Technical Musings !!

We frequently use asList method of Arrays class to convert an array to a list. In order to use this method properly we need to know what is happening behind the scene. The complete signature of this method is:   

      public static <T> List<T> asList(T... a)
 

This method is declared and defined in java.util.Arrays class. The method returns an object of ArrayList class. But here is one difference which we have to note and take care while using this.     

The object being returned does not belong to java.util.ArrayList class rather it belongs to java.util.Arrays$ArrayList class. Surprised! Actually the Arrays class defines an private static inner class named ArrayList. Following is the signature of the ArrayList class:     

      private static class ArrayList<E> extends AbstractList<E>implements RandomAccess, 
      java.io.Serializable 

Now let us see what the difference between the two ArrayList classes is.  

The Arrays$ArrayList class is sometimes refered as the view of java.util.ArrayList or a fixed size list. This class does not override all the methods of AbstractList class. To be more precise, following are the methods it overrides or declares:     

      public int size()  

      public Object[] toArray()

      public <T> T[] toArray(T[] a)

      public E get(int index)

      public E set(int index, E element)

      public int indexOf(Object o)

      public boolean contains(Object o)

Other methods if being called refer to the AbstractList class default implementation.  

For example if we try to call the add or remove method of Arrays$ArrayList class, we will get an UnsupportedOperationException . The reason behind this is the implementation of these methods in the AbstractList class which is:  

      public void add(int index, E element) {
                 throw new UnsupportedOperationException();
      }  

      public E remove(int  index) {
                 throw new  UnsupportedOperationException();
      }  

I hope this clears the understanding of how views are being used in java.

Let us conclude this article with the javadoc comments of Arrays.asList method: 

Returns a fixed-size list backed by the specified array.  (Changes to the returned list “write through” to the array.)  This method acts as bridge between array-based and collection-based APIs, in combination with Collection#toArray.  The returned list is serializable and implements RandomAccess.

This method also provides a convenient way to create a fixed-size list initialized to contain several elements:
List<String> stooges = Arrays.asList(“Larry”, “Moe”, “Curly”); 

Enjoy Learning,
Kamlesh