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Convert list to array in Java

Posted by: admin November 2, 2017 Leave a comment

Questions:

How can I convert a List to an Array in Java?

Check the code below:

ArrayList<Tienda> tiendas;
List<Tienda> tiendasList; 
tiendas = new ArrayList<Tienda>();

Resources res = this.getBaseContext().getResources();
XMLParser saxparser =  new XMLParser(marca,res);

tiendasList = saxparser.parse(marca,res);
tiendas = tiendasList.toArray();

this.adaptador = new adaptadorMarca(this, R.layout.filamarca, tiendas);
setListAdapter(this.adaptador);  

I need to populate the array tiendas with the values of tiendasList.

Answers:

Either:

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

or:

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

Note that this works only for arrays of reference types. For arrays of primitive types, use the traditional way:

List<Integer> list = ...;
int[] array = new int[list.size()];
for(int i = 0; i < list.size(); i++) array[i] = list.get(i);

Questions:
Answers:

An alternative in Java 8:

String[] strings = list.stream().toArray(String[]::new);

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I think this is the simplest way:

Foo[] array = list.toArray(new Foo[0]);

Questions:
Answers:

I came across this code snippet that solves it.

//Creating a sample ArrayList 
List<Long> list = new ArrayList<Long>();

//Adding some long type values
list.add(100l);
list.add(200l);
list.add(300l);

//Converting the ArrayList to a Long
Long[] array = (Long[]) list.toArray(new Long[list.size()]);

//Printing the results
System.out.println(array[0] + " " + array[1] + " " + array[2]);

The conversion works as follows:

  1. It creates a new Long array, with the size of the original list
  2. It converts the original ArrayList to an array using the newly created one
  3. It casts that array into a Long array (Long[]), which I appropriately named ‘array’
Questions:
Answers:

For ArrayList the following works:

ArrayList<Foo> list = new ArrayList<Foo>();

//... add values

Foo[] resultArray = new Foo[list.size()];
resultArray = list.toArray(resultArray);

Questions:
Answers:

Example taken from this page: http://www.java-examples.com/copy-all-elements-java-arraylist-object-array-example

import java.util.ArrayList;

public class CopyElementsOfArrayListToArrayExample {

  public static void main(String[] args) {
    //create an ArrayList object
    ArrayList arrayList = new ArrayList();

    //Add elements to ArrayList
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");

    /*
      To copy all elements of java ArrayList object into array use
      Object[] toArray() method.
    */

    Object[] objArray = arrayList.toArray();

    //display contents of Object array
    System.out.println("ArrayList elements are copied into an Array.
                                                  Now Array Contains..");
    for(int index=0; index < objArray.length ; index++)
      System.out.println(objArray[index]);
  }
}

/*
Output would be
ArrayList elements are copied into an Array. Now Array Contains..
1
2
3
4
5

Questions:
Answers:

This is works. Kind of.

public static Object[] toArray(List<?> a) {
    Object[] arr = new Object[a.size()];
    for (int i = 0; i < a.size(); i++)
        arr[i] = a.get(i);
    return arr;
}

Then the main method.

public static void main(String[] args) {
    List<String> list = new ArrayList<String>() {{
        add("hello");
        add("world");
    }};
    Object[] arr = toArray(list);
    System.out.println(arr[0]);
}

Questions:
Answers:

This (Ondrej’s answer):

Foo[] array = list.toArray(new Foo[0]);

Is the most common idiom I see. Those who are suggesting that you use the actual list size instead of “0” are misunderstanding what’s happening here. The toArray call does not care about the size or contents of the given array – it only needs its type. It would have been better if it took an actual Type in which case “Foo.class” would have been a lot clearer. Yes, this idiom generates a dummy object, but including the list size just means that you generate a larger dummy object. Again, the object is not used in any way; it’s only the type that’s needed.

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You can use toArray() api as follows,

ArrayList<String> stringList = new ArrayList<String>();
stringList.add("ListItem1");
stringList.add("ListItem2");
String[] stringArray = new String[stringList.size()];
stringArray = stringList.toArray(stringList);

Values from the array are,

for(String value : stringList)
{
    System.out.println(value);
}

Questions:
Answers:
tiendas = new ArrayList<Tienda>(tiendasList);

All collection implementations have an overloaded constructor that takes another collection (with the template <T> matching). The new instance is instantiated with the passed collection.

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Try this:

List list = new ArrayList();
list.add("Apple");
list.add("Banana");

Object[] ol = list.toArray();