remove unnecessary parameters

Transaction filter hotfix

pom.xml

@@ -78,6 +78,12 @@
             <artifactId>spring-security-web</artifactId>
             <version>6.1.5</version>
         </dependency>
+        <!-- https://mvnrepository.com/artifact/org.apache.commons/commons-math3 -->
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-math3</artifactId>
+            <version>3.6.1</version>
+        </dependency>
     </dependencies>
     <build>
         <defaultGoal>package</defaultGoal>

src/main/java/com/faf223/expensetrackerfaf/config/ApplicationConfig.java

@@ -1,10 +1,12 @@
 package com.faf223.expensetrackerfaf.config;
 
+import com.faf223.expensetrackerfaf.repository.CredentialRepository;
+import com.faf223.expensetrackerfaf.repository.UserRepository;
 import com.faf223.expensetrackerfaf.security.PersonDetails;
-import com.faf223.expensetrackerfaf.service.CredentialService;
 import lombok.RequiredArgsConstructor;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Primary;
 import org.springframework.security.authentication.AuthenticationManager;
 import org.springframework.security.authentication.AuthenticationProvider;
 import org.springframework.security.authentication.dao.DaoAuthenticationProvider;
@@ -18,11 +20,11 @@ import org.springframework.security.crypto.password.PasswordEncoder;
 @RequiredArgsConstructor
 public class ApplicationConfig {
 
-    private final CredentialService credentialService;
+    private final CredentialRepository credentialRepository;
 
     @Bean
     public UserDetailsService userDetailsService() {
-        return username -> new PersonDetails(credentialService.findByEmail(username).orElseThrow((() -> new UsernameNotFoundException("User not found"))));
+        return username -> new PersonDetails(credentialRepository.findByEmail(username).orElseThrow(() -> new UsernameNotFoundException("User not found")));
     }
 
     @Bean
@@ -42,4 +44,10 @@ public class ApplicationConfig {
     public PasswordEncoder passwordEncoder() {
         return new BCryptPasswordEncoder();
     }
+    @Bean
+    @Primary
+    public JwtAuthenticationFilter customJwtAuthenticationFilter(JwtService jwtService, UserDetailsService userDetailsService) {
+        return new JwtAuthenticationFilter(jwtService, userDetailsService);
+    }
+
 }

src/main/java/com/faf223/expensetrackerfaf/config/JwtAuthenticationFilter.java

@@ -62,7 +62,7 @@ public class JwtAuthenticationFilter extends OncePerRequestFilter {
             response.setContentType("application/json");
 
             ErrorResponse errorResponse = new ErrorResponse("Your session has expired. Refresh your token.");
-            ObjectMapper objectMapper = new ObjectMapper(); // You may need to import ObjectMapper
+            ObjectMapper objectMapper = new ObjectMapper();
             response.getWriter().write(objectMapper.writeValueAsString(errorResponse));
 
 

src/main/java/com/faf223/expensetrackerfaf/config/JwtService.java

@@ -5,6 +5,7 @@ import io.jsonwebtoken.Jwts;
 import io.jsonwebtoken.SignatureAlgorithm;
 import io.jsonwebtoken.io.Decoders;
 import io.jsonwebtoken.security.Keys;
+import lombok.RequiredArgsConstructor;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.security.core.userdetails.UserDetails;
 import org.springframework.stereotype.Service;
@@ -16,6 +17,7 @@ import java.util.Map;
 import java.util.function.Function;
 
 @Service
+@RequiredArgsConstructor
 public class JwtService {
 
     @Value("${application.security.jwt.secret-key}")
@@ -25,6 +27,7 @@ public class JwtService {
     @Value("${application.security.jwt.refresh-token.expiration}")
     private long refreshExpiration;
 
+
     public String extractUsername(String token) {
         return extractClaim(token, Claims::getSubject);
     }

src/main/java/com/faf223/expensetrackerfaf/config/SecurityConfiguration.java

@@ -1,31 +1,43 @@
 package com.faf223.expensetrackerfaf.config;
 
