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How to write equals and hashCode methods in java for attribute that is a list of other attributes

I have a HashMap where the key is a class and value is an integer. I need to check if an object of the class already exists in the map. I use containsKey(), but for some reason it does not work when I include attribute sideDish in the equals() and hashCode(). Here is my code for the classes:
OrderItem class:
#ToString
#Entity
#Table(name="OrderItem")
public class OrderItem implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Getter #Setter
private Long id;
#ManyToOne
#Getter #Setter
private Food food;
#ManyToMany
#Getter #Setter
private List<SideDish> sideDishes;
public OrderItem() {}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((food == null) ? 0 : food.hashCode());
result = prime * result + ((sideDishes == null) ? 0 : sideDishes.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
OrderItem other = (OrderItem) obj;
if (food == null) {
if (other.food != null)
return false;
} else if (!food.equals(other.food))
return false;
if (sideDishes == null) {
if (other.sideDishes != null)
return false;
} else if (!sideDishes.equals(other.sideDishes))
return false;
return true;
}
}
Food class:
#ToString
#Entity
#Table(name="Food")
public class Food implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Getter #Setter
private Long id;
#Column(nullable = false, unique = true)
#NotNull(message = "Name cannot be null.")
#Getter #Setter
private String name;
#ManyToMany
#Getter #Setter
private List<SideDish> sidedishes;
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((foodtype == null) ? 0 : foodtype.hashCode());
result = prime * result + ((id == null) ? 0 : id.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Food other = (Food) obj;
if (foodtype == null) {
if (other.foodtype != null)
return false;
} else if (!foodtype.equals(other.foodtype))
return false;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
SideDish class:
#Entity
#ToString(exclude= {"id","dishtype"})
#Table(name="SideDish")
public class SideDish implements Serializable, Comparable<SideDish>{
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Getter #Setter
private Long id;
#Getter #Setter
#Column(nullable = false, unique = true)
private String name;
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
SideDish other = (SideDish) obj;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
For some reason, if I removethe sideDish attribute from equals() and hashCode() in the OrderItem class, it works perfectly.
But I also need sideDish to be checked as part of the object identity.
Here is how I use it:
HashMap<OrderItem, Integer> orderItemsToSend = new HashMap<OrderItem, Integer>();
for (Order order : orders) {
for (OrderItem orderItem : order.getOrderItems()) {
int numSimilarOrders = getNumOfSimilarOrders(orderItem, orders);
if(!orderItemsToSend.containsKey(orderItem)) {
orderItemsToSend.put(orderItem, numSimilarOrders);
}else {
System.out.println("Vec je dodat item koji isti kao: " + orderItem.getFood().getName());
}
}
}

In your OrderItem class, both your hashCode() and equals() depend on the property List<SideDish> sideDishes.
Thus, if sideDishes changes, so does the hashCode() (and so does equality).
A HashMap uses both hashCode() and equals() to store and find the object which is the key. It uses a concept called "hash buckets". If you put a key into a HashMap, and then the hashCode() changes, that object will be in the wrong hash bucket, and you won't be able to find it again.
A key is something which is used for lookup purposes - that's what the word "key" means. An important quality of a key, whether in a database, or a hashmap, is immutability. So in Java, that means an object which changes its hashCode() makes for a bad key.
It's a bit like if a file system did lookups by the hash of the filename, but then you changed the filename, but it didn't update the hash. You'd only find the file by doing a lookup with the old name.
This simple test program will illustrate the point.
We store 2 objects in a HashMap, and then change the hashCode(). The map still contains both objects, but now one of them cannot be found or used for lookup.
The solution is use some simple immutable object as the key, such as a Long of its database ID.
Sample output is below the code.
public class HashTest {
static class Hashable {
String name;
#Override
public int hashCode() {
return ((name == null) ? 0 : name.hashCode());
}
#Override
public boolean equals(Object object) {
return (object instanceof Hashable) && equals((Hashable) object);
}
private boolean equals(Hashable that) {
return Objects.equals(this.name, that.name);
}
#Override
public String toString() {
// Use identityHashCode() so we can really see which object is which
return "[" + name + ":" + System.identityHashCode(this) + "]";
}
}
public static void main(String[] args) {
Hashable one = new Hashable();
one.name = "one";
Hashable two = new Hashable();
two.name = "one";
print(one, two);
two.name = "two";
print(one, two);
HashMap<Hashable, Integer> map = new HashMap<>();
map.put(one, 1);
map.put(two, 2);
find(map, one, two);
one.name = "two"; // Let's confuse things
print(one, two);
find(map, one, two);
}
private static void print(Hashable one, Hashable two) {
System.out.print("Names:" + one.name + ":" + two.name);
System.out.print("\tHashcodes:" + one.hashCode() + ":" + two.hashCode());
System.out.println("\tEquals:" + one.equals(two));
}
private static void find(HashMap<Hashable, Integer> map, Hashable one, Hashable two) {
System.out.print(map);
System.out.print("\tFound: " + map.get(one));
System.out.println("\tFound: " + map.get(two));
}
}
Sample output:
Names:one:one Hashcodes:110182:110182 Equals:true
Names:one:two Hashcodes:110182:115276 Equals:false
{[one:366712642]=1, [two:1829164700]=2} Found: 1 Found: 2
Names:two:two Hashcodes:115276:115276 Equals:true
{[two:366712642]=1, [two:1829164700]=2} Found: 2 Found: 2

