无法在类中实例化通用数据类型

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.

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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;
    }
    .........
}

Answer 6

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;
    }
}