diff --git a/src/test/java/com/application/munera/services/PersonServiceTest.java b/src/test/java/com/application/munera/services/PersonServiceTest.java
new file mode 100644
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--- /dev/null
+++ b/src/test/java/com/application/munera/services/PersonServiceTest.java
@@ -0,0 +1,184 @@
+package com.application.munera.services;
+
+import com.application.munera.data.Expense;
+import com.application.munera.data.Person;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.mockito.junit.jupiter.MockitoSettings;
+import org.mockito.quality.Strictness;
+
+import java.math.BigDecimal;
+import java.util.Collections;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+@MockitoSettings(strictness = Strictness.LENIENT)
+class PersonServiceTest {
+
+    @Mock
+    private ExpenseService expenseService;
+
+    @InjectMocks
+    private PersonService personService;
+
+    @Test
+    public void calculateDeb_whenExpensesAreMixed() {
+        Person person = new Person();
+        person.setLastName("first");
+        person.setFirstName("person");
+
+        Person person2 = new Person();
+        person2.setLastName("second");
+        person2.setFirstName("person");
+
+        Person person3 = new Person();
+        person3.setLastName("third");
+        person3.setFirstName("person");
+
+        Expense expense1 = mock(Expense.class);
+        when(expense1.getPayer()).thenReturn(person);
+        when(expense1.getBeneficiary()).thenReturn(person2);
+        when(expense1.getCost()).thenReturn(new BigDecimal("110.00"));
+        when(expense1.getIsPaid()).thenReturn(false);
+
+        Expense expense2 = mock(Expense.class);
+        when(expense2.getPayer()).thenReturn(person);
+        when(expense2.getBeneficiary()).thenReturn(person);
+        when(expense2.getCost()).thenReturn(new BigDecimal("50.00"));
+        when(expense2.getIsPaid()).thenReturn(false);
+
+        Expense expense3 = mock(Expense.class);
+        when(expense3.getPayer()).thenReturn(person);
+        when(expense3.getBeneficiary()).thenReturn(person);
+        when(expense3.getCost()).thenReturn(new BigDecimal("50.00"));
+        when(expense3.getIsPaid()).thenReturn(true);
+
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(List.of(expense1, expense2, expense3));
+
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        assertEquals(new BigDecimal("110.00"), totalDebt);
+    }
+    @Test
+    void calculateDebt_NoExpenses() {
+        // Arrange
+        Person person = new Person();
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(Collections.emptyList());
+
+        // Act
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        // Assert
+        assertEquals(BigDecimal.ZERO, totalDebt);
+    }
+
+    @Test
+    void calculateDebt_AllExpensesPaid() {
+        // Arrange
+        Person person = new Person();
+
+        Expense expense1 = mock(Expense.class);
+        when(expense1.getPayer()).thenReturn(person);
+        when(expense1.getBeneficiary()).thenReturn(new Person());
+        when(expense1.getCost()).thenReturn(new BigDecimal("100.00"));
+        when(expense1.getIsPaid()).thenReturn(true);
+
+        Expense expense2 = mock(Expense.class);
+        when(expense2.getPayer()).thenReturn(person);
+        when(expense2.getBeneficiary()).thenReturn(new Person());
+        when(expense2.getCost()).thenReturn(new BigDecimal("50.00"));
+        when(expense2.getIsPaid()).thenReturn(true);
+
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(List.of(expense1, expense2));
+
+        // Act
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        // Assert
+        assertEquals(BigDecimal.ZERO, totalDebt);
+    }
+
+    @Test
+    void calculateDebt_ExpensesWithSamePayerAndBeneficiary() {
+        // Arrange
+        Person person = new Person();
+
+        Expense expense1 = mock(Expense.class);
+        when(expense1.getPayer()).thenReturn(person);
+        when(expense1.getBeneficiary()).thenReturn(person); // Same person as payer
+        when(expense1.getCost()).thenReturn(new BigDecimal("100.00"));
+        when(expense1.getIsPaid()).thenReturn(false);
+
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(List.of(expense1));
+
+        // Act
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        // Assert
+        assertEquals(BigDecimal.ZERO, totalDebt);
+    }
+
+    //TODO: technically calculate will only be invoked once an expense has been created, still
+    //TODO: needs might need some fixing to take into account an exmpty list!!!
+    @Test
+    void calculateDebt_NullOrEmptyExpensesList() {
+        // Arrange
+        Person person = new Person();
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(null); // Null case
+
+        // Act
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        // Assert
+        assertEquals(BigDecimal.ZERO, totalDebt);
+    }
+
+    @Test
+    void calculateDebt_ExpensesWithNullAttributes() {
+        // Arrange
+        Person person = new Person();
+
+        Expense expense1 = mock(Expense.class);
+        when(expense1.getPayer()).thenReturn(person);
+        when(expense1.getBeneficiary()).thenReturn(new Person());
+        when(expense1.getCost()).thenReturn(null); // Null cost
+        when(expense1.getIsPaid()).thenReturn(false);
+
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(List.of(expense1));
+
+        // Act
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        // Assert
+        assertEquals(BigDecimal.ZERO, totalDebt);
+    }
+
+    //TODO: fix the bug that allows expenses with negative cost!!!
+    @Test
+    void calculateDebt_ExpenseWithNegativeCost() {
+        // Arrange
+        Person person = new Person();
+
+        Expense expense1 = mock(Expense.class);
+        when(expense1.getPayer()).thenReturn(person);
+        when(expense1.getBeneficiary()).thenReturn(new Person());
+        when(expense1.getCost()).thenReturn(new BigDecimal("-50.00")); // Negative cost
+        when(expense1.getIsPaid()).thenReturn(false);
+
+        when(expenseService.findExpensesWherePayer(person)).thenReturn(List.of(expense1));
+
+        // Act
+        BigDecimal totalDebt = personService.calculateDebt(person);
+
+        // Assert
+        assertEquals(new BigDecimal("-50.00"), totalDebt);
+    }
+
+}
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