StringTest.java

import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class StringTest {

    @Test
    public void testAllTheSame() {
        assertEquals(true, allDuplicateChars("cccccc"));
    }

    @Test
    public void testNotTheSame() {
        assertEquals(false, allDuplicateChars("abcdef"));
    }

    public boolean allDuplicateChars(String input) {
        return input.matches("(.)\\1*");
    }
}