-import lombok.RequiredArgsConstructor;
+import com.faf223.expensetrackerfaf.controller.auth.JwtAuthenticationSuccessHandler;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.HttpStatus;
 import org.springframework.security.authentication.AuthenticationProvider;
 import org.springframework.security.config.Customizer;
 import org.springframework.security.config.annotation.method.configuration.EnableMethodSecurity;
 import org.springframework.security.config.annotation.web.builders.HttpSecurity;
 import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
 import org.springframework.security.config.annotation.web.configurers.AbstractHttpConfigurer;
-import org.springframework.security.config.http.SessionCreationPolicy;
+import org.springframework.security.oauth2.client.registration.ClientRegistrationRepository;
+import org.springframework.security.web.AuthenticationEntryPoint;
 import org.springframework.security.web.SecurityFilterChain;
-import org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter;
+import org.springframework.security.web.authentication.HttpStatusEntryPoint;
 import org.springframework.web.cors.CorsConfiguration;
 import org.springframework.web.cors.CorsConfigurationSource;
 import org.springframework.web.cors.UrlBasedCorsConfigurationSource;
 
 import java.util.Arrays;
 
+import static org.springframework.security.config.Customizer.withDefaults;
+
 @Configuration
 @EnableWebSecurity
-@RequiredArgsConstructor
 @EnableMethodSecurity
 public class SecurityConfiguration {
 
     private final JwtAuthenticationFilter jwtAuthFilter;
     private final AuthenticationProvider authenticationProvider;
+    private final ClientRegistrationRepository clientRegistrationRepository;
+
+    public SecurityConfiguration(JwtAuthenticationFilter jwtAuthFilter,
+                                 AuthenticationProvider authenticationProvider,
+                                 ClientRegistrationRepository clientRegistrationRepository) {
+        this.jwtAuthFilter = jwtAuthFilter;
+        this.authenticationProvider = authenticationProvider;
+        this.clientRegistrationRepository = clientRegistrationRepository;
+    }
 
     @Bean
     public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
@@ -33,15 +45,17 @@ public class SecurityConfiguration {
                 .cors(Customizer.withDefaults())
                 .csrf(AbstractHttpConfigurer::disable)
                 .authorizeHttpRequests(auth -> auth
-                        .requestMatchers("/api/v1/auth/**").permitAll()
                         .anyRequest().authenticated()
                 )
-                .sessionManagement(session -> session.sessionCreationPolicy(SessionCreationPolicy.STATELESS))
+                .oauth2Login(withDefaults());
-                .authenticationProvider(authenticationProvider)
+
-                .addFilterBefore(jwtAuthFilter, UsernamePasswordAuthenticationFilter.class); // will be executed before UsernamePasswordAuthenticationFilter
         return http.build();
     }
 
+    @Bean
+    public JwtAuthenticationSuccessHandler jwtAuthenticationSuccessHandler() {
+        return new JwtAuthenticationSuccessHandler();
+    }
     @Bean
     public CorsConfigurationSource corsConfigurationSource() {
         CorsConfiguration configuration = new CorsConfiguration();
@@ -54,4 +68,8 @@ public class SecurityConfiguration {
         return source;
     }
 
+    @Bean
+    public AuthenticationEntryPoint authenticationEntryPoint() {
+        return new HttpStatusEntryPoint(HttpStatus.UNAUTHORIZED);
+    }
 }

src/main/java/com/faf223/expensetrackerfaf/controller/ExpenseController.java

@@ -9,11 +9,9 @@ import com.faf223.expensetrackerfaf.model.User;
 import com.faf223.expensetrackerfaf.service.ExpenseCategoryService;
 import com.faf223.expensetrackerfaf.service.ExpenseService;
 import com.faf223.expensetrackerfaf.service.UserService;
+import com.faf223.expensetrackerfaf.util.DataExtender;
 import com.faf223.expensetrackerfaf.util.errors.ErrorResponse;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionDoesNotBelongToTheUserException;
+import com.faf223.expensetrackerfaf.util.exceptions.*;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionNotCreatedException;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionNotUpdatedException;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionsNotFoundException;
 import jakarta.validation.Valid;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.HttpStatus;
@@ -26,6 +24,7 @@ import org.springframework.transaction.annotation.Transactional;
 import org.springframework.validation.BindingResult;
 import org.springframework.web.bind.annotation.*;
 
