Returning an Array from a generic class

I get a ClassCastException error when I run this because of the implicit cast of d to a double when I run the code. However, if I change reference to d in to Object[] then it can't be a parameter to the set function. If I change the set function to accept an Object[] then everything works fine, but then the class will fail at runtime if anyone calls set on an object that isn't of type N .

I need a solution that can get() the old array (or a clone thereof) and that can set() data to a new array.

public class Foo<N> {

    public static void main(String[] args) {
        Foo<Double> foo = new Foo<Double>();
        Double[] d = foo.get();

        // do stuff to d ...

        foo.set(d);
    }

    N[] data;

    public Foo() {
        data = (N[]) new Object[2];
    }

    public N[] get() {
        return (N[]) data;
    }

    public void set(N[] data) {
        this.data = data;
    }

}

---

To create an array with the right runtime type requires some kind of runtime representation of that type. A generic class like Foo<N> has no runtime representation of N .

There are two solutions:

Use a List<N> instead. This is best if it is possible!

Manually add a runtime representation of N by passing in a Class<N> to Foo , use java.lang.reflect.Array.newInstance to create the array.

Code for the second solution:

public Foo(Class<N> newDataClass) {
    data = (N[]) Array.newInstance(newDataClass, 2);
}

Foo<Double> foo = new Foo<>(Double.class);

---

EDIT:

If what you want to do is instead to get a copy of an existing Double array you can do that with (N[]) data.clone() . Setting it will not be a problem.

---

There's a trick to this.

class Foo<N> {

    // Deliberately no parameters.
    N[] myArray = makeArray();

    private N[] makeArray(N... ns) {
        return ns;
    }

    public N[] get() {
        return myArray;
    }
}

This may look like there is no casting (which is a good thing) but actually there is, it is just being done by the varargs system.

---

一个方便（但是详细）的方法是修改构造函数以使用数组类型的伪参数，因为您知道它在主方法中是双重的。

  public class Foo<N> {
         N[] data;

         public Foo(N inType) {
             data = (N[]) Array.newInstance(inType.getClass(), 2) ;
         }

         public N[] get() {
             return (N[]) data;
         }

         public void set(N[] data) {
             this.data = data;
         }

         public static void main(String[] args) {
             Foo<Double> foo = new Foo<Double>(new Double(0));
             Double[] d = foo.get();
             // do stuff to d ...
             foo.set(d);
         }

  }

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