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How can I convert List<List<String>> into String[][]?

In my project I have a List that contains Lists of Strings (List<List<String>>) and now I have to convert this List into an array of String arrays (String[][]).
If I had a single List<String> (for example myList) then I could do this to convert to String[]:
String[] myArray = new String[myList.size()];
myArray = myList.toArray(myArray);
But in this case I have lots of List of String inside a List (of List of Strings).
How can I do this? I've spent lots of time but I didn't find a solution..

I wrote a generic utility method few days back for my own usage, which converts a List<List<T>> to T[][]. Here's the method. You can use it for your purpose:
public static <T> T[][] multiListToArray(final List<List<T>> listOfList,
final Class<T> classz) {
final T[][] array = (T[][]) Array.newInstance(classz, listOfList.size(), 0);
for (int i = 0; i < listOfList.size(); i++) {
array[i] = listOfList.get(i).toArray((T[]) Array.newInstance(classz, listOfList.get(i).size()));
}
return array;
}
There I've created the array using Array#newInstance() method, since you cannot create an array of type parameter T directly.
Since we're creating a 2-D array, and we don't yet know how many columns you will be having, I've given the column size as 0 initially. Anyways, the code is initializing each row with a new array inside the for loop. So, you don't need to worry about that.
This line:
array[i] = listOfList.get(i).toArray((T[]) Array.newInstance(classz, listOfList.get(i).size()));
uses the List#toArray(T[]) method to convert the inner list to an array, as you already did. I'm just passing an array as a parameter to the method so that the return value which I get is T[], which I can directly assign to the current row - array[i].
Now you can use this method as:
String[][] arr = multiListToArray(yourList, String.class);
Thanks to #arshaji, you can modify the generic method to pass the uninitialized array as 2nd parameter, instead of Class<T>:
public static <T> void multiListToArray(final List<List<T>> listOfList,
final T[][] arr) {
for (int i = 0; i < listOfList.size(); ++i) {
arr[i] = listOfList.get(i).toArray(arr[i]);
}
}
But then, you have to pass the array to this method like this:
List<List<String>> list = new ArrayList<>();
// Initialize list
String[][] arr = new String[list.size()][0];
multiListToArray(list, arr);
Note that, since now we are passing the array as argument, you no longer need to return it from your method. The modification done in the array will be reflected to the array in the calling code.

String[][] myArray = new String[myList.size()][];
for (int i = 0; i < myList.size(); i++) {
List<String> row = myList.get(i);
myArray[i] = row.toArray(new String[row.size()]);
}

Here is one way
List<List<String>> list = new ArrayList<List<String>>();
String[][] array = new String[list.size()][];
int counter1 = 0;
for(List<String> outterList : list)
{
array[counter1] = new String[outterList.size()];
int counter2 = 0;
for(String s : outterList)
{
array[counter1][counter2] = s;
counter2++;
}
counter1++;
}

To init String[][], you should init the list(array) of String[]
So you need to define a list
List<String[]> string = new List<String[]>();
for (int i = 0;....)
{
String[] _str = new String[numbers];
_str[0] = ...
string.append(_str);
}

How can I transform a List of List (multidimensional List) to an Array of Array(multidimensional Array)?

I have written this code, but at run time I have this error:
[Ljava.lang.Object; cannot be cast to [[Ljava.lang.String;
please help me, thanks!!!
public java.util.List<String> concatAll(java.util.List<java.util.List<String>> mergedList) {
java.lang.String [][] mergedArray = (String[][])mergedList.toArray();
Iterator<java.util.List<String>> itr = mergedList.iterator();
java.util.List<String> list1 = itr.next();
java.lang.String [] firstArray = (String[])list1.toArray();
int totalLength = firstArray.length;
for (String[] array : mergedArray) {
totalLength += array.length;
}
String[] result = Arrays.copyOf(firstArray, totalLength);
int offset = firstArray.length;
for (String[] array : mergedArray) {
System.arraycopy(array, 0, result, offset, array.length);
offset += array.length;
}
java.util.List<String> finalList = Arrays.asList(result);
for (String list : finalList)
System.out.println(list);
return finalList;
}

