Cannot instantiate generic data type in class

Question

I have an immutable class , with the following layout ,

public final class A <T> { private ArrayList<T> myList; private A(){ myList = new ArrayList<T>(); } public A<T> addData(T t){ A newOb = // make a new list and instantiate using overloaded constructor T newT = new T(); ***********ERROR HERE************* newOb.myList.add(newT); return newOb; } ......... }

The error that I get here is cannot instantiate type T . Now , I think this is related to maybe type erasure in Java.

How can I overcome this ? I want to add a new copy of the argument that is being passed to addData into my list.

possible duplicate of How to obtain class instance generic argument type — Joachim Sauer
– Joachim Sauer, Commented Aug 23, 2012 at 14:10
Objects are created at Runtime. So we need proper byte code at run time. at max Java erasure can do convert T newT = new T(); to Object newT = new Object() to be available on runtime. But that also wrong since T can be anything extends Object not only Object. Hence compiler error. — Kanagavelu Sugumar
– Kanagavelu Sugumar, Commented Jun 8, 2016 at 14:58

Ransom Briggs · Accepted Answer · 2012-08-23 14:22:23Z

8

T newT = (T) t.getClass().newInstance() // assuming zero args constructor and you'll // have to catch some reflection exceptions

answered Aug 23, 2012 at 14:22

Ransom Briggs

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3 Comments

h4ck3d Over a year ago

I used this , T was Integer in my class , it threw exceptions at runtime.

newacct Over a year ago

@sTEAK.: so if you could do new T(), what would it mean when T is Integer anyway?

h4ck3d Over a year ago

At complile time T is replaced by Object

rustyx · Accepted Answer · 2017-09-27 09:25:34Z

In Java 8 you can pass a factory lambda which would create a new instance of the desired type:

public final class A <T> { private ArrayList<T> myList; private A(){ myList = new ArrayList<T>(); } public A<T> addData(Supplier<T> factory){ A newOb = // make a new list and instantiate using overloaded constructor T newT = factory.get(); newOb.myList.add(newT); return newOb; } ......... }

Use it like this for example:

A<Integer> a = new A<>(); a.addData( () -> new Integer(0) );

The built-in no-argument Supplier interface can be used as the wrapper for the callback.

This should be the way to go, without workarounds and/or assumptions...
Should be the accepted answer. I am not pro in Java but I guess explicit type casting is rarely a good sign... What do the pros say ?
Hello, great solution! I came up with a doubt, hoping you could share some light, why do I need to create a new A just because it is declared as final? Or is it a best practice to create a new class?

Joachim Sauer · Accepted Answer · 2012-08-23 14:14:42Z

1

In java language generics are implemented by erasure, so it is impossible to instantiate a generic type. Also it is impossible to instiate an array of generic type and so on.

How can I overcome this ? I want to add a new copy of the argument that is being passed to addData into my list.

You can try to use Cloneable interface as a type bound or add your own similar interface.

edited Aug 23, 2012 at 14:14

Joachim Sauer

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answered Aug 23, 2012 at 14:09

gkuzmin

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5 Comments

newacct Over a year ago

Cloneable as type bound is completely useless

gkuzmin Over a year ago

Unless it is used right and clone() is implemented, I suppose.

newacct Over a year ago

in order to call a method on an expression, the static type of the expression must have this method

gkuzmin Over a year ago

So T extends Object & Cloneable would be more applicable. Nice addition, I did not notice that.

newacct Over a year ago

everything automatically extends Object. That doesn't matter. Object's clone is not public and is only meant for a subclass to use to call the super implementation. There is no abstract type that specifies a public clone method in Java.

Vitaly · Accepted Answer · 2015-01-03 21:46:15Z

I looked for solution of similar problem. Here is the solution I made:

public final class ImmutableA<T> { private ArrayList<T> myList; private Class<T> clazz; public static <E> ImmutableA<E> createEmpty(Class<E> clazz) { return new ImmutableA<>(clazz); } private ImmutableA(Class<T> clazz) { this.myList = new ArrayList<T>(); this.clazz = clazz; } public ImmutableA<T> addData(T t) { ImmutableA<T> newOb = new ImmutableA<>(clazz); try { Constructor<T> constructor = clazz.getDeclaredConstructor(clazz); T newT = constructor.newInstance(t); newOb.myList.add(newT); return newOb; } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) { e.printStackTrace(); } return null; } public ArrayList<T> getMyList() { return myList; } public static void main(String[] args) throws InstantiationException, IllegalAccessException { ImmutableA<String> immutableA = createEmpty(String.class); System.out.print(immutableA.getMyList().toString()); immutableA = immutableA.addData("a"); System.out.print(immutableA.getMyList().toString()); immutableA = immutableA.addData("b"); System.out.print(immutableA.getMyList().toString()); immutableA = immutableA.addData("c"); System.out.print(immutableA.getMyList().toString()); }

}

Hope this will help to somebody.

jolivier · Accepted Answer · 2012-08-23 14:15:29Z

0

You can use clone() method like this:

public final class A <T extends Cloneable> { private ArrayList<T> myList; private A(){ myList = new ArrayList<T>(); } public A<T> addData(T t){ T newT = t.clone(); newOb.myList.add(newT); return newOb; } ......... }

answered Aug 23, 2012 at 14:15

jolivier

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5 Comments

h4ck3d Over a year ago

Assume my T is Date , and I do Date d = new Date(); A a = new A(); a.addData(d); d.setDate(10); , then will the value in the List be changed or not ?\

jolivier Over a year ago

Yes, you call a setter of a mutable object you added into the list so you change also the value in the list.

h4ck3d Over a year ago

Okay . That was the main purpose behind creating a new copy of the data that was to be inserted. But turns out it doesn't work.

jolivier Over a year ago

if you make myList.add(d.clone()) and then call d.setX() this will not change the value into the list

h4ck3d Over a year ago

@jolivier The idea is nice. But I think the clone method won't work on a generic.

Luca Corradi · Accepted Answer · 2014-01-07 14:14:09Z

you can get the type of T doing

Type type = new TypeToken<T>(){}.getType();

then get an instance of T doing

type.getClass().newInstance();

Complete example

public T foo() { try { return (T) type.getClass().newInstance(); } catch (Exception e) { return null; } }

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