last element of recursive list

[rokko]

Hey,

i have a problem to find the last element of my recursive list

import java.util.NoSuchElementException;
import list.List;


public class Lists {
	
	/**
	 * 
	 * 
	 *
	 * Returns a string representation of the list.
	 * 
	 * @return a comma-separated list of the list values
	 */
	public static String toString(List list)
	{
		if (list == null) return "null";
		
		StringBuffer buf = new StringBuffer();
		buf.append(list.value).append(", ").append(toString(list.next));
		return buf.toString();
	}
	
	/**
	 * Adds a value to at the beginning of a list.
	 * 
	 * @param list the list to which the value is added
	 * @param value the value which is added to the list
	 * @return a new list with the new element first followed by the given list
	 */
	public static List add(List list, int value)
	{
		List newlist = new List(value, list);
		list = newlist;
		return list;
	}
	
	/**
	 * Retrieves the size of a list.
	 * 
	 * @param list the list in question
	 * @return the number of values in the list.
	 */
	public static int size(List list)
	{
		return list == null ? 0 : 1 + size(list.next);
		
	}
	/**
	 * Aggregates the sum of all list values.
	 * 
	 * @param list the list in question
	 * @return the sum of all list values
	 */
	public static int sum(List list)
	{
		return list == null ? 0 : list.value + sum(list.next);
		
	}	

	/**
	 * Retrieves the last element of a list.
	 * 
	 * The last element of the empty list is the empty list.
	 * 
	 * @param list the list in question.
	 * @return the last element of the given list.
	 */
	public static List last(List list) 
    {
		
			if (list != null && list.next == null) {
				
			return list.next;
			} else { 
				return list;
			}

				
			}

it has to pass a jUnit test,

import org.junit.Assert;


import u5a1.Lists;

public class Tests {
	public void check(List list, String expected) 
	{ 
		Assert.assertEquals(expected, Lists.toString(list));	
	}
	
	public List create(int... values) 
	{
		List list = null;
		for (int i=0; i<values.length; i++) {
			list = Lists.add(list, values[i]);
		}
		return list;
	}
}

-----

private void checkLast(List list, String expected)
	{
		check(Lists.last(list), expected);
	}
	
	@Test
	public void last()
	{
		checkLast(null, "null");
		checkLast(create(1), "1, null");
		//checkLast(create(1,2), "1, null");
		checkLast(create(1,2,3), "1, null");
	}

but i receive the whole list, not only the last element and i dont find the failure
if its list.next == null, it means i am at the end of the list, so i can return list.next, or not?

i hope somebody can help me

gruss

[Norm]

Can you post the program's output and explain what is wrong with the output and show what the output should be?

Do you have a testing program? I do not see a main() method in the code you posted.

[Tolls]

It's JUnit, which executes in its own framework.
The entry point (as far as this is concerned) is the method marked as @Test.

[rokko]

Can you post the program's output and explain what is wrong with the output and show what the output should be?

Do you have a testing program? I do not see a main() method in the code you posted.

the first code block is my code, which i have to test with junit (second code block), i have to return a list with the last element, the test-program makes a string of this result and checks it with junit

the output of junit is:

org.junit.ComparisonFailure: expected:<[1, ]null> but was:<[]null>

[Norm]

So this code can't be executed as a java program for testing?

[JosAH]

I'd say your last( ... ) method is goofy; the (recursive) logic has to be:

List last(List list) {
   if (list == null) return null;
   if (list.next == null) return list;
   return last(list.next);
}

kind regards,

Jos

[rokko]

thank you, it works!

[rokko]

And to find a sublist

/**
	 * Retrieves a sublist of a list.
	 * 
	 * @param list the list in question
	 * @param index a position in the list starting at zero for the head value
	 * @return the sublist with the element indexed by index as head element
	 * @throws IndexOutOfBoundsException if the list is smaller than index
	 */
	public static List sublist(List list, int index) throws IndexOutOfBoundsException
	{
		index = size(list)-x;
		return sublist(list, index);
		
	}

i have to use size(list), right?

[Norm]

What happens when you execute the code? Does it do what you want it to do?

[rokko]

no no, it cant, because f.e. x is undefined... it was just a question to see if im on the right way or if this idea is completley wrong

[Norm]

If it does not compile, then you must be doing something wrong.

[rokko]

org.junit.ComparisonFailure: expected:<[1, ]null> but was:<[]null>

[Norm]

How can the posted code be executed for testing?

[rokko]

In eclipse you can run it as Junit-Test, the code and the test are in the same package

[Norm]

I don't use the IDE so I'd need a class with a main() method to test the code.

[rokko]

of course you need the whole test

[Norm]

Where is the starting class? The one with the main() method.

[rokko]

i doubt there is a main method^^

[Norm]

The code can not be executed for testing as an application without a class with a main() method.

[Tolls]

	/**
	 * Retrieves a sublist of a list.
	 * 
	 * @param list the list in question
	 * @param index a position in the list starting at zero for the head value
	 * @return the sublist with the element indexed by index as head element
	 * @throws IndexOutOfBoundsException if the list is smaller than index
	 */
	public static List sublist(List list, int index) throws IndexOutOfBoundsException
	{
		if (list == null)
			return list;
		if (list.next == null){
			index = size(list);
			return list.next;
		}
		return list;
		
	}

it does run, but dont pass the junit test:

@Test(expected = IndexOutOfBoundsException.class)
	public void sublistFail1()
	{
		Lists.sublist(null, 0);
	}

org.junit.ComparisonFailure: expected:<[1, ]null> but was:<[]null>

You should be able to say what that failure comes from.
JUnit tells you which test has failed.

I suspect it's the one I've posted here anyway.
If 'list' is null then all you do is return null. You do not throw an IndexOutOfBoundsException, so that test will fail.