My design

mock/machine/coding/pkg3/Board.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mock.machine.coding.pkg3;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ *
+ * @author rajeshkumar.yadav
+ */
+public class Board extends Item{
+
+    //Board: Each board should have a id, name, privacy (PUBLIC/PRIVATE), url, members, lists
+
+    private Privacy privacy;
+    private String url;
+    private List<User> userList;
+    private List<ListItem> list;
+
+    public Board(Privacy privacy, String name) {
+
+        super(name);
+        this.privacy = privacy;
+        url = URLGenerator.getURL(getId());
+        userList = new ArrayList<>();
+        list = new ArrayList<>();
+    }
+
+    public Privacy getPrivacy() {
+        return privacy;
+    }
+
+    public String getUrl() {
+        return url;
+    }
+
+    public void setPrivacy(Privacy privacy) {
+        this.privacy = privacy;
+    }
+
+    public void setUrl(String url) {
+        this.url = url;
+    }
+
+    public void addListItemToBoard(ListItem l){
+        list.add(l);
+        l.setBoard(this);
+    }
+
+    public void removeListFromBoard(ListItem l){
+        if(list.contains(l))
+            list.remove(l);
+    }
+
+    public void addUserToBoard(User u){
+        userList.add(u);
+    }
+
+    public void removeUserToBoard(User u){
+        if(userList.contains(u))
+            userList.remove(u);
+    }
+
+    public String getPrintableString(){
+        String s = "{ Id: " + getId() + ", Name:  " + getName() + " privacy: " + getPrivacy() + ", URL:  " + getUrl();
+        if(userList.size() > 0){
+            s = s + ", Users: [";
+            for(int i=0; i<userList.size(); i++){
+                s = s + userList.get(i).getPrintableString();
+                if(i<userList.size()-1)
+                    s = s+", ";
+            }
+            s = s + "]";
+        }
+        if(list.size() > 0){
+            s = s + ", List: [";
+            for(int i=0; i<list.size(); i++){
+                s = s + list.get(i).getPrintableString();
+                if(i<list.size()-1)
+                    s = s+", ";
+            }
+            s = s + "]";
+        }
+        s = s + " }";
+        return s;
+    }
+
+    public void show(){
+        System.out.println(getPrintableString());
+    }
+
+}

mock/machine/coding/pkg3/Card.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mock.machine.coding.pkg3;
+
+/**
+ *
+ * @author rajeshkumar.yadav
+ */
+public class Card extends Item{
+
+    //Card: Each card should have a id, name, description, assigned user
+    private String description;
+    private User assigendUser;
+    private ListItem litem;
+
+    public Card(String name) {
+        super(name);
+        //this.assigendUser = assigendUser;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public void setDescription(String description) {
+        this.description = description;
+    }
+
+    public User getAssigendUser() {
+        return assigendUser;
+    }
+
+    public void setAssigendUser(User assigendUser) {
+        this.assigendUser = assigendUser;
+    }
+
+    public void unAssigendUser() {
+        this.assigendUser = null;
+    }
+
+    public ListItem getLitem() {
+        return litem;
+    }
+
+    public void setLitem(ListItem litem) {
+        this.litem = litem;
+    }
+
+    public String getPrintableString(){
+        String s = " { id: " + getId() + ", name: " + getName() + ", description:  " + getDescription() + ",  AssignedToUser: " + getAssigendUser().getName() + " }";
+        return s;
+    }
+
+    public void show(){
+        System.out.println(getPrintableString());    
+    }
+
+
+}

mock/machine/coding/pkg3/IdCreator.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mock.machine.coding.pkg3;
+
+/**
+ *
+ * @author rajeshkumar.yadav
+ */
+public class IdCreator {
+
+    private static int inc = 0;
+    private static IdCreator idCreator = null;
+
+    private IdCreator(){
+
+    }
+
+    public static IdCreator getInstance(){
+        if(idCreator == null)
+            idCreator = new IdCreator();
+        return  idCreator;
+    }
+
+    public static String getNewId(){
+
+        String s = String.valueOf(System.currentTimeMillis()).substring(1,9);
+        inc = (inc + 1)%10;
+        s = s + inc;
+        return s;
+    }
+
+
+}

mock/machine/coding/pkg3/Item.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mock.machine.coding.pkg3;
+
+/**
+ *
+ * @author rajeshkumar.yadav
+ */
+public class Item {
+
+    private String id;
+    private String name;
+
+    public Item(String name) {
+        this.id = IdCreator.getNewId();
+        this.name = name;
+    }
+
+    public String getId() {
+        return id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public void show(){
+
+    }
+    public String getPrintableString(){
+        return "";
+    }
+}

mock/machine/coding/pkg3/ListItem.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mock.machine.coding.pkg3;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ *
+ * @author rajeshkumar.yadav
+ */
+public class ListItem extends Item{
+
+    // List: Each list should have a id, name and cards
+    private List<Card> cards;
+    private Board board;
+
+    public ListItem(String name) {
+        super(name);
+        cards = new ArrayList<>();
+    }
+
+    public List<Card> getCards() {
+        return cards;
+    }
+
+    //We should also be able to move cards across lists in the same board
+    public void AddCardToList(Card c){
+        if(c.getLitem() != null && this.getBoard() != c.getLitem().getBoard())
+            return;
+        cards.add(c);
+        c.setLitem(this);
+    }
+
+    // We should also be able to remove cards from lists 
+    public void removeCardFromList(Card c){
+        if(c.getLitem() != null)
+            return;
+        cards.remove(c);
+        c.setLitem(null);
+    }
+
+    public Board getBoard() {
+        return board;
+    }
+
+    public void setBoard(Board board) {
+        this.board = board;
+    }
+
+    public String getPrintableString(){
+        String s = "{ Id: " + getId() + ", Name:  " + getName();
+        if(cards.size() > 0){
+            s = s + ", cards: [";
+            for(int i=0; i<cards.size(); i++){
+                s = s + cards.get(i).getPrintableString();
+                if(i<cards.size()-1)
+                    s = s+", ";
+            }
+        }
+        return s;
+    }
+
+    public void show(){
+        System.out.println(getPrintableString());
+    }
+
+}