Converting String ArrayList into String array is very common programming
task in Java. you often need to convert
Array to Array List in Java and
vice-versa. In this Java program we will How
to convert String ArrayList to String array. This is also a common
programming exercise which is asked to many Java programmers in various Java
related courses. It’s also worth noting that ArrayList in Java is internally
backed by array and Array in Java are objects much like String
which is also an Object in Java. In this Java program we first create an ArrayList
which stores name of months as String e.g. Jan, Feb and Mar. Later we use
ArrayList toArray() method to convert ArrayList into
array.
How to convert String ArrayList to Array in Java
Here is full code example of Java test program which convert String ArrayList to String Array in
Java.In this Java program we first create an ArrayList which stores name of month in String format and then we convert that into an String array.
package arrayListEx;
import java.util.ArrayList;
import java.util.Arrays;
public class ArrayListtoArray {
public static void main(String args[]){
//ArrayList containing string objects
ArrayList<String> aListMonth = new ArrayList<String>();
aListMonth.add("Jan");
aListMonth.add("Feb");
aListMonth.add("mar");
/*
* To convert ArrayList containing String elements to String array, use
* Object[] toArray() method of ArrayList class.
*/
//First Step: convert ArrayList to an Object array.
Object[] objMnt = aListMonth.toArray();
//Second Step: convert Object array to String array
String[] strMnts = Arrays.copyOf(objMnt, objMnt.length, String[].class);
System.out.println("ArrayList converted to String array");
//print elements of String array
for(int i=0; i < strMnts.length; i++){
System.out.println(strMnts[i]);
}
}
}
import java.util.ArrayList;
import java.util.Arrays;
public class ArrayListtoArray {
public static void main(String args[]){
//ArrayList containing string objects
ArrayList<String> aListMonth = new ArrayList<String>();
aListMonth.add("Jan");
aListMonth.add("Feb");
aListMonth.add("mar");
/*
* To convert ArrayList containing String elements to String array, use
* Object[] toArray() method of ArrayList class.
*/
//First Step: convert ArrayList to an Object array.
Object[] objMnt = aListMonth.toArray();
//Second Step: convert Object array to String array
String[] strMnts = Arrays.copyOf(objMnt, objMnt.length, String[].class);
System.out.println("ArrayList converted to String array");
//print elements of String array
for(int i=0; i < strMnts.length; i++){
System.out.println(strMnts[i]);
}
}
}
That’s all on how to convert String
ArrayList to String Array in Java. This is general way of convert
ArrayList to Array in Java and by using this example you can even convert
an Integer ArrayList or Double ArrayList to
corresponding Array in Java. Just remember that ArrayList
is not synchronized and this program can not be used in multi-threaded
environment in Java. If you want to use synchronized collection, consider using
Vector
over ArrayList.
List of Java homework exercise program from java67 blog
Thanks for article.
ReplyDeleteOne question:
if each element into objMnt is really a Object reference to String instance ( and I can make a downcast such String e = (String)objMnt[0] )
Why I cannot make a downcast
String[] a = (String[]) objMnt
?
Arrays are covariant
Regards
Your code is not HTML safe use pastebin or stuff like that !
ReplyDelete@jmzc: The object array objMnt is of type Object[] and contains String references, that's why you can downcast a reference in the array. You can store String references in an Object[] because the class String is derived from the class Object and therefore you can substitute objects of type String for objects with type Object.
ReplyDeleteThe Object[] itself cannot be downcasted to String[] because String[] is a derived class of Object[]. This means even tough the Object[] object contains references to Strings, the Object[] object itself is _NOT_ a String[] object.
But back to the problem: You could also convert the ArrayList of strings in an easier way:
String[] strMnts = aListMonth.toArray( new String[aListMonth.size()] );
This method exists (AFAIK) because of the exact reason mentioned above. By passing in the pre-allocated array of type String[], the actual return type of toArray is of type String[] and not of type Object[].