Form the Inheritance concept, If some perticular method is not available in the current class it will search for that method in super classes. If available it executes.
It executes AbstractList<E> class add() method which throws UnsupportedOperationException.
When you are converting from an Array to a Collection Obejct. i.e., array-based to collection-based API then it is going to provide you fixed-size collection object, because Array's behaviour is of Fixed size.
java.util.Arrays.asList( T... a )
Souce samples for conformation.
public class Arrays {
public static <T> List<T> asList(T... a) {
return new java.util.Arrays.ArrayList.ArrayList<>(a); // Arrays Inner Class ArrayList
}
//...
private static class ArrayList<E> extends AbstractList<E> implements RandomAccess, java.io.Serializable {
//...
}
}
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
public E set(int index, E element) {
throw new UnsupportedOperationException();
}
public E remove(int index) {
throw new UnsupportedOperationException();
}
public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
//...
}
public ListIterator<E> listIterator() {
return listIterator(0);
}
private class ListItr extends Itr implements ListIterator<E> {
//...
}
}
Form the above Source you may observe that java.util.Arrays.ArrayList class doesn't @Override add(index, element), set(index, element), remove(index). So, From inheritance it executes super AbstractList<E> class add() function which throws UnsupportedOperationException.
As AbstractList<E> is an abstract class it provides the implementation to iterator() and listIterator(). So, that we can iterate over the list object.
List<String> list_of_Arrays = Arrays.asList(new String[] { "a", "b" ,"c"});
try {
list_of_Arrays.add("Yashwanth.M");
} catch(java.lang.UnsupportedOperationException e) {
System.out.println("List Interface executes AbstractList add() fucntion which throws UnsupportedOperationException.");
}
System.out.println("Arrays → List : " + list_of_Arrays);
Iterator<String> iterator = list_of_Arrays.iterator();
while (iterator.hasNext()) System.out.println("Iteration : " + iterator.next() );
ListIterator<String> listIterator = list_of_Arrays.listIterator();
while (listIterator.hasNext()) System.out.println("Forward iteration : " + listIterator.next() );
while(listIterator.hasPrevious()) System.out.println("Backward iteration : " + listIterator.previous());
You can even create Fixed-Size array form Collections class Collections.unmodifiableList(list);
Sample Source:
public class Collections {
public static <T> List<T> unmodifiableList(List<? extends T> list) {
return (list instanceof RandomAccess ?
new UnmodifiableRandomAccessList<>(list) :
new UnmodifiableList<>(list));
}
}
A Collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data.
@see also
