Java Interface As Argument

I have 2 files which are as follows:

ICharacterReader.java::

import java.io.EOFException;
public interface ICharacterReader {
char GetNextChar() throws EOFException;
void Dispose();
}

And another file:

SimpleCharacterReader.java::
import java.io.EOFException;
import java.util.Random;
public class SimpleCharacterReader implements ICharacterReader {
private int m_Pos = 0;

[Code] ....

My task is as follows:

1.) Write a class that takes an ICharacterReader interface as an argument and returns a list of word frequencies ordered by word count and then alphabetically.

And also write a main method of a console application that exercises this class using a SimpleCharacterReader, and prints the output to the console.

For example, if the stream returns "It was the best of times, it was the worst of times" then the output will be:

it - 2;
of - 2;
the - 2;
times -2;
was - 2;
best - 1;
worst - 1;

2.) Test the answers in part 1, by writing unit test cases.

How to pass interface as an argument and what is meant by "writing unit test case"?