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](http://stackoverflow.com/questions/1372432/how-to-obtain-class-instance-generic-argument-type) — Joachim Sauer, Aug 23 '12 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, Jun 08 '16 at 14:58

score 8 · Answer 1 · answered Aug 23 '12 at 14:22

8

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

answered Aug 23 '12 at 14:22

Ransom Briggs

3,025
3
32
46

I used this , T was Integer in my class , it threw exceptions at runtime. – h4ck3d Aug 23 '12 at 15:01
@sTEAK.: so if you could do `new T()`, what would it mean when `T` is `Integer` anyway? – newacct Aug 23 '12 at 21:51
2

At complile time T is replaced by Object – h4ck3d Aug 24 '12 at 08:51

rustyx · Answer 2 · 2017-09-27T09:25:34.727

5

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.

edited Sep 27 '17 at 09:25

answered Sep 27 '17 at 09:20

rustyx

80,671
25
200
267

1

This should be the way to go, without workarounds and/or assumptions... – gustavodidomenico Jan 07 '18 at 04:53
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 ? – FLBzh Aug 05 '20 at 14:32
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? – DTK Aug 17 '21 at 14:56

score 1 · Accepted Answer · edited Aug 23 '12 at 14:14

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 '12 at 14:14

Joachim Sauer

302,674
57
556
614

answered Aug 23 '12 at 14:09

gkuzmin

2,414
17
24

`Cloneable` as type bound is completely useless – newacct Aug 23 '12 at 18:10
Unless it is used right and `clone()` is implemented, I suppose. – gkuzmin Aug 23 '12 at 18:16
in order to call a method on an expression, the static type of the expression must have this method – newacct Aug 23 '12 at 21:50
So `T extends Object & Cloneable` would be more applicable. Nice addition, I did not notice that. – gkuzmin Aug 23 '12 at 21:52
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. – newacct Aug 23 '12 at 22:28

score 1 · Answer 4 · answered Jan 03 '15 at 21:46

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.

score 0 · Answer 5 · answered Aug 23 '12 at 14:15

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 '12 at 14:15

jolivier

7,380
3
29
47

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 ?\ – h4ck3d Aug 23 '12 at 14:51
Yes, you call a setter of a mutable object you added into the list so you change also the value in the list. – jolivier Aug 23 '12 at 15:03
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. – h4ck3d Aug 23 '12 at 15:05
if you make myList.add(d.clone()) and then call d.setX() this will not change the value into the list – jolivier Aug 23 '12 at 15:07
@jolivier The idea is nice. But I think the clone method won't work on a generic. – h4ck3d Aug 23 '12 at 18:35

score -1 · Answer 6 · answered Jan 07 '14 at 14:14

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;
    }
}

Cannot instantiate generic data type in class

6 Answers6

Linked

Related