+import java.math.BigDecimal;
 import java.time.LocalDate;
 import java.time.Month;
 import java.util.Collections;
@@ -134,6 +133,32 @@ public class ExpenseController {
         throw new TransactionsNotFoundException("The expenses have not been found");
     }
 
+    @GetMapping("/extend-data")
+    public ResponseEntity<List<BigDecimal>> extendData(@RequestParam Optional<String> extendValue,
+                                                       @RequestParam Optional<Integer> extrapolationCount) {
+        if(extendValue.isEmpty())
+            throw new RequiredParamMissingException("Extend value has not been specified");
+
+        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
+        User user;
+
+        if (authentication != null && authentication.getPrincipal() instanceof UserDetails userDetails)
+            user = userService.getUserByEmail(userDetails.getUsername());
+        else
+            user = null;
+
+        if(user == null)
+            throw new UserNotFoundException("User with the specified UUID has not been found");
+
+        if(extendValue.get().equalsIgnoreCase("i"))
+            return ResponseEntity.ok(DataExtender.interpolate(user.getExpenses()));
+
+        if(extendValue.get().equalsIgnoreCase("e"))
+            return extrapolationCount.map(integer -> ResponseEntity.ok(DataExtender.extrapolate(user.getExpenses(), integer))).orElseGet(() -> ResponseEntity.ok(DataExtender.extrapolate(user.getExpenses(), 10)));
+
+        throw new WrongParamValueException("Wrong extend value has been specified (use either \"i\" or \"e\")");
+    }
+
     @GetMapping("/categories")
     public ResponseEntity<List<ExpenseCategory>> getAllCategories() {
         List<ExpenseCategory> categories = expenseCategoryService.getAllCategories();

src/main/java/com/faf223/expensetrackerfaf/controller/GlobalExceptionHandler.java

@@ -12,72 +12,47 @@ public class GlobalExceptionHandler {
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleDoesNotBelongException(TransactionDoesNotBelongToTheUserException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.FORBIDDEN);
-                e.getMessage(),
-                System.currentTimeMillis()
-        );
-
-        return new ResponseEntity<>(response, HttpStatus.FORBIDDEN);
     }
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleTransactionNotCreatedException(TransactionNotCreatedException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.NOT_MODIFIED);
-                e.getMessage(),
-                System.currentTimeMillis()
-        );
-
-        return new ResponseEntity<>(response, HttpStatus.NOT_MODIFIED);
     }
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleTransactionsNotFoundException(TransactionsNotFoundException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.NOT_FOUND);
-                e.getMessage(),
-                System.currentTimeMillis()
-        );
-
-        return new ResponseEntity<>(response, HttpStatus.NOT_FOUND);
     }
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleUserNotFoundException(UserNotFoundException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.NOT_FOUND);
-                e.getMessage(),
-                System.currentTimeMillis()
-        );
-
-        return new ResponseEntity<>(response, HttpStatus.NOT_FOUND);
     }
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleTransactionNotUpdatedException(TransactionNotUpdatedException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.NOT_MODIFIED);
-                e.getMessage(),
-                System.currentTimeMillis()
-        );
-
-        return new ResponseEntity<>(response, HttpStatus.NOT_MODIFIED);
     }
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleUserNotAuthenticatedException(UserNotAuthenticatedException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.FORBIDDEN);
-                e.getMessage(),
-                System.currentTimeMillis()
-        );
-
-        return new ResponseEntity<>(response, HttpStatus.FORBIDDEN);
     }
 
