Instantiating a generic class in Java

Question

I know Java's generics are somewhat inferior to .Net's.

I have a generic class Foo<T>, and I really need to instantiate a T in Foo using a parameter-less constructor. How can one work around Java's limitation?

Answer 1

One option is to pass in Bar.class (or whatever type you're interested in - any way of specifying the appropriate Class<T> reference) and keep that value as a field:

public class Test {
    public static void main(String[] args) throws IllegalAccessException,
            InstantiationException {
        Generic<Bar> x = new Generic<>(Bar.class);
        Bar y = x.buildOne();
    }
}

public class Generic<T> {
    private Class<T> clazz;

    public Generic(Class<T> clazz) {
        this.clazz = clazz;
    }

    public T buildOne() throws InstantiationException, IllegalAccessException {
        return clazz.newInstance();
    }
}

public class Bar {
    public Bar() {
        System.out.println("Constructing");
    }
}

Another option is to have a "factory" interface, and you pass a factory to the constructor of the generic class. That's more flexible, and you don't need to worry about the reflection exceptions.

Answer 2

And this is the Factory implementation, as Jon Skeet suggested:

interface Factory<T> {
    T factory();
}

class Araba {
    //static inner class for Factory<T> implementation
    public static class ArabaFactory implements Factory<Araba> {
        public Araba factory() {
            return new Araba();
        }
    }
    public String toString() { return "Abubeee"; }
}

class Generic<T> {
    private T var;

    Generic(Factory<T> fact) {
        System.out.println("Constructor with Factory<T> parameter");
        var = fact.factory();
    }
    Generic(T var) {
        System.out.println("Constructor with T parameter");
        this.var = var;
    }
    T get() { return var; }
}

public class Main {
    public static void main(String[] string) {
        Generic<Araba> gen = new Generic<Araba>(new Araba.ArabaFactory());
        System.out.print(gen.get());
    }
}

Output:

Constructor with Factory<T> parameter
Abubeee

Answer 3

Here's a rather contrived way to do it without explicitly using an constructor argument. You need to extend a parameterized abstract class.

public class Test {   
    public static void main(String [] args) throws Exception {
        Generic g = new Generic();
        g.initParameter();
    }
}

import java.lang.reflect.ParameterizedType;
public abstract class GenericAbstract<T extends Foo> {
    protected T parameter;

    @SuppressWarnings("unchecked")
    void initParameter() throws Exception, ClassNotFoundException, 
        InstantiationException {
        // Get the class name of this instance's type.
        ParameterizedType pt
            = (ParameterizedType) getClass().getGenericSuperclass();
        // You may need this split or not, use logging to check
        String parameterClassName
            = pt.getActualTypeArguments()[0].toString().split("\\s")[1];
        // Instantiate the Parameter and initialize it.
        parameter = (T) Class.forName(parameterClassName).newInstance();
    }
}

public class Generic extends GenericAbstract<Foo> {
}

public class Foo {
    public Foo() {
        System.out.println("Foo constructor...");
    }
}

Answer 4

I really need to instantiate a T in Foo using a parameter-less constructor

Simple answer is "you cant do that" java uses type erasure to implment generics which would prevent you from doing this.

How can one work around Java's limitation?

One way (there could be others) is to pass the object that you would pass the instance of T to the constructor of Foo<T>. Or you could have a method setBar(T theInstanceofT); to get your T instead of instantiating in the class it self.

Answer 5

From https://stackoverflow.com/a/2434094/848072. You need a default constructor for T class.

import java.lang.reflect.ParameterizedType;

class Foo<T> {
  
  public bar() {
    ParameterizedType superClass = (ParameterizedType) getClass().getGenericSuperclass();
    Class<T> type = (Class<T>) superClass.getActualTypeArguments()[0];
    try {
      T t = (T) type.getDeclaredConstructor().newInstance();
      //Do whatever with t
    } catch (Exception e) {
      // Oops, no default constructor
      throw new RuntimeException(e);
    } 
  }
}

Answer 6

For Java 8 ....

There is a good solution at https://stackoverflow.com/a/36315051/2648077 post.

