Create instance of generic type in Java?

You are correct. You can't do new E(). But you can change it to

private static class SomeContainer<E> {
    E createContents(Class<E> clazz) {
        return clazz.newInstance();
    }
}

It's a pain. But it works. Wrapping it in the factory pattern makes it a little more tolerable.

I don't know if this helps, but when you subclass (including anonymously) a generic type, the type information is available via reflection. e.g.,

public abstract class Foo<E> {

  public E instance;  

  public Foo() throws Exception {
    instance = ((Class)((ParameterizedType)this.getClass().
       getGenericSuperclass()).getActualTypeArguments()[0]).newInstance();
    ...
  }

}

So, when you subclass Foo, you get an instance of Bar e.g.,

// notice that this in anonymous subclass of Foo
assert( new Foo<Bar>() {}.instance instanceof Bar );

But it's a lot of work, and only works for subclasses. Can be handy though.

You'll need some kind of abstract factory of one sort or another to pass the buck to:

interface Factory<E> {
    E create();
}

class SomeContainer<E> {
    private final Factory<E> factory;
    SomeContainer(Factory<E> factory) {
        this.factory = factory;
    }
    E createContents() {
        return factory.create();
    }
}

Here is an option I came up with, it may help:

public static class Container<E> {
    private Class<E> clazz;

    public Container(Class<E> clazz) {
        this.clazz = clazz;
    }

    public E createContents() throws Exception {
        return clazz.newInstance();
    }
}

EDIT: Alternatively you can use this constructor (but it requires an instance of E):

@SuppressWarnings("unchecked")
public Container(E instance) {
    this.clazz = (Class<E>) instance.getClass();
}

If you want not to type class name twice during instantiation like in:

new SomeContainer<SomeType>(SomeType.class);

You can use factory method:

<E> SomeContainer<E> createContainer(Class<E> class); 

Like in:

public class Container<E> {

    public static <E> Container<E> create(Class<E> c) {
        return new Container<E>(c);
    }

    Class<E> c;

    public Container(Class<E> c) {
        super();
        this.c = c;
    }

    public E createInstance()
            throws InstantiationException,
            IllegalAccessException {
        return c.newInstance();
    }

}

You can use:

Class.forName(String).getConstructor(arguments types).newInstance(arguments)

But you need to supply the exact class name, including packages, eg. java.io.FileInputStream. I used this to create a math expressions parser.

Hope this's not too late to help!!!

Java is type-safe, meaning that only Objects are able to create instances.

In my case I cannot pass parameters to the createContents method. My solution is using extends unlike the answer below.

private static class SomeContainer<E extends Object> {
    E e;
    E createContents() throws Exception{
        return (E) e.getClass().getDeclaredConstructor().newInstance();
    }
}

This is my example case in which I can't pass parameters.

public class SomeContainer<E extends Object> {
    E object;

    void resetObject throws Exception{
        object = (E) object.getClass().getDeclaredConstructor().newInstance();
    }
}

Using reflection create run time error, if you extends your generic class with none object type. To extends your generic type to object convert this error to compile time error.

Use the TypeToken<T> class:

public class MyClass<T> {
    public T doSomething() {
        return (T) new TypeToken<T>(){}.getRawType().newInstance();
    }
}

what you can do is -

1. First declare the variable of that generic class

   2.Then make a constructor of it and instantiate that object

2. Then use it wherever you want to use it

example-

1

private Class<E> entity;

2

public xyzservice(Class<E> entity) {
        this.entity = entity;
    }



public E getEntity(Class<E> entity) throws InstantiationException, IllegalAccessException {
        return entity.newInstance();
    }

3.

E e = getEntity(entity);

As you said, you can't really do it because of type erasure. You can sort of do it using reflection, but it requires a lot of code and lot of error handling.

