如何编写用于测试字符串中重复元素计数的Junit测试用例

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

public class StringDuplicateCounterTest {
    @Test
    public void testCountDuplicates() {
        // 测试用例代码
    }
}

@Test
public void testCountDuplicates() {
    String input = "abcaabbcc";
    int expected = 4;
    
    int actual = StringDuplicateCounter.countDuplicates(input);
    
    assertEquals(expected, actual);
}

public class StringDuplicateCounter {
    public static int countDuplicates(String input) {
        int count = 0;
        char[] chars = input.toCharArray();
        Set<Character> uniqueChars = new HashSet<>();
        
        for (char c : chars) {
            if (!uniqueChars.add(c)) {
                count++;
            }
        }
        
        return count;
    }
}