This uses Java 8 Supplier functional interface

Answer 7

Use The Constructor.newInstance method. The Class.newInstance method has been deprecated since Java 9 to enhance compiler recognition of instantiation exceptions.

public class Foo<T> {   
    public Foo()
    {
        Class<T> newT = null; 
        instantiateNew(newT);
    }

    T instantiateNew(Class<?> clsT)
    {
        T newT;
        try {
            newT = (T) clsT.getDeclaredConstructor().newInstance();
        } catch (InstantiationException | IllegalAccessException | IllegalArgumentException
            | InvocationTargetException | NoSuchMethodException | SecurityException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
        return newT;
    }
}

Answer 8

Generics in Java are generally more powerful than in C#.

If you want to construct an object but without hardwiring a constructor/static method, use an abstract factory. You should be able to find detailed information and tutorials on the Abstract Factory Pattern in any basic design patterns book, introduction to OOP or all over the interwebs. It's not worth duplicating code here, other than to mention that Java's closure syntax sucks.

IIRC, C# has a special case for specifying a generic type has a no-args constructor. This irregularity, by definition, presupposes that client code wants to use this particular form of construction and encourages mutability.

Using reflection for this is just wrongheaded. Generics in Java are a compile-time, static-typing feature. Attempts to use them at runtime are a clear indication of something going wrong. Reflection causes verbose code, runtime failures, unchecked dependencies and security vulnerabilities. (Class.forName is particularly evil.)

Answer 9

I could do this in a JUnit Test Setup.

I wanted to test a Hibernate facade so I was looking for a generic way to do it. Note that the facade also implements a generic interface. Here T is the database class and U the primary key. Ifacade<T,U> is a facade to access the database object T with the primary key U.

public abstract class GenericJPAController<T, U, C extends IFacade<T,U>>

{
    protected static EntityManagerFactory emf;

    /* The properties definition is straightforward*/
    protected T testObject;
    protected C facadeManager;

    @BeforeClass
    public static void setUpClass() {


        try {
            emf = Persistence.createEntityManagerFactory("my entity manager factory");

        } catch (Throwable ex) {
            System.err.println("Failed to create sessionFactory object." + ex);
            throw new ExceptionInInitializerError(ex);
        }

    }

    @AfterClass
    public static void tearDownClass() {
    }

    @Before
    public void setUp() {
    /* Get the class name*/
        String className = ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[2].getTypeName();

        /* Create the instance */
        try {
            facadeManager = (C) Class.forName(className).newInstance();
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException ex) {
            Logger.getLogger(GenericJPAController.class.getName()).log(Level.SEVERE, null, ex);
        }
        createTestObject();
    }

    @After
    public void tearDown() {
    }

    /**
     * Test of testFindTEntities_0args method, of class
     * GenericJPAController<T, U, C extends IFacade<T,U>>.
     * @throws java.lang.ClassNotFoundException
     * @throws java.lang.NoSuchMethodException
     * @throws java.lang.InstantiationException
     * @throws java.lang.IllegalAccessException
     */
    @Test
    public void  testFindTEntities_0args() throws ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException {

        /* Example of instance usage. Even intellisense (NetBeans) works here!*/
        try {
            List<T> lista = (List<T>) facadeManager.findAllEntities();
            lista.stream().forEach((ct) -> {
                System.out.println("Find all: " + stringReport());
            });
        } catch (Throwable ex) {
            System.err.println("Failed to access object." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }


    /**
     *
     * @return
     */
    public abstract String stringReport();

    protected abstract T createTestObject();
    protected abstract T editTestObject();
    protected abstract U getTextObjectIndex();
}

Answer 10

Quick solution that worked for me. I see there is already an answer for this and this may not even be the best way to go about it. Also, for my solution you'll need Gson.

However, I ran into a situation where I needed to create an instance of a generic class of type java.lang.reflect.Type.

The following code will create an instance of the class you want with null instance variables.

T object = new Gson().fromJson("{}", myKnownType);

Where myKnownType is known before hand and obtained via TypeToken.getType().

You can now set appropriate properties on this object. Again, this may not be the best way to do this but it works as a quick solution if that's what you need.