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Create instance of generic type in Java?

Posted by: admin November 2, 2017 Leave a comment

Questions:

Is it possible to create an instance of a generic type in Java? I’m thinking based on what I’ve seen that the answer is no (due to type erasure), but I’d be interested if anyone can see something I’m missing:

class SomeContainer<E>
{
    E createContents()
    {
        return what???
    }
}

EDIT: It turns out that Super Type Tokens could be used to resolve my issue, but it requires a lot of reflection-based code, as some of the answers below have indicated.

I’ll leave this open for a little while to see if anyone comes up with anything dramatically different than Ian Robertson’s Artima Article.

Answers:

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.

Questions:
Answers:

Dunno 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.

Questions:
Answers:

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();
    }
}

Questions:
Answers:

In Java 8 you can use the Supplier functional interface to achieve this pretty easily:

class SomeContainer<E> {
  private Supplier<E> supplier;

  SomeContainer(Supplier<E> supplier) {
    this.supplier = supplier;
  }

  E createContents() {
    return supplier.get();
  }
}

You would construct this class like this:

SomeContainer<String> stringContainer = new SomeContainer<>(String::new);

The syntax String::new on that line is a constructor reference.

If your constructor takes arguments you can use a lambda expression instead:

SomeContainer<BigInteger> bigIntegerContainer
    = new SomeContainer<>(() -> new BigInteger(1));

Questions:
Answers:
package org.foo.com;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

/**
 * Basically the same answer as noah's.
 */
public class Home<E>
{

    @SuppressWarnings ("unchecked")
    public Class<E> getTypeParameterClass()
    {
        Type type = getClass().getGenericSuperclass();
        ParameterizedType paramType = (ParameterizedType) type;
        return (Class<E>) paramType.getActualTypeArguments()[0];
    }

    private static class StringHome extends Home<String>
    {
    }

    private static class StringBuilderHome extends Home<StringBuilder>
    {
    }

    private static class StringBufferHome extends Home<StringBuffer>
    {
    }   

    /**
     * This prints "String", "StringBuilder" and "StringBuffer"
     */
    public static void main(String[] args) throws InstantiationException, IllegalAccessException
    {
        Object object0 = new StringHome().getTypeParameterClass().newInstance();
        Object object1 = new StringBuilderHome().getTypeParameterClass().newInstance();
        Object object2 = new StringBufferHome().getTypeParameterClass().newInstance();
        System.out.println(object0.getClass().getSimpleName());
        System.out.println(object1.getClass().getSimpleName());
        System.out.println(object2.getClass().getSimpleName());
    }

}

Questions:
Answers:

If you need a new instance of a type argument inside a generic class then make your constructors demand its class…

public final class Foo<T> {

    private Class<T> typeArgumentClass;

    public Foo(Class<T> typeArgumentClass) {

        this.typeArgumentClass = typeArgumentClass;
    }

    public void doSomethingThatRequiresNewT() throws Exception {

        T myNewT = typeArgumentClass.newInstance();
        ...
    }
}

Usage:

Foo<Bar> barFoo = new Foo<Bar>(Bar.class);
Foo<Etc> etcFoo = new Foo<Etc>(Etc.class);

Pros:

  • Much simpler (and less problematic) than Robertson’s Super Type Token (STT) approach.
  • Much more efficient than the STT approach (which will eat your cellphone for breakfast).

Cons:

  • Can’t pass Class to a default constructor (which is why Foo is final). If you really do need a default constructor you can always add a setter method but then you must remember to give her a call later.
  • Robertson’s objection… More Bars than a black sheep (although specifying the type argument class one more time won’t exactly kill you). And contrary to Robertson’s claims this does not violate the DRY principal anyway because the compiler will ensure type correctness.
  • Not entirely Foo<L>proof. For starters… newInstance() will throw a wobbler if the type argument class does not have a default constructor. This does apply to all known solutions though anyway.
  • Lacks the total encapsulation of the STT approach. Not a big deal though (considering the outrageous performance overhead of STT).
Questions:
Answers:

You can do this now and it doesn’t require a bunch of reflection code.

import com.google.common.reflect.TypeToken;

public class Q26289147
{
    public static void main(final String[] args) throws IllegalAccessException, InstantiationException
    {
        final StrawManParameterizedClass<String> smpc = new StrawManParameterizedClass<String>() {};
        final String string = (String) smpc.type.getRawType().newInstance();
        System.out.format("string = \"%s\"",string);
    }

    static abstract class StrawManParameterizedClass<T>
    {
        final TypeToken<T> type = new TypeToken<T>(getClass()) {};
    }
}

Of course if you need to call the constructor that will require some reflection, but that is very well documented, this trick isn’t!

Here is the JavaDoc for TypeToken.

Questions:
Answers:

Think about a more functional approach: instead of creating some E out of nothing (which is clearly a code smell), pass a function that knows how to create one, i.e.