     @ExceptionHandler
     private ResponseEntity<ErrorResponse> handleUserNotCreatedException(UserNotCreatedException e) {
-        ErrorResponse response = new ErrorResponse(
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.NOT_MODIFIED);
-                e.getMessage(),
+    }
-                System.currentTimeMillis()
-        );
 
-        return new ResponseEntity<>(response, HttpStatus.NOT_MODIFIED);
+    @ExceptionHandler
+    private ResponseEntity<ErrorResponse> handleRequiredParamMissingException(RequiredParamMissingException e) {
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.BAD_REQUEST);
+    }
+
+    @ExceptionHandler
+    private ResponseEntity<ErrorResponse> handleWrongParamValueException(WrongParamValueException e) {
+        return new ResponseEntity<>(new ErrorResponse(e.getMessage()), HttpStatus.BAD_REQUEST);
     }
 
 }

src/main/java/com/faf223/expensetrackerfaf/controller/IncomeController.java

@@ -9,11 +9,9 @@ import com.faf223.expensetrackerfaf.model.User;
 import com.faf223.expensetrackerfaf.service.IncomeCategoryService;
 import com.faf223.expensetrackerfaf.service.IncomeService;
 import com.faf223.expensetrackerfaf.service.UserService;
+import com.faf223.expensetrackerfaf.util.DataExtender;
 import com.faf223.expensetrackerfaf.util.errors.ErrorResponse;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionDoesNotBelongToTheUserException;
+import com.faf223.expensetrackerfaf.util.exceptions.*;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionNotCreatedException;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionNotUpdatedException;
-import com.faf223.expensetrackerfaf.util.exceptions.TransactionsNotFoundException;
 import jakarta.validation.Valid;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.HttpStatus;
@@ -26,6 +24,7 @@ import org.springframework.transaction.annotation.Transactional;
 import org.springframework.validation.BindingResult;
 import org.springframework.web.bind.annotation.*;
 
+import java.math.BigDecimal;
 import java.time.LocalDate;
 import java.time.Month;
 import java.util.Collections;
@@ -134,6 +133,32 @@ public class IncomeController {
         throw new TransactionsNotFoundException("The expenses have not been found");
     }
 
+    @GetMapping("/extend-data")
+    public ResponseEntity<List<BigDecimal>> extendData(@RequestParam Optional<String> extendValue,
+                                                       @RequestParam Optional<Integer> extrapolationCount) {
+        if(extendValue.isEmpty())
+            throw new RequiredParamMissingException("Extend value has not been specified");
+
+        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
+        User user;
+
+        if (authentication != null && authentication.getPrincipal() instanceof UserDetails userDetails)
+            user = userService.getUserByEmail(userDetails.getUsername());
+        else
+            user = null;
+
+        if(user == null)
+            throw new UserNotFoundException("User with the specified UUID has not been found");
+
+        if(extendValue.get().equalsIgnoreCase("i"))
+            return ResponseEntity.ok(DataExtender.interpolate(user.getIncomes()));
+
+        if(extendValue.get().equalsIgnoreCase("e"))
+            return extrapolationCount.map(integer -> ResponseEntity.ok(DataExtender.extrapolate(user.getIncomes(), integer))).orElseGet(() -> ResponseEntity.ok(DataExtender.extrapolate(user.getIncomes(), 10)));
+
+        throw new WrongParamValueException("Wrong extend value has been specified (use either \"i\" or \"e\")");
+    }
+
     @GetMapping("/categories")
     public ResponseEntity<List<IncomeCategory>> getAllCategories() {
         List<IncomeCategory> categories = incomeCategoryService.getAllCategories();

src/main/java/com/faf223/expensetrackerfaf/controller/auth/JwtAuthenticationSuccessHandler.java

