How to convert linkedlist to array using `toArray()`?

Question

I do not know how to convert a linked list of doubles to array. Please help me find the error.

import java.util.*;
public class StackCalculator {


  private   LinkedList<Double> values;
  double value1 , value2 ;

  public StackCalculator()
  {
     values = new LinkedList<Double>();
  }


    void push(double x)
    {
         values.addFirst(x);
    }
    double pop()
    {
       return values.removeFirst();
    }
    double add()
    {
        value1=pop();
        value2=pop();
        return (value1 + value2);
    }

    double mult()
    {
        value1=pop();
        value2=pop();
        return (value1 * value2);
    }


    double[] v = new double[10];
    double[] getValues()
    {
        return   values.toArray(v);
    }

}

Answer 1

The List#toArray() of a List<Double> returns Double[]. The Double[] isn't the same as double[]. As arrays are objects, not primitives, the autoboxing rules doesn't and can't apply here.

Either use Double[] instead:

Double[] array = list.toArray(new Double[list.size()]);

...or create double[] yourself using a simple for loop:

double[] array = new double[list.size()];
for (int i = 0; i < list.size(); i++) {
    array[i] = list.get(i); // Watch out for NullPointerExceptions!
}

Answer 2

The problem is the type of your list, which is Double (object), while you're trying to return an array of type double (primitive type). Your code should compile if you change getValues() to return a Double[].

For your other methods, it's not a problem, because Java automatically converts Doubles to doubles and back (called autoboxing). It cannot do that for array types.