Examples for an initialized list to replace element on a particular position.

ListTest.java

package de.rouel;

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

import junit.framework.Assert;

import org.junit.Test;

/**
 * How can you setup a list with one or more elements at a particular position (index), without having to add elements
 * to these position with <code>List.add(T element)</code>?
 * 
 * @see java.util.List#add(Object)
 * 
 * @author André Rouél
 */
public class ListTest {

	/**
	 * This test shows an easier initialization to replace elements on an expected position in a <code>List</code>.
	 */
	@Test
	public void testGetByIndexInitiallyFilled() {
		final String[] l = new String[11];
		final List<String> list = new ArrayList<String>(Arrays.asList(l));

		// adds an element for index 3, and now you can replace it with set(idx, string)
		list.set(3, "test on index 3");

		// adds an element for index 10, and now you can replace it with set(idx, string)
		list.set(10, "test on index 10");

		Assert.assertEquals("test on index 10", list.get(10));
		Assert.assertEquals("test on index 3", list.get(3));

		Assert.assertNull(list.get(0));
		Assert.assertNull(list.get(2));
		Assert.assertNull(list.get(7));

		list.remove(3);
		Assert.assertNull(list.remove(3));
	}

	/**
	 * Of course an empty list throws an <code>IndexOutOfBoundsException</code> if you want to get an element of an
	 * non-existent position.
	 */
	@Test(expected = IndexOutOfBoundsException.class)
	public void testGetByIndexOnEmptyList() {
		final List<String> list = new ArrayList<String>();
		list.get(3);
	}

	/**
	 * This way you get an initialized list of strings with 11 <code>null</code>-elements.
	 */
	@Test
	public void testGetByIndexOnInitializedList() {
		final String[] l = new String[11];
		final List<String> list = new ArrayList<String>(Arrays.asList(l));
		Assert.assertNull(list.get(10));
	}

	/**
	 * This test shows an complicated way to replace elements on an expected position in a <code>List</code>.<br>
	 * <br>
	 * <b>Attention</b>: This way is not recommended!
	 */
	@Test
	public void testGetByIndexOnManuallyFilledList() {
		final List<String> list = new ArrayList<String>(10);
		list.add(null);
		list.add(null);
		list.add(null);

		// adds an element for index 3, and now you can replace it with set(idx, string)
		list.add(null);
		list.set(3, "test on index 3");
		list.add(null);
		list.add(null);
		list.add(null);
		list.add(null);
		list.add(null);
		list.add(null);

		// adds an element for index 10, and now you can replace it with set(idx, string)
		list.add(null);
		list.set(10, "test on index 10");

		Assert.assertEquals("test on index 10", list.get(10));
		Assert.assertEquals("test on index 3", list.get(3));

		Assert.assertNull(list.get(0));
		Assert.assertNull(list.get(2));
		Assert.assertNull(list.get(7));

		list.remove(3);
		Assert.assertNull(list.remove(3));
	}

}