mergedList.toArray() creates a singly indexed array typed as objects.
Each of the objects it contains is in fact a (singly-indexed) list of strings, though with this call syntax the type is not known at compile-time. It is not an array of strings, as would be needed for your cast to work.
Since your concatAll is trying to convert a List<List<String>> into a List<String> by some sort of concatenation operation, it may be best to do this without ever converting to a String[][] at all, but if you do want that conversion, it can be done as follows:
private String[][] toDoubleIndexArray(List<List<String>> mergedList) {
String[][] result = new String[mergedList.size()][];
for (int i = 0; i< mergedList.size(); i++) {
List<String> currentList = mergedList.get(i);
result[i] = currentList.toArray(new String[currentList.size()]);
}
return result;
}
Original answer, not quite correct as noted by Xavi Lopez in comments:
Since mergedList has type List<List<String>>,
mergedList.toArray() has type List<String>[], i.e., it's an array of lists, and not a doubly indexed array.

There's no out-of-the-box method, but it's fairly straightforward to do by hand:
// Create the outer dimension of the array, with the same size as the total list
String[][] mergedArray = new String[mergedList.size()][];
// Now iterate over each nested list and convert them into the String[]
// instances that form the inner dimension
for (int i = 0; i < mergedList.size(); i++) {
mergedArray[i] = mergedList.get(i).toArray(new String[0]);
}
A slightly more efficient version of the loop body would be
List<String> innerList = mergedList.get(i);
String[] innerAsArray = innerList.toArray(new String[innerList.size()]);
mergedArray[i] = innerAsArray;
as this avoids the array resizing that would be required in my initial example (the new String[0] isn't large enough to hold the list elements). But quite frankly, unless this was a performance critical loop, I'd prefer the first version as I find it slightly clearer to see what's going on.

Hey you cannot convert the Multi dimentional String list to String array directly. Add the below code before trying to use the mergedArray:
/** Create Array **/
String [][] mergedArray = new String[mergedList.size()][];
/** Initialize array from list **/
for(int i=0; i< mergedList.size(); i++){
mergedArray[i] = mergedList.get(i).toArray(new String[0]);
}
This should do the trick

I think your return type is wrong if your intention is to return an array of array.
Try this:
public String[][] concatAll(java.util.List<java.util.List<String>> mergedList) {
//java.lang.String [][] mergedArray = (String[][])mergedList.toArray();
java.lang.String[][] mergedArray = new String[mergedList.size()][];
Iterator<java.util.List<String>> itr = mergedList.iterator();
int count = 0;
while (itr.hasNext())
{
java.util.List<String> list1 = itr.next();
String[] array1 = list1.toArray(new String[list1.size()]);
mergedArray[count++] = array1;
}
return mergedArray;
}

You can't convert a
List<List<String>>
to a String[][] by using the out of the box toArray functionality. This would only work if you had:
List<String[]>.

Converting ArrayList to Array in java

I have an ArrayList with values like "abcd#xyz" and "mnop#qrs". I want to convert it into an Array and then split it with # as delimiter and have abcd,mnop in an array and xyz,qrs in another array. I tried the following code:
String dsf[] = new String[al.size()];
for(int i =0;i<al.size();i++){
dsf[i] = al.get(i);
}
But it failed saying "Ljava.lang.String;#57ba57ba"

You don't need to reinvent the wheel, here's the toArray() method:
String []dsf = new String[al.size()];
al.toArray(dsf);

List<String> list=new ArrayList<String>();
list.add("sravan");
list.add("vasu");
list.add("raki");
String names[]=list.toArray(new String[list.size()])

List<String> list=new ArrayList<String>();
list.add("sravan");
list.add("vasu");
list.add("raki");
String names[]=list.toArray(new String[0]);
if you see the last line (new String[0]), you don't have to give the size, there are time when we don't know the length of the list, so to start with giving it as 0 , the constructed array will resize.

import java.util.*;
public class arrayList {
public static void main(String[] args) {
Scanner sc=new Scanner(System.in);
ArrayList<String > x=new ArrayList<>();
//inserting element
x.add(sc.next());
x.add(sc.next());
x.add(sc.next());
x.add(sc.next());
x.add(sc.next());
//to show element
System.out.println(x);
//converting arraylist to stringarray
String[]a=x.toArray(new String[x.size()]);
for(String s:a)
System.out.print(s+" ");
}
}

