Suppose I have a simple Java Enum:
public Enum itemType
{
FRUITS("fru"),
VEGETABLES("veg"),
LIQUOURS("liq"),
SODAS("sod");
private String dbCode;
public ItemType(String dbCode){
this.dbCode = dbCode;
}
public String getDbCode(){
return this.dbCode;
}
}
I would now like to introduce a "category" to this enum, for example to make the distinction between liquid items and solid items. I found two ways of doing this within the enum class, see below. However, both suffer from the same anti-pattern: if the amount of categories or amount of items ever increases/decreases (imagine 100 item types with 10 categories!), I've got a lot of updating to do. What patterns can I use to design this enum as cleanly and re-usable as possible?
First approach: Add additional properties to the enum
public Enum itemType
{
FRUITS("fru",false),
VEGETABLES("veg",false),
LIQUOURS("liq",true),
SODAS("sod",true);
private String dbCode;
private boolean liquid;
public ItemType(String dbCode, boolean liquid){
this.dbCode = dbCode;
this.liquid = liquid;
}
public String getDbCode(){
return this.dbCode;
}
public boolean isLiquid(){
return this.liquid;
}
}
Second approach: Use static methods to ask about subcategories
public Enum itemType
{
FRUITS("fru"),
VEGETABLES("veg"),
LIQUOURS("liq"),
SODAS("sod");
private String dbCode;
public ItemType(String dbCode){
this.dbCode = dbCode;
}
public String getDbCode(){
return this.dbCode;
}
public static boolean isLiquid(ItemType type){
switch(t){
case SODA:
case LIQOURS: return true;
default: return false;
}
}
How about using an EnumSet for that?
public enum ItemType
{
FRUITS("fru"),
VEGETABLES("veg"),
LIQUOURS("liq"),
SODAS("sod");
public static final EnumSet<ItemType> LIQUIDS = EnumSet.of(LIQUOURS, SODAS);
// ...
}
Then you can use ItemType.LIQUIDS.contains(someItemType) to check if someItemType is a "liquid".
I would do something like:
enum Category {
LIQUID, SOLID;
}
enum ItemType {
FRUITS("fru", SOLID),
VEGETABLES("veg", SOLID),
LIQUOURS("liq", LIQUID),
SODAS("sod", LIQUID);
private String dbCode;
private Category category;
public ItemType(String dbCode, Category category){
this.dbCode = dbCode;
this.category = category;
}
/* getters / setters */
}
That would allow, for example, that you can add new products and categories (e.g. BUTANE("but", GAS)) without having to modify the existing code (as would happen in Approach 2).
On the other hand, if the number of categories and items is long and changing, I would consider to use a SQL database.
Since you are modeling something that has no logic that can be encoded in an algorithmic way (i.e. there's no algorithm that would figure out that "sod" is liquid and "veg" is not) there is no way around enumerating all related pairs of (item, category) in one way or the other.
There are three approaches to implementing it:
Enumerate categories on item's side - this is what your code does in both cases, or
Enumerate items on category's side - this would build an enum of categories, and attach a full list of items to each of them, or
Enumerate item+category pairs independently - this approach may be useful when storing item/category mapping in the database or in a configuration file.
I would recommend taking the third approach as it is the most "symmetric" one. Make a table for categories with category codes, and add a "cross-table" (or a cross-file) that has all pairs of categories and their corresponding items. Read the cross table/file at startup, and set up the dependencies on both sides.
public Enum ItemType {
FRUITS("fru")
, VEGETABLES("veg")
, LIQUOURS("liq")
, SODAS("sod");
public void addCategory(ItemCategory category) ...;
public EnumSet<ItemCategory> getItemCategories() ...;
}
public Enum ItemCategory {
LIQUIDS("liq")
, SNACKS("snk")
, FAST("fst");
public void addItem(ItemType type) ...;
public EnumSet<ItemType> getItemTypes() ...;
}
Cross-file or cross-table may look like this:
liq liq
sod liq
fru snk
fru fst
sod fst
You process it by enumerating pairs, and calling addCategory on the pair's item side, and calling addItem on the pair's category side.
