Made the steps in the initial test a bit more explicit.

aes-encryptor-test.java

public class AESEncryptorTest {
    private final AESEncryptor aesExample = new AESEncryptor();
    private static String SAMPLE_TEXT = "Original Plaintext";

    @Test
    public void decryptingCiphertextShouldReturnOriginalPlaintext() throws Exception {
        String cipherText = aesExample.encrypt(SAMPLE_TEXT);
        String plainText = aesExample.decrypt(cipherText);
        assertEquals(SAMPLE_TEXT, plainText);
    }

}