@@ -0,0 +1,19 @@
+package com.faf223.expensetrackerfaf.controller.auth;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.springframework.security.core.Authentication;
+import org.springframework.security.web.authentication.AuthenticationSuccessHandler;
+import org.springframework.security.web.authentication.SimpleUrlAuthenticationSuccessHandler;
+
+import java.io.IOException;
+
+public class JwtAuthenticationSuccessHandler extends SimpleUrlAuthenticationSuccessHandler implements AuthenticationSuccessHandler {
+
+    @Override
+    public void onAuthenticationSuccess(HttpServletRequest request, HttpServletResponse response, Authentication authentication) throws IOException, ServletException {
+
+        super.onAuthenticationSuccess(request, response, authentication);
+    }
+}
+

src/main/java/com/faf223/expensetrackerfaf/controller/auth/OAuth2SuccessController.java

@@ -0,0 +1,24 @@
+package com.faf223.expensetrackerfaf.controller.auth;
+
+import com.faf223.expensetrackerfaf.service.AuthenticationService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.security.core.annotation.AuthenticationPrincipal;
+import org.springframework.security.oauth2.core.user.OAuth2User;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequiredArgsConstructor
+public class OAuth2SuccessController {
+
+    private final AuthenticationService jwtService;
+
+    @GetMapping()
+    public AuthenticationResponse getUser(@AuthenticationPrincipal OAuth2User oAuth2User) {
+
+        AuthenticationResponse response = jwtService.register(oAuth2User);
+        System.out.println("Response: " + response);
+
+        return response;
+    }
+}

src/main/java/com/faf223/expensetrackerfaf/service/AuthenticationService.java

@@ -17,6 +17,7 @@ import org.springframework.security.authentication.AuthenticationManager;
 import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
 import org.springframework.security.core.userdetails.UserDetails;
 import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.security.oauth2.core.user.OAuth2User;
 import org.springframework.stereotype.Service;
 
 import java.util.Optional;
@@ -30,6 +31,7 @@ public class AuthenticationService {
     private final PasswordEncoder passwordEncoder;
     private final JwtService jwtService;
     private final AuthenticationManager authenticationManager;
+    private final PasswordGenerator passwordGenerator;
 
     public AuthenticationResponse register(RegisterRequest request) {
 
@@ -53,6 +55,56 @@ public class AuthenticationService {
                 .build();
     }
 
+    public AuthenticationResponse register(OAuth2User oAuth2User) {
+        String userEmail = oAuth2User.getAttribute("email");
+
+        // Check if the user is already registered
+        Optional<Credential> existingCredential = credentialRepository.findByEmail(userEmail);
+        if (existingCredential.isPresent()) {
+            UserDetails userDetails = new PersonDetails(existingCredential.get());
+            String jwtToken = jwtService.generateToken(userDetails);
+            String refreshToken = jwtService.generateRefreshToken(userDetails);
+
+            return AuthenticationResponse.builder()
+                    .accessToken(jwtToken)
+                    .refreshToken(refreshToken)
+                    .build();
+        }
+
+
+        String givenName = oAuth2User.getAttribute("given_name");
+        String familyName = oAuth2User.getAttribute("family_name");
+        String email = oAuth2User.getAttribute("email");
+
+        User user = User.builder()
+                .firstName(givenName)
+                .lastName(familyName)
+                .username(email)
+                .build();
+
+        String randomPassword = passwordGenerator.generateRandomPassword(8);
+
+        user.setPassword(passwordEncoder.encode(randomPassword));
+
+        userRepository.save(user);
+        Credential credential = new Credential(user, email, passwordEncoder.encode(randomPassword));
+        credentialRepository.save(credential);
+
+        UserDetails userDetails = new PersonDetails(credential);
+        String jwtToken = jwtService.generateToken(userDetails);
+        String refreshToken = jwtService.generateRefreshToken(userDetails);
+
+        System.out.println("New user: " + user);
+        System.out.println("New credentials: " + credential);
+
+        return AuthenticationResponse.builder()
+                .accessToken(jwtToken)
+                .refreshToken(refreshToken)
+                .build();
+    }
+
+
+
     public AuthenticationResponse authenticate(AuthenticationRequest request) {
         authenticationManager.authenticate(new UsernamePasswordAuthenticationToken(request.getEmail(), request.getPassword()));
 