String[] values = new String[arrayList.size()];
for (int i = 0; i < arrayList.size(); i++) {
values[i] = arrayList.get(i).type;
}

What you did with the iteration is not wrong from what I can make of it based on the question. It gives you a valid array of String objects. Like mentioned in another answer it is however easier to use the toArray() method available for the ArrayList object => http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ArrayList.html#toArray%28%29
Just a side note. If you would iterate your dsf array properly and print each element on its own you would get valid output. Like this:
for(String str : dsf){
System.out.println(str);
}
What you probably tried to do was print the complete Array object at once since that would give an object memory address like you got in your question. If you see that kind of output you need to provide a toString() method for the object you're printing.

package com.v4common.shared.beans.audittrail;
import java.util.ArrayList;
import java.util.List;
public class test1 {
public static void main(String arg[]){
List<String> list = new ArrayList<String>();
list.add("abcd#xyz");
list.add("mnop#qrs");
Object[] s = list.toArray();
String[] s1= new String[list.size()];
String[] s2= new String[list.size()];
for(int i=0;i<s.length;i++){
if(s[i] instanceof String){
String temp = (String)s[i];
if(temp.contains("#")){
String[] tempString = temp.split("#");
for(int j=0;j<tempString.length;j++) {
s1[i] = tempString[0];
s2[i] = tempString[1];
}
}
}
}
System.out.println(s1.length);
System.out.println(s2.length);
System.out.println(s1[0]);
System.out.println(s1[1]);
}
}

Here is the solution for you given scenario -
List<String>ls = new ArrayList<String>();
ls.add("dfsa#FSDfsd");
ls.add("dfsdaor#ooiui");
String[] firstArray = new String[ls.size()];
firstArray =ls.toArray(firstArray);
String[] secondArray = new String[ls.size()];
for(int i=0;i<ls.size();i++){
secondArray[i]=firstArray[i].split("#")[0];
firstArray[i]=firstArray[i].split("#")[1];
}

This is the right answer you want and this solution i have run my self on netbeans
ArrayList a=new ArrayList();
a.add(1);
a.add(3);
a.add(4);
a.add(5);
a.add(8);
a.add(12);
int b[]= new int [6];
Integer m[] = new Integer[a.size()];//***Very important conversion to array*****
m=(Integer[]) a.toArray(m);
for(int i=0;i<a.size();i++)
{
b[i]=m[i];
System.out.println(b[i]);
}
System.out.println(a.size());

This can be done using stream:
List<String> stringList = Arrays.asList("abc#bcd", "mno#pqr");
List<String[]> objects = stringList.stream()
.map(s -> s.split("#"))
.collect(Collectors.toList());
The return value would be arrays of split string.
This avoids converting the arraylist to an array and performing the operation.

NameOfArray.toArray(new String[0])
This will convert ArrayList to Array in java

// A Java program to convert an ArrayList to arr[]
import java.io.*;
import java.util.List;
import java.util.ArrayList;
class Main {
public static void main(String[] args)
{
List<Integer> al = new ArrayList<Integer>();
al.add(10);
al.add(20);
al.add(30);
al.add(40);
Integer[] arr = new Integer[al.size()];
arr = al.toArray(arr);
for (Integer x : arr)
System.out.print(x + " ");
}
}

Convert array list items to integer

I have an arraylist, say arr. Now this arraylist stores numbers as strings. now i want to convert this arraylist to integer type. So how can i do that???
ArrayList arr = new ArrayList();
String a="Mode set - In Service", b="Mode set - Out of Service";
if(line.contains(a) || line.contains(b)) {
StringTokenizer st = new StringTokenizer(line, ":Mode set - Out of Service In Service");
while(st.hasMoreTokens()) {
arr.add(st.nextToken());
}
}

