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Implement rest-api exercise (exercism#1818)
* Initial skeleton for rest-api implementation. * Continue work on rest-api. * Add initial files for rest-api src. * Complete initial implementation of rest-api. * Fix style violations in rest-api exercise. * Fix the tests to use the constructor properly for rest-api. * Fix the tests and reference implementation for rest-api.
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‎config.json

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"matrices"
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]
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},
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{
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"slug": "rest-api",
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"uuid": "809c0e3d-3494-4a85-843d-2bafa8752ce8",
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"core": false,
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"unlocked_by": "robot-name",
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"difficulty": 6,
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"topics": [
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"strings",
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"parsing"
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]
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},
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{
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"slug": "roman-numerals",
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"uuid": "3e728cd4-5e5f-4c69-8a53-bc36d020fcdb",

‎exercises/rest-api/.meta/src/reference/java/Iou.java

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/**
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* POJO representing an IOU.
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*
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* If this is in a user's "owed", then the user owes the person with {@link name} this {@link amount}.
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* If this is in a user's "owedBy", then {@link name} owes the user this {@link amount}.
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*/
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public class Iou {
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public final String name;
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public final double amount;
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public Iou(String name, double amount) {
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this.name = name;
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this.amount = amount;
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}
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}

‎exercises/rest-api/.meta/src/reference/java/RestApi.java

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import org.json.JSONArray;
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import org.json.JSONObject;
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import java.util.ArrayList;
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import java.util.Comparator;
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import java.util.HashMap;
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import java.util.List;
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import java.util.Map;
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public class RestApi {
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private final Map<String, User> usersByName = new HashMap<>();
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public RestApi(User... users) {
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for (User user : users) {
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usersByName.put(user.name(), user);
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}
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}
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public String get(String url) {
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if (url.equals("/users")) {
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return getAllUsers();
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}
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return "";
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}
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private String getAllUsers() {
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JSONObject result = new JSONObject();
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JSONArray users = new JSONArray();
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usersByName.values().stream()
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.sorted(Comparator.comparing(User::name))
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.map(User::toJson)
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.forEach(users::put);
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result.put("users", users);
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return result.toString();
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}
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public String get(String url, JSONObject payload) {
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if (url.equals("/users") && payload.has("users")) {
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return getUsers(payload.getJSONArray("users"));
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}
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return "";
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}
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private String getUsers(JSONArray users) {
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return getUsers(getUserNames(users));
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}
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private String getUsers(List<String> userNames) {
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JSONObject result = new JSONObject();
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JSONArray users = new JSONArray();
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userNames.stream()
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.sorted()
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.filter(usersByName::containsKey)
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.map(userName -> usersByName.get(userName).toJson())
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.forEach(users::put);
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result.put("users", users);
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return result.toString();
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}
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private static List<String> getUserNames(JSONArray users) {
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List<String> names = new ArrayList<>();
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for (int i = 0; i < users.length(); i++) {
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names.add(users.getString(i));
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}
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return names;
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}
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public String post(String url, JSONObject payload) {
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if (url.equals("/add")) {
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return addUser(payload);
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}
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if (url.equals("/iou")) {
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return addIou(payload);
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}
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return "";
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}
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private String addUser(JSONObject payload) {
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if (payload.has("user")) {
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return addUser(payload.getString("user"));
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}
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return "";
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}
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private String addUser(String name) {
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if (usersByName.containsKey(name)) {
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return "";
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}
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User user = User.builder().setName(name).build();
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usersByName.put(name, user);
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return user.toJson().toString();
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}
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private String addIou(JSONObject payload) {
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if (payload.has("lender")
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&& payload.has("borrower")
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&& payload.has("amount")) {
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return addIou(
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payload.getString("lender"),
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payload.getString("borrower"),
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payload.getDouble("amount"));
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}
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return "";
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}
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private String addIou(String lender, String borrower, double amount) {
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if (!usersByName.containsKey(lender)
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|| !usersByName.containsKey(borrower)
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|| amount <= 0.0) {
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return "";
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}
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updateLender(lender, borrower, amount);
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updateBorrower(lender, borrower, amount);
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return getUsers(List.of(lender, borrower));
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}
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private void updateLender(String lenderName, String borrowerName, double amount) {
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User user = usersByName.get(lenderName);
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User.Builder lender = User.builder().setName(lenderName);
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copyUnaffectedEntries(lenderName, borrowerName, user, lender);
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double owedToBorrower = user.getAmountOwedTo(borrowerName);
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double owedByBorrower = user.getAmountOwedBy(borrowerName) + amount;
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if (owedToBorrower > owedByBorrower) {
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lender.owes(borrowerName, owedToBorrower - owedByBorrower);
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} else if (owedByBorrower > owedToBorrower) {
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lender.owedBy(borrowerName, owedByBorrower - owedToBorrower);
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}
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usersByName.put(lenderName, lender.build());
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}
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private void updateBorrower(String lenderName, String borrowerName, double amount) {
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User user = usersByName.get(borrowerName);
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User.Builder borrower = User.builder().setName(borrowerName);
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copyUnaffectedEntries(lenderName, borrowerName, user, borrower);
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double owedToLender = user.getAmountOwedTo(lenderName) + amount;
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double owedByLender = user.getAmountOwedBy(lenderName);
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if (owedToLender > owedByLender) {
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borrower.owes(lenderName, owedToLender - owedByLender);
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} else if (owedByLender > owedToLender) {
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borrower.owedBy(lenderName, owedByLender - owedToLender);
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}
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usersByName.put(borrowerName, borrower.build());
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}
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private static void copyUnaffectedEntries(String lender, String borrower, User user, User.Builder updated) {
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user.owes().stream()
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.filter(iou -> !isAffected(lender, borrower, iou))
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.forEach(iou -> updated.owes(iou.name, iou.amount));
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user.owedBy().stream()
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.filter(iou -> !isAffected(lender, borrower, iou))
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.forEach(iou -> updated.owedBy(iou.name, iou.amount));
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}
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private static boolean isAffected(String lender, String borrower, Iou iou) {
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return iou.name.equals(lender) || iou.name.equals(borrower);
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}
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}