src/main/java/com/faf223/expensetrackerfaf/service/PasswordGenerator.java

@@ -0,0 +1,18 @@
+package com.faf223.expensetrackerfaf.service;
+
+import org.springframework.stereotype.Service;
+
+import java.security.SecureRandom;
+import java.util.Base64;
+
+@Service
+public class PasswordGenerator {
+
+    private final SecureRandom secureRandom = new SecureRandom();
+
+    public String generateRandomPassword(int length) {
+        byte[] randomBytes = new byte[length];
+        secureRandom.nextBytes(randomBytes);
+        return Base64.getUrlEncoder().withoutPadding().encodeToString(randomBytes);
+    }
+}

src/main/java/com/faf223/expensetrackerfaf/util/DataExtender.java

@@ -0,0 +1,63 @@
+package com.faf223.expensetrackerfaf.util;
+
+import com.faf223.expensetrackerfaf.model.IMoneyTransaction;
+import org.apache.commons.math3.analysis.interpolation.LinearInterpolator;
+import org.apache.commons.math3.analysis.polynomials.PolynomialSplineFunction;
+
+import java.math.BigDecimal;
+import java.util.ArrayList;
+import java.util.List;
+
+public class DataExtender {
+
+    public static List<BigDecimal> interpolate(List<? extends IMoneyTransaction> transactions) {
+        List<BigDecimal> values = new ArrayList<>();
+
+        // Perform linear interpolation on the amount field
+        if (transactions.size() > 1) {
+            LinearInterpolator interpolator = new LinearInterpolator();
+
+            double[] xValues = new double[transactions.size()];
+            double[] yValues = new double[transactions.size()];
+
+            for (int i = 0; i < transactions.size(); i++) {
+                xValues[i] = i;
+                yValues[i] = transactions.get(i).getAmount().doubleValue();
+            }
+
+            PolynomialSplineFunction splineFunction = interpolator.interpolate(xValues, yValues);
+
+            // Interpolate values between the first and last data points
+            for (int i = 1; i < transactions.size() - 1; i++)
+                values.add(BigDecimal.valueOf(splineFunction.value(i + 0.5))); // Interpolate at the midpoint
+        }
+
+        return values;
+    }
+
+    public static List<BigDecimal> extrapolate(List<? extends IMoneyTransaction> transactions, int count) {
+        List<BigDecimal> values = new ArrayList<>();
+
+        // Perform linear extrapolation on the amount field
+        if (transactions.size() > 1) {
+            LinearInterpolator interpolator = new LinearInterpolator();
+
+            double[] xValues = new double[transactions.size()];
+            double[] yValues = new double[transactions.size()];
+
+            for (int i = 0; i < transactions.size(); i++) {
+                xValues[i] = i;
+                yValues[i] = transactions.get(i).getAmount().doubleValue();
+            }
+
+            PolynomialSplineFunction splineFunction = interpolator.interpolate(xValues, yValues);
+
+            // Extrapolate values beyond the last data point
+            for (int i = transactions.size(); i < transactions.size() + count; i++)
+                values.add(BigDecimal.valueOf(splineFunction.value(i))); // Extrapolate by extending the x-axis
+        }
+
+        return values;
+    }
+
+}

src/main/java/com/faf223/expensetrackerfaf/util/exceptions/RequiredParamMissingException.java

@@ -0,0 +1,7 @@
+package com.faf223.expensetrackerfaf.util.exceptions;
+
+public class RequiredParamMissingException extends RuntimeException {
+    public RequiredParamMissingException(String message) {
+        super(message);
+    }
+}

src/main/java/com/faf223/expensetrackerfaf/util/exceptions/WrongParamValueException.java

@@ -0,0 +1,7 @@
+package com.faf223.expensetrackerfaf.util.exceptions;
+
+public class WrongParamValueException extends RuntimeException {
+    public WrongParamValueException(String message) {
+        super(message);
+    }
+}