If you mean new E() then it is impossible. And I would add that it is not always correct - how do you know if E has public no-args constructor? But you can always delegate creation to some other class that knows how to create an instance - it can be Class<E> or your custom code like this

interface Factory<E>{
    E create();
}    

class IntegerFactory implements Factory<Integer>{    
  private static int i = 0; 
  Integer create() {        
    return i++;    
  }
}

You can achieve this with the following snippet:

import java.lang.reflect.ParameterizedType;

public class SomeContainer<E> {
   E createContents() throws InstantiationException, IllegalAccessException {
      ParameterizedType genericSuperclass = (ParameterizedType)
         getClass().getGenericSuperclass();
      @SuppressWarnings("unchecked")
      Class<E> clazz = (Class<E>)
         genericSuperclass.getActualTypeArguments()[0];
      return clazz.newInstance();
   }
   public static void main( String[] args ) throws Throwable {
      SomeContainer< Long > scl = new SomeContainer<>();
      Long l = scl.createContents();
      System.out.println( l );
   }
}

Here is an improved solution, based on ParameterizedType.getActualTypeArguments, already mentioned by @noah, @Lars Bohl, and some others.

First small improvement in the implementation. Factory should not return instance, but a type. As soon as you return instance using Class.newInstance() you reduce a scope of usage. Because only no-arguments constructors can be invoke like this. A better way is to return a type, and allow a client to choose, which constructor he wants to invoke:

public class TypeReference<T> {
  public Class<T> type(){
    try {
      ParameterizedType pt = (ParameterizedType) this.getClass().getGenericSuperclass();
      if (pt.getActualTypeArguments() == null || pt.getActualTypeArguments().length == 0){
        throw new IllegalStateException("Could not define type");
      }
      if (pt.getActualTypeArguments().length != 1){
        throw new IllegalStateException("More than one type has been found");
      }
      Type type = pt.getActualTypeArguments()[0];
      String typeAsString = type.getTypeName();
      return (Class<T>) Class.forName(typeAsString);

    } catch (Exception e){
      throw new IllegalStateException("Could not identify type", e);
    }

  }
}

Here is a usage examples. @Lars Bohl has shown only a signe way to get reified geneneric via extension. @noah only via creating an instance with {}. Here are tests to demonstrate both cases:

import java.lang.reflect.Constructor;

public class TypeReferenceTest {

  private static final String NAME = "Peter";

  private static class Person{
    final String name;

    Person(String name) {
      this.name = name;
    }
  }

  @Test
  public void erased() {
    TypeReference<Person> p = new TypeReference<>();
    Assert.assertNotNull(p);
    try {
      p.type();
      Assert.fail();
    } catch (Exception e){
      Assert.assertEquals("Could not identify type", e.getMessage());
    }
  }

  @Test
  public void reified() throws Exception {
    TypeReference<Person> p = new TypeReference<Person>(){};
    Assert.assertNotNull(p);
    Assert.assertEquals(Person.class.getName(), p.type().getName());
    Constructor ctor = p.type().getDeclaredConstructor(NAME.getClass());
    Assert.assertNotNull(ctor);
    Person person = (Person) ctor.newInstance(NAME);
    Assert.assertEquals(NAME, person.name);
  }

  static class TypeReferencePerson extends TypeReference<Person>{}

  @Test
  public void reifiedExtenension() throws Exception {
    TypeReference<Person> p = new TypeReferencePerson();
    Assert.assertNotNull(p);
    Assert.assertEquals(Person.class.getName(), p.type().getName());
    Constructor ctor = p.type().getDeclaredConstructor(NAME.getClass());
    Assert.assertNotNull(ctor);
    Person person = (Person) ctor.newInstance(NAME);
    Assert.assertEquals(NAME, person.name);
  }
}

Note: you can force the clients of TypeReference always use {} when instance is created by making this class abstract: public abstract class TypeReference<T>. I've not done it, only to show erased test case.

Note that a generic type in kotlin could come without a default constructor.

 implementation("org.objenesis","objenesis", "3.2")

    val fooType = Foo::class.java
    var instance: T = try {
        fooType.newInstance()
    } catch (e: InstantiationException) {
//            Use Objenesis because the fooType class has not a default constructor
        val objenesis: Objenesis = ObjenesisStd()
        objenesis.newInstance(fooType)
    }

• Withou default constructor
• Objenesis

I was inspired with Ira's solution and slightly modified it.

abstract class SomeContainer<E>
{
    protected E createContents() {
        throw new NotImplementedException();
    }

    public void doWork(){
        E obj = createContents();
        // Do the work with E 
     }
}

class BlackContainer extends SomeContainer<Black>{
    // this method is optional to implement in case you need it
    protected Black createContents() {
        return new Black();
    }
}

In case you need E instance you can implement createContents method in your derived class (or leave it not implemented in case you don't need it.