These were three excellent answers, but I think I can combine all three in one nice package:
public enum ItemType {
FRUITS("fru",PERISHABLE),
VEGETABLES("veg",PERISHABLE),
LIQUOURS("liq",LIQUIDS),
SODAS("sod",LIQUIDS),
FRESH_SQUEEZED_ORANGE_JUICE("orgj",LIQUIDS,PERISHABLE);
private final String dbCode;
private final EnumSet<ItemCategory> categories;
private static final Map<ItemCategory,Set<ItemType>> INDEX_BY_CATEGORY = new EnumMap<>(ItemCategory.class);
ItemType(String dbcode,ItemCategory... categories) {
this.dbCode = dbcode;
this.categories = EnumSet.copyOf(Arrays.asList(categories));
//for (ItemCategory c:categories) {
// // Illegal Reference to Static Field!
// INDEX_BY_CATEGORY.put(c, this);
//}
}
static {
for (ItemCategory c:ItemCategory.values()) {
INDEX_BY_CATEGORY.put(c, EnumSet.noneOf(ItemType.class));
}
for (ItemType t:values()) {
for (ItemCategory c:t.categories) {
INDEX_BY_CATEGORY.get(c).add(t);
}
}
}
public boolean is(ItemCategory c) {
return INDEX_BY_CATEGORY.get(c).contains(this);
}
public Set<ItemType> getAll(ItemCategory c) {
return EnumSet.copyOf(INDEX_BY_CATEGORY.get(c));
}
public String getDbCode() {
return dbCode;
}
}
Now,
we can easily ask about additional subcategories without writing the code for it: boolean isVegetableLiquid = VEGETABLES.is(LIQUIDS);
we can easily assign not only one, but multiple categories to an item as you can see for FRESH_SQUEEZED_ORANGE_JUICE.
we are using EnumSet and EnumMap for performance, including their methods like contains.
we absolutely are minimizing the amount of code required to add an additional item. This could be further minimized by setting this up by database or configuration. However, in that case we would have to avoid the use of Enum as well.
I have a class called SalesOrder (SO), that allows users to buy several items in a single order. SO has an order number.
class SalesOrder {
public String orderNumber;
}
Each SO has many items in it, so I have created a new class OrderItem which has the item name and price.
class OrderItem {
public String name;
public double price;
}
Each SO has a order header, include user name and address. It also has a field called total price, which hold the sum of all items prices
class OrderHeader {
public String username;
public String address;
public double totalPrice;
}
After that, I added two fields to SO:
class SalesOrder {
...
public List<OrderItem> items;
public OrderHeader header;
}
Because OrderItem and OrderHeader are always used with SalesOrder and the header should return all items prices, I converted them to be be inner classes of SalesOrder.
class SalesOrder {
...
public SalesOrder() {
this.items = new ArrayList<>();
this.header = new OrderHeader();
}
public class OrderItem {
...
}
public class OrderHeader {
...
public double getTotalPrice() {
double total = 0.0;
// loop SalesOrder.items
total += items[i].price;
return total;
}
}
}
My question is whether using inner classes like this is good OOP design? If not, how should they be designed?
======= Update Some information =======
I'm very sorry that I haven't give more inforamtion.
Header and Item make they construe method private, other object can't create them without SalesOrder.
SalesOrder have a factory method
class SalesOrder {
...
public SalesOrder parseOrder(Xml xml) {
//init header and items from xml
this.header = new OrderHeader(valueFromXml, valueFromXml);
}
public class OrderHeader {
....
private OrderHeader(username, address) { ... }
}
public Class OrderItem {
...
private OrderItem(name, price) { ... }
}
}
And other object use them like this
Xml xml = orderXmlData;
SalesOrder order = SalesOrder.parseOrder(orderXmlData);
OrderItem item = order.item;
OrderHeader header = order.header;
There are a few suggestion I would have that might improve your design. Firstly, it seems unlikely to me that the totalPrice should be part of the header. It seems more likely that it is derived from the order items rather than being a component of the header. Secondly, unless you want clients of the class to create order items independent of the order then there seems no need to define them as a class. Better to convert to an interface returned from the order. Thirdly, there's no reason why Header can't be interface - this allows a client to use any class they want as a header as long as it has name and address.