Since you're using an untyped List arr, you'll need to cast to String before performing parseInt:
List<Integer> arrayOfInts = new ArrayList<Integer>();
for (Object str : arr) {
arrayOfInts.add(Integer.parseInt((String)str));
}
I recommend that you define arr as follows:
List<String> arr = new ArrayList<String>();
That makes the cast in the conversion unnecessary.

run the below code,i hope it meets you requirement.
import java.util.*;
import java.lang.*;
import java.io.*;
class Ideone
{
public static void main (String[] args) throws java.lang.Exception
{
ArrayList<String> strArrayList= new ArrayList<String>();
strArrayList.add("1");
strArrayList.add("11");
strArrayList.add("111");
strArrayList.add("12343");
strArrayList.add("18475");
int[] ArrayRes = new int[strArrayList.size()];
int i = 0;
int x = 0;
for (String s : strArrayList)
{
ArrayRes[i] = Integer.parseInt(s);
System.out.println(ArrayRes[i]);
i++;
}
}
}
Output:
1
11
111
12343
18475

To convert to an integer array, you will input as a string array then go through each one and change it to an int.
public int[] convertStringArraytoIntArray(String[] sarray) throws Exception {
if (sarray != null) {
//new int for each string
int intarray[] = new int[sarray.length];
//for each int blah blah to array length i
for (int i = 0; i < sarray.length; i++) {
intarray[i] = Integer.parseInt(sarray[i]);
}
return intarray;
}
return null;
}

final List<String> strs = new ArrayList();
strs.add("1");
strs.add("2");
Integer[] ints = new Integer[strs.size()];
for (int i = 0; i<strs.size(); i++){
ints[i] = Integer.parseInt(strs.get(i));
}

use the Integer.parseInt() method.
http://www.java2s.com/Code/Java/Language-Basics/Convertstringtoint.htm

If you know that you have an arraylist of string but in your you wil use the same list as list of integer so better while initializing array list specify that the array list must insert only int type of data
instead of writing ArrayList arr = new ArrayList();
you could have written ArrayList<Integer> arr = new ArrayList<Integer>();
Alternate solution
If you want to convert that list into Integer ArrayList then use following code
How to convert String ArrayList into ArrayList of int
ArrayList<String> oldList = new ArrayList<String>();
oldList.add(""+5);
oldList.add(""+5);
ArrayList<Integer> newList = new ArrayList<Integer>(oldList.size());
for (String myInt : oldList) {
newList.add(Integer.parseInt(myInt));
}

Converting 'ArrayList<String> to 'String[]' in Java

How might I convert an ArrayList<String> object to a String[] array in Java?

List<String> list = ..;
String[] array = list.toArray(new String[0]);
For example:
List<String> list = new ArrayList<String>();
//add some stuff
list.add("android");
list.add("apple");
String[] stringArray = list.toArray(new String[0]);
The toArray() method without passing any argument returns Object[]. So you have to pass an array as an argument, which will be filled with the data from the list, and returned. You can pass an empty array as well, but you can also pass an array with the desired size.
Important update: Originally the code above used new String[list.size()]. However, this blogpost reveals that due to JVM optimizations, using new String[0] is better now.

An alternative in Java 8:
String[] strings = list.stream().toArray(String[]::new);
Java 11+:
String[] strings = list.toArray(String[]::new);

Starting from Java-11, one can use the API Collection.toArray(IntFunction<T[]> generator) to achieve the same as:
List<String> list = List.of("x","y","z");
String[] arrayBeforeJDK11 = list.toArray(new String[0]);
String[] arrayAfterJDK11 = list.toArray(String[]::new); // similar to Stream.toArray

You can use the toArray() method for List:
ArrayList<String> list = new ArrayList<String>();
list.add("apple");
list.add("banana");
String[] array = list.toArray(new String[list.size()]);
Or you can manually add the elements to an array:
ArrayList<String> list = new ArrayList<String>();
list.add("apple");
list.add("banana");
String[] array = new String[list.size()];
for (int i = 0; i < list.size(); i++) {
array[i] = list.get(i);
}
Hope this helps!

ArrayList<String> arrayList = new ArrayList<String>();
Object[] objectList = arrayList.toArray();
String[] stringArray = Arrays.copyOf(objectList,objectList.length,String[].class);
Using copyOf, ArrayList to arrays might be done also.

