Convert an arraylist to array

Question

I have an Arraylist that I want to convert it into an array. I am using this:

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

but, it does not work. Any idea how I can fix it? Thanks in advance. It seems it returns object.

So where is your list? Did you know that collection can contain only wrappers for primitives? — skuntsel, Apr 02 '13 at 19:57

score 4 · Accepted Answer · answered Apr 02 '13 at 19:57

For converting a List<Double> to a double[], or anything where you need the primitive type, there's no alternative to doing a traditional for loop (or calling a third-party library method that does a traditional for loop, such as Guava's Doubles.toArray).

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

Alternately, you could do

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

which would get you the array of boxed Doubles, which might suffice for your purposes anyway.

score 3 · Answer 2 · answered Apr 02 '13 at 19:58

A List can't contain primitive types. Assuming it contains instances of Double, the only way is to iterate over the list and unbox each element (hoping there's no null in the list):

double[] array = new double[list.size()];
int i = 0;
for (Double d : list) {
    array[i] = d;
    i++;
}

Eng.Fouad · Answer 3 · 2013-04-02T20:04:33.950

0

It should be:

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

and if you want an array of primitives, iterate over each element in the array:

double[] array = new double[list.size()];
for(int i = 0; i < list.size(); i++) array[i] = list.get(i).doubleValue();

edited Apr 02 '13 at 20:04

answered Apr 02 '13 at 19:57

Eng.Fouad

115,165
71
313
417

Have you tried this? The way generics work doesn't work like this. – Louis Wasserman Apr 02 '13 at 19:58
@LouisWasserman You are right, I missed that :) .. updated my answer. – Eng.Fouad Apr 02 '13 at 19:59

score 0 · Answer 4 · answered Apr 02 '13 at 19:59

0

Please read the java docs:

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ArrayList.html

HTH.

answered Apr 02 '13 at 19:59

John Brown

189
1
7

score 0 · Answer 5 · answered Apr 02 '13 at 19:59

0

Use the Object Double not the primitive double:

ArrayList<Double> list = new ArrayList<Double>();
...add data
Double[] array=list.toArray(new Double[list.size()]);

answered Apr 02 '13 at 19:59

bluevoid

1,274
13
29

I doubt that is what the goal of the asker is.. They probably want to perform operations on those `doubles` as primitives. – Nora Powers Apr 02 '13 at 20:02

score -1 · Answer 6 · answered May 02 '13 at 13:40

-1

       StringBuffer strBuffer = new StringBuffer();
        for(Object o:list){
            strBuffer.append(o);
        }
        double []x = new double[]{strBuffer.toString()};

I consider this should work

answered May 02 '13 at 13:40

CodeMadness

1
4

1

This answer does not work. This is the same as your [other answer here](http://stackoverflow.com/a/16336520/922184). Please make sure your answers actually work before you post them. – Mysticial May 03 '13 at 08:20

Convert an arraylist to array

6 Answers6