Added and rewrote tests

src/test/java/de/rwu/easydrop/service/processing/TransactionHandlerTest.java

@@ -0,0 +1,50 @@
+package de.rwu.easydrop.service.processing;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import de.rwu.easydrop.api.dto.TransactionDTO;
+import de.rwu.easydrop.data.connector.TransactionPersistenceInterface;
+import de.rwu.easydrop.model.Offer;
+import de.rwu.easydrop.model.Product;
+import de.rwu.easydrop.model.Transaction;
+import de.rwu.easydrop.service.mapping.TransactionMapper;
+
+class TransactionHandlerTest {
+
+    @Mock
+    private TransactionPersistenceInterface mockPersistence;
+
+    public TransactionHandlerTest() {
+        MockitoAnnotations.openMocks(this);
+    }
+
+    @Test
+    void turnOfferToTransaction_ValidOffer_TransactionCreatedAndWrittenToPersistence() {
+        // Arrange
+        Offer offer = new Offer();
+        Product product = new Product();
+        offer.setSourceProduct(product);
+        offer.setTargetProduct(product);
+
+        TransactionHandler handler = new TransactionHandler(mockPersistence);
+
+        // Act
+        handler.turnOfferToTransaction(offer);
+
+        // Assert
+        ArgumentCaptor<TransactionDTO> transactionDtoCaptor = ArgumentCaptor.forClass(TransactionDTO.class);
+        verify(mockPersistence, times(1)).writeTX(transactionDtoCaptor.capture());
+
+        TransactionDTO capturedTransactionDto = transactionDtoCaptor.getValue();
+        Transaction capturedTransaction = TransactionMapper.mapTXFromDTO(capturedTransactionDto);
+
+        assertEquals(offer.getOfferId(), capturedTransaction.getOfferId());
+    }
+}