An example of a Java program that uses a while loop to check if a character is in a string of no more than 20 characters, and allows the user to continue searching for other characters or finish the process. The class is tested using two functional testing techniques: Equivalence Partitioning, Boundary Value Analysis and Category Partition.

CharacterSearch.java

import java.util.Scanner;

public class CharacterSearch {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String inputString;
        char searchChar;
        String searchAgain = "y";

        while (searchAgain.equals("y")) {
            // get the input string
            System.out.print("Enter a string of no more than 20 characters: ");
            inputString = scanner.nextLine();
            while(inputString.length()>20){
                System.out.println("String is too long, please enter a string of no more than 20 characters:");
                inputString = scanner.nextLine();
            }
            // get the character to search for
            System.out.print("Enter a character to search for: ");
            searchChar = scanner.nextLine().charAt(0);

            // search for the character in the string
            int charIndex = inputString.indexOf(searchChar);
            if (charIndex == -1) {
                System.out.println("The character '" + searchChar + "' was not found in the string '" + inputString + "'.");
            } else {
                System.out.println("The character '" + searchChar + "' was found at position " + (charIndex + 1) + " in the string '" + inputString + "'.");
            }

            // ask the user if they want to search for another character
            System.out.print("Search for another character? (y/n): ");
            searchAgain = scanner.nextLine();
        }
        System.out.println("Thank you for using the Character Search program!");
    }
    
    public int indexOfCharInString(String inputString, char searchChar) {
        if (inputString.length() > 20) {
            return -1;
        }
        return inputString.indexOf(searchChar);
    }
}

CharacterSearchBoundaryValueAnalysisTest.java

import org.junit.Test;
import static org.junit.Assert.*;

public class CharacterSearchTest {

    CharacterSearch cs = new CharacterSearch();

      @Test
    public void testIndexOfCharInString2() {
        // Test case for the minimum length of string
        String inputString = "a";
        char searchChar = 'a';
        int expectedResult = 0;
        int result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for the maximum length of string
        inputString = "abcdefghijklmnopqrsz";
        searchChar = 'z';
        expectedResult = 19;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for the first character of the string
        inputString = "abcdefghijklmnopqrst";
        searchChar = 'a';
        expectedResult = 0;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for the last character of the string
        inputString = "abcdefghijklmnopqrsz";
        searchChar = 'z';
        expectedResult = 19;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for special characters
        inputString = "!@#abcdefghijklmnopq";
        searchChar = '#';
        expectedResult = 2;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);
    }
}

CharacterSearchCategoryPartitionTest.java

import org.junit.Test;
import static org.junit.Assert.*;

public class CharacterSearchTest {

    CharacterSearch cs = new CharacterSearch();

    @Test
    public void testIndexOfCharInString3() {
        // Test case for strings with length less than 20
        String inputString = "abc";
        char searchChar = 'b';
        int expectedResult = 1;
        int result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for strings with length equal to 20
        inputString = "abcdefghijklmnopqrsx";
        searchChar = 'x';
        expectedResult = 19;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for strings with length greater than 20
        inputString = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";
        searchChar = 'z';
        expectedResult = -1;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);
    }
}

CharacterSearchEquivalencePartitioningTest.java

import org.junit.Test;
import static org.junit.Assert.*;

public class CharacterSearchTest {

    CharacterSearch cs = new CharacterSearch();

    @Test
    public void testIndexOfCharInString() {
        //Positive Test Case
        String inputString = "abcdefghijklmnopqrst";
        char searchChar = 'a';
        int expectedResult = 0;
        int result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        //Negative Test Case
        inputString = "abcdefghijklmnopqrstuvwxyz";
        searchChar = 'z';
        expectedResult = -1;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for max length of string
        inputString = "abcdefghijklmnopqrsz";
        searchChar = 'z';
        expectedResult = 19;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for min length of string
        inputString = "a";
        searchChar = 'a';
        expectedResult = 0;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);

        // Test case for special characters
        inputString = "!@#abcdefghijklmnopq";
        searchChar = '#';
        expectedResult = 2;
        result = cs.indexOfCharInString(inputString, searchChar);
        assertEquals(expectedResult, result);
    }
}