How can I fill a array with the data provided by one List?
For example, I have a List with Strings:
List l = new ArrayList<String>();
l.add("a");
l.add("b");
l.add("c");
then I want to copy this data into a String array:
String[] array = ?
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5 Answers
There's a toArray() method in list...
You have to first allocate an array of an appropriate size (typically list.size()), and then pass it to the toArray method as a parameter. The array will be initialized with the list content.
For example:
String[] myArray = new String[myList.size]();
myList.toArray(myArray);
You can also do the new call inside the parentheses of toArray
4 Comments
Hardcoded
For safety it should be
myArray = myList.toArray(myArray);
Uri
@Hardcoded: I've always wondered about that one. Why is there even a return type here? I always hate methods that change their input and return it unless you chain them.
Hardcoded
The method first checks, if the given array is big enough. If it isn't, a new one will created. The only reason for the array parameter is, that there were no information about the type of the array at runtime. It could have been taken a Class-object as well.
Kevin Day
This is minor, but the standard idiom here is to pass in a zero length array of the desired class. If the list contains 0 elements, it will be returned (0 length arrays are immutable and safe to share). If the list is longer than 0 elements, a new array is created. This is cleaner than creating the array up-front with the desired size. So String[] myArray = myList.toArray(new String[0]);
First, you have to specify the concrete type of the List - List<String> (don't leave it raw).
Then, you can do
String[] array = list.toArray(new String[list.size()]);
Comments
You can use the toArray(T[] a) method:
String[] arr = list.toArray(new String[0]);
Or alternately:
String[] arr = list.toArray(new String[list.size()]);
The difference between the two is that the latter may not need to allocate a new array.
Effective Java 2nd Edition: Item 23: Don't use raw types in new code.
The use of raw types is allowed only as a concession to compatibility of legacy code. The use of raw types in code written after the introduction of genericity into the Java programming language is strongly discouraged. It is possible that future versions of the Java programming language will disallow the use of raw types.
Don't make a habit of naming an identifier l. It looks an awful lot like 1.
answered Mar 29, 2010 at 14:53
polygenelubricants
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Comments
Syntax is a little goofy; you have to provide your own created array as input.
String[] array = l.toArray(new String[l.size()]);
Comments
String[] array = new String[l.size()];
l.toArray(array);
