CODEWITHSUNDEEP
java
kathy
kathy

Reputation: 131

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

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);
    }

}

Upvotes: 13

Views: 47622

Answers (3)

BalusC
BalusC

Reputation: 1108632

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!
}

Upvotes: 28

sepp2k
sepp2k

Reputation: 370102

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[].

Upvotes: 1

user8681
user8681

Reputation:

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.

Upvotes: 4

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