$Simulation.<init>(Test.java:55)
at test.Test$ComplexSimulation.<init>(Test.java:37)
at test.Test.go(Test.java:12)
at test.Test.main(Test.java:6)
Caused by: java.lang.NoSuchMethodException: test.Test$ComplexEntity.<init>()
at java.lang.Class.getConstructor0(Unknown Source)

The zero argument constructor cannot be the problem because my entities have them... Does anybody know what the problem can be?

The exception should be telling you what is wrong. Post the stacktrace. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Oct 5, 2015 at 16:26
You are using a generic class without erasure, the generic type doesn't now the actual type. — user2173738
– user2173738, Commented Oct 5, 2015 at 16:27
The problem is that you're using inner classes. Move each class into a proper top class or mark them static. — Luiggi Mendoza
– Luiggi Mendoza, Commented Oct 5, 2015 at 16:27

Community · Accepted Answer · 2017-05-23 11:59:22Z

3

The problem is that you're using inner classes. You cannot create an instance of an inner class without an instance of the outer class, not even if you call Class<InnerClass>.newInstance().

Just make these inner classes to be top classes and your example should work as expected.

If you really want/need to initialize non-static inner classes using reflection, see here: How to instantiate inner class with reflection in Java?

edited May 23, 2017 at 11:59

CommunityBot

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answered Oct 5, 2015 at 16:29

Luiggi Mendoza

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5 Comments

user2173738 Over a year ago

this is also not the case

marczoid Over a year ago

Yes it is the case nikpon! Thanks Luiggi, your solution solved my problem.

Luiggi Mendoza Over a year ago

@nikpon yes it is. Just look at the stacktrace.

Bohemian Over a year ago

It there a way to make invoking newInstance() (or similar), on an inner class work?

Luiggi Mendoza Over a year ago

@Bohemian yes, this is covered in other Q/A so I've posted the link here in my answer.

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