Java List size Puzzle

This is a Java puzzle involving the Collection List interface. This is a Java beginner level puzzle and will help understand how the size works in List element. What is the output for the following Java code snippet?

package com.javapapers.java.puzzle;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ListPuzzle {
	public static void main(String args[]) {
		List<String> circus1 = new ArrayList<String>();
		circus1.add("Monkey");
		circus1.add("Elephant");

		List<String> zoo1 = new ArrayList<String>(circus1);
		zoo1.add("Lion");
		System.out.println("zoo1 size: " + zoo1.size());

		String[] circus2 = { "Monkey", "Elephant" };

		List<String> zoo2 = Arrays.asList(circus2);
		zoo2.add("Tiger");
		System.out.println("zoo2 size: " + zoo2.size());
	}

}

Solution

In this Java program we are creating two different List instances zoo1 and zoo2. zoo1 is created using the ArrayList constructor which allows to pass a collection. zoo2 is created using the utility class Arrays which is used for manipulating arrays. There is no issue in the zoo1 instantiation and addition of an element to it.

The problem is on line where we add an element to the zoo2 object. zoo2 instance is a fixed size List instance. The method asList() of Arrays returns a fixed size List instance. This instance serves as a bridge for an array to a collection. We cannot add a new element or remove an element from the returned instance. We can set the value of an already existing element using its index. Any change we do will be propagated to the array source.

So we will get UnSupportedOperationException at the line where we add an element to zoo2. This is one of the popular errors faced a by a beginner Java programmer.

Program Output

zoo1 size: 3
Exception in thread "main" java.lang.UnsupportedOperationException
	at java.util.AbstractList.add(AbstractList.java:148)
	at java.util.AbstractList.add(AbstractList.java:108)
	at com.javapapers.java.puzzle.ListPuzzle.main(ListPuzzle.java:20)