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);
    }

}

thank you .That helped me resolve the problem – kathy Feb 21 '10 at 01:27 — kathy, Feb 21 '10 at 01:27

score 28 · Answer 1 · answered Feb 21 '10 at 00:05

28

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!
}

answered Feb 21 '10 at 00:05

BalusC

1,082,665
372
3,610
3,555

6

ow, O(n^2) for loop? why not `int i = 0; for (Double v : list) array[i++] = v;` – amara Jul 03 '11 at 12:13
Yeah that second option is pretty terrible. Sparkleshy's suggestion would be much better. – sage88 Jul 23 '13 at 02:32

score 4 · Answer 2 · answered Feb 21 '10 at 00:01

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.

score 1 · Answer 3 · answered Feb 21 '10 at 00:01

1

toArray is defined to return T[] where T is a generic argument (the one-argument version is anyway). Since primitives can't be used as generic arguments, toArray can't return double[].

answered Feb 21 '10 at 00:01

sepp2k

363,768
54
674
675

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

3 Answers3

Linked