Java Interface as argument help

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Java

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;

  public static final char lf = '\n';

  private String m_Content =
    "It was the best of times, it was the worst of times," +lf +
    "it was the age of wisdom, it was the age of foolishness,";

  Random m_Rnd = new Random();

  public char GetNextChar() throws EOFException {

    if (m_Pos >= m_Content.length()) {
        throw new EOFException();
    }

    return m_Content.charAt(m_Pos++);

  }

  public void Dispose() {
    // do nothing
  }
}

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.

Could you help me with passing interface as an argument and what is meant by "writing unit test case"?