‎exercises/rest-api/.meta/src/reference/java/User.java

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import static java.util.Collections.unmodifiableList;
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import java.util.ArrayList;
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import java.util.List;
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import org.json.JSONObject;
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/** POJO representing a User in the database. */
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public class User {
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private final String name;
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private final List<Iou> owes;
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private final List<Iou> owedBy;
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private User(String name, List<Iou> owes, List<Iou> owedBy) {
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this.name = name;
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this.owes = new ArrayList<>(owes);
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this.owedBy = new ArrayList<>(owedBy);
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}
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public String name() {
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return name;
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}
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/** IOUs this user owes to other users. */
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public List<Iou> owes() {
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return unmodifiableList(owes);
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}
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public double getAmountOwedTo(String other) {
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return owes().stream()
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.filter(iou -> iou.name.equals(other))
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.map(iou -> iou.amount)
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.findFirst()
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.orElse(0.0);
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}
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/** IOUs other users owe to this user. */
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public List<Iou> owedBy() {
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return unmodifiableList(owedBy);
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}
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public double getAmountOwedBy(String other) {
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return owedBy().stream()
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.filter(iou -> iou.name.equals(other))
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.map(iou -> iou.amount)
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.findFirst()
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.orElse(0.0);
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}
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public static Builder builder() {
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return new Builder();
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}
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public JSONObject toJson() {
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JSONObject object =
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new JSONObject()
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.put("name", name())
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.put("owes", owesToJson())
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.put("owedBy", owedByToJson())
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.put("balance", balance());
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return object;
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}
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private JSONObject owesToJson() {
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return iousToJson(owes());
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}
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private JSONObject owedByToJson() {
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return iousToJson(owedBy());
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}
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private static JSONObject iousToJson(List<Iou> ious) {
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JSONObject object = new JSONObject();
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for (Iou iou : ious) {
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object.put(iou.name, iou.amount);
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}
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return object;
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}
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private double balance() {
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double balance = 0.0;
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for (Iou iou : owes()) {
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balance -= iou.amount;
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}
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for (Iou iou : owedBy()) {
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balance += iou.amount;
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}
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return balance;
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}
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public static class Builder {
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private String name;
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private final List<Iou> owes = new ArrayList<>();
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private final List<Iou> owedBy = new ArrayList<>();
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public Builder setName(String name) {
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this.name = name;
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return this;
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}
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public Builder owes(String name, double amount) {
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owes.add(new Iou(name, amount));
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return this;
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}
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public Builder owedBy(String name, double amount) {
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owedBy.add(new Iou(name, amount));
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return this;
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}
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public User build() {
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return new User(name, owes, owedBy);
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}
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}
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}