E createContents(Callable<E> makeone) {
     return makeone.call(); // most simple case clearly not that useful
}

Questions:
Answers:

From Java Tutorial – Restrictions on Generics:

Cannot Create Instances of Type Parameters

You cannot create an instance of a type parameter. For example, the following code causes a compile-time error:

public static <E> void append(List<E> list) {
    E elem = new E();  // compile-time error
    list.add(elem);
}

As a workaround, you can create an object of a type parameter through reflection:

public static <E> void append(List<E> list, Class<E> cls) throws Exception {
    E elem = cls.newInstance();   // OK
    list.add(elem);
}

You can invoke the append method as follows:

List<String> ls = new ArrayList<>();
append(ls, String.class);

Questions:
Answers:

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();
}

Questions:
Answers:

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();
    }

}

Questions:
Answers:

When you are working with E at compile time you don’t really care the actual generic type “E” (either you use reflection or work with base class of generic type) so let the subclass provide instance of E.

Abstract class SomeContainer<E>
{

    abstract protected  E createContents();
    public doWork(){
        E obj = createContents();
        // Do the work with E 

     }
}


**BlackContainer extends** SomeContainer<Black>{
    Black createContents() {
        return new  Black();
    }
}

Questions:
Answers:

I thought I could do that, but quite disappointed: it doesn’t work, but I think it still worths sharing.

Maybe someone can correct:

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

interface SomeContainer<E> {
    E createContents();
}

public class Main {

    @SuppressWarnings("unchecked")
    public static <E> SomeContainer<E> createSomeContainer() {
        return (SomeContainer<E>) Proxy.newProxyInstance(Main.class.getClassLoader(),
                new Class[]{ SomeContainer.class }, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                Class<?> returnType = method.getReturnType();
                return returnType.newInstance();
            }
        });
    }

    public static void main(String[] args) {
        SomeContainer<String> container = createSomeContainer();

    [*] System.out.println("String created: [" +container.createContents()+"]");

    }
}

It produces:

Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.String
    at Main.main(Main.java:26)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Line 26 is the one with the [*].

The only viable solution is the one by @JustinRudd

Questions:
Answers:

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 );
   }
}

Questions:
Answers:

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.

Questions:
Answers:

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.

Questions:
Answers:

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++;    
  }
}

Questions:
Answers:
return   (E)((Class)((ParameterizedType)this.getClass().getGenericSuperclass()).getActualTypeArguments()[0]).newInstance();

Questions:
Answers:

An imporovement of @Noah’s answer.

Reason for Change

a] Is safer if more then 1 generic type is used in case you changed the order.

b] A class generic type signature changes from time to time so that you will not be surprised by unexplained exceptions in the runtime.

Robust Code

public abstract class Clazz<P extends Params, M extends Model> {

    protected M model;

    protected void createModel() {
    Type[] typeArguments = ((ParameterizedType) this.getClass().getGenericSuperclass()).getActualTypeArguments();
    for (Type type : typeArguments) {
        if ((type instanceof Class) && (Model.class.isAssignableFrom((Class) type))) {
            try {
                model = ((Class<M>) type).newInstance();
            } catch (InstantiationException | IllegalAccessException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

Or use the one liner

One Line Code

model = ((Class<M>) ((ParameterizedType) this.getClass().getGenericSuperclass()).getActualTypeArguments()[1]).newInstance();

Questions:
Answers:

There are various libraries that can resolve E for you using techniques similar to what the Robertson article discussed. Here’s an implemenation of createContents that uses TypeTools to resolve the raw class represented by E:

E createContents() throws Exception {
  return TypeTools.resolveRawArgument(SomeContainer.class, getClass()).newInstance();
}

This assumes that getClass() resolves to a subclass of SomeContainer and will fail otherwise since the actual parameterized value of E will have been erased at runtime if it’s not captured in a subclass.

Questions:
Answers:

Here’s an implementation of createContents that uses TypeTools to resolve the raw class represented by E:

E createContents() throws Exception {
  return TypeTools.resolveRawArgument(SomeContainer.class, getClass()).newInstance();
}

This approach only works if SomeContainer is subclassed so the actual value of E is captured in a type definition:

class SomeStringContainer extends SomeContainer<String>

Otherwise the value of E is erased at runtime and is not recoverable.

Questions:
Answers:

You can with a classloader and the class name, eventually some parameters.

final ClassLoader classLoader = ...
final Class<?> aClass = classLoader.loadClass("java.lang.Integer");
final Constructor<?> constructor = aClass.getConstructor(int.class);
final Object o = constructor.newInstance(123);
System.out.println("o = " + o);

Questions:
Answers:

Java unfortunatly does not allow what you want to do. See http://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#createObjects

You cannot create an instance of a type parameter. For example, the following code causes a compile-time error:

public static <E> void append(List<E> list) {
    E elem = new E();  // compile-time error
    list.add(elem);
}

As a workaround, you can create an object of a type parameter through reflection:

public static <E> void append(List<E> list, Class<E> cls) throws Exception {
    E elem = cls.newInstance();   // OK
    list.add(elem);
}

You can invoke the append method as follows:

List<String> ls = new ArrayList<>();
append(ls, String.class);