So my suggestion would be something like:
class Order {
interface Item {
String getName();
double getPrice();
}
interface Header {
String getName();
Address getAddress();
}
public Order(Header header) {
...
}
public double getTotalPrice() {
return streamItems().mapToDouble(Item::getPrice).sum();
}
public void addItem(String name, double price) {
...
}
public Stream<Item> streamItems() {
...
}
}
The use of nested classes depends on the requirements. I do not see any need for the use of nested classes in your code. If it is a Java Beans class, then you should keep the classes separated so they can be reusable. Other than your last block of code with the nested classes, your design is perfect.
I have to implement the following class diagram to the java code. This diagram is very complicated and some parts creates confusion. This question definitely going to help me a lot as well as any reader because it contains several important aspects of UML diagram.
class Book{
String isbn;
String publisher;
String publishDate;
int pages;
}
class BookItem extends Book{
String barcode;
boolean isReferenceOnly;
}
class Author{
String name;
String biography;
Collection<Book> book;
}
class Account{
String number;
List<History> history;
String openDate;
AccountState state;
public Account(AccountState state){
this.state = state;
}
}
enum AccountState{
Active,
Frozen,
Closed
}
class Catalog implements Search, Manage{
List<BookItem> bookItem;
/* Implement the methods of Manage interface */
void add(BookItem item){ }
void remove(BookItem item){ }
/* Implement the methods of Search interface */
int search(BookItem item){ }
}
class Account{
String number;
List<History> history;
Student student = new Student();
void setStudent(Student student){
this.student = student;
}
}
interface Search{
int search(BookItem item);
}
interface Manage{
void add(BookItem item);
void remove(BookItem item);
}
class Student{
String name;
String address;
Search searchBook = new Catalog();
}
class Librarian{
String name;
String address;
String position;
Search searchBook = new Catalog();
Manage manage = new Catalog();
Account account = new Account();
void setAccount(Account account){
this.account = account;
}
class Library{
String name;
String Address;
List<BookItem> bookItem = new ArrayList<BookItem>();
Catalog catalog = new catalog();
List<Account> accounts = new ArrayList<Account>();
Library(Catalog catalog){
this.catalog = catalog;
}
void setBookItem(List<BookItem> bookItem){
this.bookItem = bookItem;
}
void setAccounts(List<Account> accounts){
this.accounts = accounts;
}
}
I implemented in the following way but confusion arise in various cases:
How to implement Class Student use the interface Search.
How to implement Class Librarian use the interfaces Search and Manage.
Why we are not use association instead of usage dependency.
How to implement that Enumeration data type in this case with usage dependency [I have just considered AccountState as a class, i the it is a wrong implementation].
How to use AccountState in the Account [I have just created a object of AccountState].
After read many blogs still unable to implement Aggregation and Composition confidently. Note: In this diagram 3 Aggregations and 1 Composition Exist. Those are:
(a) Library consists of many Account. {Aggregation}
(b) Many Book Item is the part of Library. {Aggregation}
(c) An Account is the part of a Student. {Aggregation}
(d) Library must have a Catalog. {Composition}
Please give your valuable advice so i can learn it well. Thanking you.
Since this question is homework for learning purposes, I will post only examples of how to implement the things you need to review and won't give a direct answer about how to apply them to your current design.