In Java 8:
String[] strings = list.parallelStream().toArray(String[]::new);

In Java 8, it can be done using
String[] arrayFromList = fromlist.stream().toArray(String[]::new);

If your application is already using Apache Commons lib, you can slightly modify the accepted answer to not create a new empty array each time:
List<String> list = ..;
String[] array = list.toArray(ArrayUtils.EMPTY_STRING_ARRAY);
// or if using static import
String[] array = list.toArray(EMPTY_STRING_ARRAY);
There are a few more preallocated empty arrays of different types in ArrayUtils.
Also we can trick JVM to create en empty array for us this way:
String[] array = list.toArray(ArrayUtils.toArray());
// or if using static import
String[] array = list.toArray(toArray());
But there's really no advantage this way, just a matter of taste, IMO.

You can use Iterator<String> to iterate the elements of the ArrayList<String>:
ArrayList<String> list = new ArrayList<>();
String[] array = new String[list.size()];
int i = 0;
for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); i++) {
array[i] = iterator.next();
}
Now you can retrive elements from String[] using any Loop.

Generics solution to covert any List<Type> to String []:
public static <T> String[] listToArray(List<T> list) {
String [] array = new String[list.size()];
for (int i = 0; i < array.length; i++)
array[i] = list.get(i).toString();
return array;
}
Note You must override toString() method.
class Car {
private String name;
public Car(String name) {
this.name = name;
}
public String toString() {
return name;
}
}
final List<Car> carList = new ArrayList<Car>();
carList.add(new Car("BMW"))
carList.add(new Car("Mercedes"))
carList.add(new Car("Skoda"))
final String[] carArray = listToArray(carList);

List<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");
String [] strArry= list.stream().toArray(size -> new String[size]);
Per comments, I have added a paragraph to explain how the conversion works.
First, List is converted to a String stream. Then it uses Stream.toArray to convert the elements in the stream to an Array. In the last statement above "size -> new String[size]" is actually an IntFunction function that allocates a String array with the size of the String stream. The statement is identical to
IntFunction<String []> allocateFunc = size -> {
return new String[size];
};
String [] strArry= list.stream().toArray(allocateFunc);

List <String> list = ...
String[] array = new String[list.size()];
int i=0;
for(String s: list){
array[i++] = s;
}

in case some extra manipulation of the data is desired, for which the user wants a function, this approach is not perfect (as it requires passing the class of the element as second parameter), but works:
import java.util.ArrayList;
import java.lang.reflect.Array;
public class Test {
public static void main(String[] args) {
ArrayList<Integer> al = new ArrayList<>();
al.add(1);
al.add(2);
Integer[] arr = convert(al, Integer.class);
for (int i=0; i<arr.length; i++)
System.out.println(arr[i]);
}
public static <T> T[] convert(ArrayList<T> al, Class clazz) {
return (T[]) al.toArray((T[])Array.newInstance(clazz, al.size()));
}
}

In Java 11, we can use the Collection.toArray(generator) method. The following code will create a new array of strings:
List<String> list = List.of("one", "two", "three");
String[] array = list.toArray(String[]::new)
from java.base's java.util.Collection.toArray().

You can convert List to String array by using this method:
Object[] stringlist=list.toArray();
The complete example:
ArrayList<String> list=new ArrayList<>();
list.add("Abc");
list.add("xyz");
Object[] stringlist=list.toArray();
for(int i = 0; i < stringlist.length ; i++)
{
Log.wtf("list data:",(String)stringlist[i]);
}

private String[] prepareDeliveryArray(List<DeliveryServiceModel> deliveryServices) {
String[] delivery = new String[deliveryServices.size()];
for (int i = 0; i < deliveryServices.size(); i++) {
delivery[i] = deliveryServices.get(i).getName();
}
return delivery;
}

An alternate one-liner method for primitive types, such as double, int, etc.:
List<Double> coordList = List.of(3.141, 2.71);
double[] doubleArray = coordList.mapToDouble(Double::doubleValue).toArray();
List<Integer> coordList = List.of(11, 99);
int[] intArray = coordList.mapToInt(Integer::intValue).toArray();
and so on...