‎exercises/rest-api/.meta/version

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1.1.1

‎exercises/rest-api/README.md

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Implement a RESTful API for tracking IOUs.
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Four roommates have a habit of borrowing money from each other frequently, and have trouble remembering who owes whom, and how much.
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Your task is to implement a simple [RESTful API](https://en.wikipedia.org/wiki/Representational_state_transfer) that receives [IOU](https://en.wikipedia.org/wiki/IOU)s as POST requests, and can deliver specified summary information via GET requests.
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### API Specification
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#### User object
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```json
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{
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"name": "Adam",
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"owes": {
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"Bob": 12.0,
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"Chuck": 4.0,
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"Dan": 9.5
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},
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"owed_by": {
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"Bob": 6.5,
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"Dan": 2.75,
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},
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"balance": "<(total owed by other users) - (total owed to other users)>"
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}
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```
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#### Methods
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| Description | HTTP Method | URL | Payload Format | Response w/o Payload | Response w/ Payload |
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| --- | --- | --- | --- | --- | --- |
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| List of user information | GET | /users | `{"users":["Adam","Bob"]}` | `{"users":<List of all User objects>}` | `{"users":<List of User objects for <users> (sorted by name)}` |
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| Create user | POST | /add | `{"user":<name of new user (unique)>}` | N/A | `<User object for new user>` |
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| Create IOU | POST | /iou | `{"lender":<name of lender>,"borrower":<name of borrower>,"amount":5.25}` | N/A | `{"users":<updated User objects for <lender> and <borrower> (sorted by name)>}` |
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### Other Resources:
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- https://restfulapi.net/
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- Example RESTful APIs
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- [GitHub](https://developer.github.com/v3/)
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- [Reddit](https://www.reddit.com/dev/api/)

‎exercises/rest-api/build.gradle

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apply plugin: "java"
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apply plugin: "eclipse"
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apply plugin: "idea"
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repositories {
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mavenCentral()
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}
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dependencies {
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compile 'org.json:json:20190722'
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testCompile "junit:junit:4.12"
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}
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test {
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testLogging {
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exceptionFormat = 'short'
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showStandardStreams = true
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events = ["passed", "failed", "skipped"]
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}
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}

‎exercises/rest-api/src/main/java/Iou.java

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/**
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* POJO representing an IOU.
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*
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* If this is in a user's "owed", then the user owes the person with {@link name} this {@link amount}.
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* If this is in a user's "owedBy", then {@link name} owes the user this {@link amount}.
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*/
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public class Iou {
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public final String name;
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public final double amount;
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public Iou(String name, double amount) {
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this.name = name;
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this.amount = amount;
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}
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}

‎exercises/rest-api/src/main/java/RestApi.java

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/*
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Since this exercise has a difficulty of > 4 it doesn't come
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with any starter implementation.
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This is so that you get to practice creating classes and methods
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which is an important part of programming in Java.
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Please remove this comment when submitting your solution.
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*/

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