19

I need to convert from List<Object> to String[].

I made:

List<Object> lst ...
String arr = lst.toString();

But I got this string:

["...", "...", "..."]

is just one string, but I need String[]

Thanks a lot.

calbertts
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7 Answers7

28

You have to loop through the list and fill your String[].

String[] array = new String[lst.size()];
int index = 0;
for (Object value : lst) {
  array[index] = (String) value;
  index++;
}

If the list would be of String values, List then this would be as simple as calling lst.toArray(new String[0]);

Dan D.
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    Change `array[index] = (String) value`, to `array[index] = String.valueOf( value )`. The answer as it is will crash if the list contains anything, but Strings. Maybe this is what OP wants. – Alexander Pogrebnyak Feb 08 '13 at 13:13
14

You could use toArray() to convert into an array of Objects followed by this method to convert the array of Objects into an array of Strings:

Object[] objectArray = lst.toArray();
String[] stringArray = Arrays.copyOf(objectArray, objectArray.length, String[].class);
Community
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Steve Chambers
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  • Why additional array copy when `objectArray` can still be used as String[]? – sundar Feb 08 '13 at 13:26
  • See the bottom part of the linked question - casting `objectArray` to `String[]` causes a `ClassCastException`. This answer explains better: http://stackoverflow.com/a/1018774/1063716 – Steve Chambers Feb 08 '13 at 13:33
  • Yes in case of List containing objects other than string. But that happens even in your code. – sundar Feb 08 '13 at 13:35
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    `List lst = new ArrayList(); lst.add(1);` I m getting ArrayStoreException for this test case. – sundar Feb 08 '13 at 13:38
11

Java 8

Java 8 has the option of using streams as well.

List<Object> lst = new ArrayList<>(); 
lst.add("Apple"); 
String[] strings = lst.stream().toArray(String[]::new); 
System.out.println(Arrays.toString(strings));  // [Apple]

If we have a stream of Object, we would need an intermediate operation to do object-to-string conversion, and a terminal operation to collect the results. We can use Objects.toString(obj, null) or any other such implementation for string conversion.

String[] output = lst.stream()
    .map((obj) -> Objects.toString(obj, null))
    .toArray(String[]::new);
akhil_mittal
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5

If we are very sure that List<Object> will contain collection of String, then probably try this.

List<Object> lst = new ArrayList<Object>();
lst.add("sample");
lst.add("simple");
String[] arr = lst.toArray(new String[] {});
System.out.println(Arrays.deepToString(arr));
sundar
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2

Lot of concepts here which will be useful: 

List<Object> list = new ArrayList<Object>(Arrays.asList(new String[]{"Java","is","cool"}));
String[] a = new String[list.size()];
list.toArray(a);

Tip to print array of Strings:

System.out.println(Arrays.toString(a));
Srujan Kumar Gulla
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2

Using Guava

List<Object> lst ...    
List<String> ls = Lists.transform(lst, Functions.toStringFunction());
husayt
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0

There is a simple way available in Kotlin

var lst: List<Object> = ...
    
listOFStrings: ArrayList<String> = (lst!!.map { it.name })
flaxel
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Umasankar
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