Testing individual methods from a class

I do not understand and know how to test and what individual methods in the below Java class.  I don't know for each method how to pass either characters,strings for the respective methods.

import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
/**
 * Java class to add chracters &/or a string of characters to a
 * default alpha table for use by developers for testing an input
 * string to be valid for developer's objective
 *
 *
 * Creation date:     2004/02/10
 * Modification date: 0000/00/00
 * @author: TJW
 */
public class ValidCharactersChecker {

    private char[] initialChars = {'a','b','c','d','e','f','g',
                                                 'h','i','j','k','l','m','n',
                                                 'o','p','q','r','s','t','u',
                                                 'v','w','x','y','z','A','B',
                                                   'C','D','E','F','G','H','I',
                                                 'J','K','L','M','N','O','P',
                                                 'Q','R','S','T','U','V','W',
                                                 'X','Y','Z'};
    private TreeSet validCharacterSet = null;

    public ValidCharactersChecker() {
        validCharacterSet = new TreeSet();
            this.addChars(initialChars);
    } // 21 c'tor

    public void addChars(char[] c) {
            int numChars = initialChars.length;
          for (int i = 0;i <numChars;++i ){
                  Character myChar = new Character(initialChars[i]);
                  addChar(myChar);

          }
      }
    //31 input of string example: ("-,.")
    public void addChars(String s){

          int numChars = s.length();
        for (int i = 0;i < numChars;++i){
                  char k = s.charAt(i);
                  Character myChar = new Character (k);
                  addChar(myChar);
        }

    }

    public void addChar(char c) {
        validCharacterSet.add(new Character(c));
    }
    public void addChar(Character validChar) {
        validCharacterSet.add(validChar);
    }

    public void removeChar(char invalidChar) {
        validCharacterSet.remove(new Character(invalidChar));
    }

    public void removeChar(Character invalidChar) {
        validCharacterSet.remove(invalidChar);
    }

    public char[] getValidCharacters() {
        int numChars = validCharacterSet.size();
        char[] validCharArray = new char[numChars];

        Iterator iter = validCharacterSet.iterator();
        int i = 0;
        while(iter.hasNext()) {
            Character c = (Character) iter.next();
            validCharArray[i] = c.charValue();
            ++i;
        } // while
        return validCharArray;
    } // getChars

      //overloaded method
      public boolean isValid(String s) {
            int slen = s.length();
            for (int i =0;i < slen;i++){
                  char k = s.charAt(i);
                  boolean bull = this.isValid(k);
                  if (false == bull) {
                     return false;
               }
            }
            return true;
      }

      //overloaded method
      public boolean isValid(char c) {
        Character d = new Character(c);
            if (validCharacterSet.contains (d))
             return true;
          else
             return false;
      }


      public Set getValidCharacterSet() {
        return Collections.unmodifiableSet(validCharacterSet);
    } // getChars
}