Enumeration in Java is implemented by using enum.
enum WeekDays {
MONDAY,
TUESDAY,
WEDNESDAY,
THURSDAY,
FRIDAY,
SATURDAY,
SUNDAY;
}
Aggregation/Composition means to have a field of the other class. If it's a weak association (aggregation), it should be initialized by the setter or another method. If it's a strong association, it should be initialized in the class constructor since it is needed for the class to live/work.
class WeakAssociation { }
class StrongAssociation { }
class NeedWeekAndStrongAssociation {
private WeakAssociation weakAssociation;
private StrongAssociation strongAssociation;
public NeedWeekAndStrongAssociation(StrongAssociation strongAssociation) {
this.strongAssociation = strongAssociation;
}
public void setWeakAssociation(WeakAssociation weakAssociation) {
this.weakAssociation = weakAssociation;
}
}
Usage dependency means that the class/interface will use the other class/interface within one or more of its methods:
class WantToBeUsed {
public void methodToBeUsed(String data) {
//fancy implementation
}
}
class CannotDoThisAlone {
public void cannotDoItAlone(String data) {
WantToBeUsed wantToBeUsed = new WantToBeUsed();
wantToBeUsed.methodToBeUsed(data);
}
}
I am attempting to create an inventory tracking system. I have a class (in Java) called "InventoryItem" with the properties of name and quantity.
This works fine for simple objects, but what if my inventory item contains other inventory items, for example, a server with RAM?
Should I be creating my own datatype, or is there a better way to do this (linked listed maybe)? should my class extend whatever that datatype is or should I not bother creating my own class?
My class so far:
public class InventoryItem {
private String name;
private int quantity;
private InventoryItem childInventoryItem;
// CONSTRUCTORS
public InventoryItem() {
}
public InventoryItem(int quantity, String name) {
this.quantity = quantity;
this.name = name;
}
//GETTERS
public String getName() {
return name;
}
public int getQuantity() {
return quantity;
}
//SETTERS
public void setName(String name) {
this.name = name;
}
public void setQuantity(int quantity) {
this.quantity = quantity;
}
}
A tree is usually what is involved in any parent-child relationship. If you aren't doing anything complicated, you can simply maintain an internal list that is basically List<InventoryItem> which contains any child items.
So all you would add to your class is something like this:
public class InventoryItem {
...
private List<InventoryItem> composingItems = new ArrayList<>(); //if still using Java 6 this must be new ArrayList<InventoryItem>();
...
public void addComposingItem(InventoryItem composingItem) {
composingItems.add(composingItems);
}
public List<InventoryItem> getComposingItems() {
//Enforce immutability so no one can mess with the collection. However
//this doesn't guarantee immutability for the objects inside the list;
//you will have to enforce that yourself.
return Collections.umodifiableList(composingItems);
}
}
There are many ways you can do this. I think the easiest way would be to create an array list.
ArrayList<InventoryItem> childInventory = new ArrayList<InventoryItem>();
Then create a setter that adds inventory items to this array
public void addChildItem(InventoryItem child)
childInventory.add(child);
This way you would have a list of all of the child items within the item. You could also make a method to return a list of all of the child items in either an array or an ArrayList.
public ArrayList<InventoryItem> getChildItems()
return childInventory;
Using the metawidget to build some flexible UI in Java: https://sourceforge.net/projects/metawidget/
public class Cohort {
private int id;
private Project project;
private Member teamLead;
public Cohort() { }
#UiHidden
public int getId() { return id; }
public void setId(int id) { this.id = id; }
public Project getProject() { return project; }
public void setProject(Project project) { this.project = project; }
public Member getTeamLead() { return teamLead; }
public void setTeamLead(Member teamLead) { this.teamLead = teamLead; }
}
Cohort is the class inspected. However as is desirable it recursively inspects both the Project and Member classes.
When displayed on the UI, it will display all the fields for each of the classes. However I would only like to display the "Name" field of the Project and firstName + last Name of the Member.
There are a number of ways to achieve this. I'll start with one and let me know if it's sufficient for your needs:
a) mark the fields of Project/Member that you don't want to see as UiHidden (you don't say what those fields are, but you seem to have gotten the idea because you are already hiding 'Cohort.getId'). Note you can also reuse existing annotations (like JPA annotations) for this purpose.
b) mark 'Cohort.getProject' and 'Cohort.getTeamLead' as UiLabel( "" ). This will suppress the sub-label for the sub-object, and make its fields appear as if part of the original object.