How to group values in list based on id in java?

Hello folks this may be dumb question but as a beginner am struggling with this how to group values based on id in list, Now let me clarify you briefly am having set of objects like this :
ID:1,UserID:330
ID:2,UserID:303
ID:3,UserID:090
ID:1,UserID:302
ID:2,UserID:306
How my list should look like is(Json Format):
[{"ID":1,"UserID":[330,302]},{"ID":2,"UserID":[303,306]},{"ID":3,"UserID":[090]}]
Now let me post what i have tried so far:
final List<Integer>list=new ArrayList<>();
final List<SpareReturnModel>lisobj=new ArrayList<>();
int duplicate=0;
for(int i=0;i<tView.getSelected().size();i++){
Object o= tView.getSelected().get(i).getValue();
SpareReturnModel asset=(SpareReturnModel) o;
int flag=asset.getFlag();
if(flag==2) {
int warehouseid = asset.getWareHouseID();
asset.setWareHouseID(warehouseid);
int partid = asset.getSerialNoID();
list.add(partid);
}
else {
Log.d("s","no value for header");
}
if(duplicate!=asset.getWareHouseID()){
asset.setParlist(list);
asset.setWareHouseID(asset.getWareHouseID());
lisobj.add(asset);
list.clear();
}
duplicate=asset.getWareHouseID();
}
Gson gson=new Gson();
//this will convert list to json
String value=gson.toJson(listobj);
SpareReturn Model Class:
public class SpareReturnModel {
private Integer SerialNoID;
private String SerialNumber;
private List<Integer>parlist;
public List<Integer> getParlist() {
return parlist;
}
public void setParlist(List<Integer> parlist) {
this.parlist = parlist;
}
public Integer getFlag() {
return flag;
}
public void setFlag(Integer flag) {
this.flag = flag;
}
private Integer flag;
public Integer getWareHouseID() {
return WareHouseID;
}
public void setWareHouseID(Integer wareHouseID) {
WareHouseID = wareHouseID;
}
private Integer WareHouseID;
public Integer getSerialNoID() {
return SerialNoID;
}
public void setSerialNoID(Integer serialNoID) {
SerialNoID = serialNoID;
}
public String getSerialNumber() {
return SerialNumber;
}
public void setSerialNumber(String serialNumber) {
SerialNumber = serialNumber;
}
}
Can someone let me know how to achieve this am struggling with this.

I simplify your class to make solution clearer:
public class SpareReturnModel implements Comparable<SpareReturnModel> {
private Integer id;
private String userId;
public SpareReturnModel(Integer id, String userId) {
this.id = id;
this.userId = userId;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUserId() {
return userId;
}
public void setUserId(String userId) {
this.userId = userId;
}
#Override
public int compareTo(SpareReturnModel other) {
return this.getId().compareTo(other.getId());
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
SpareReturnModel model = (SpareReturnModel) o;
if (id != null ? !id.equals(model.id) : model.id != null) return false;
return userId != null ? userId.equals(model.userId) : model.userId == null;
}
#Override
public int hashCode() {
int result = id != null ? id.hashCode() : 0;
result = 31 * result + (userId != null ? userId.hashCode() : 0);
return result;
}
}
and add JsonSpareReturnModel
public class JsonSpareRuturnModel implements Comparable<JsonSpareRuturnModel> {
private final List<SpareReturnModel> modelList;
private final Integer id;
public JsonSpareRuturnModel(List<SpareReturnModel> modelList) {
this.modelList = modelList;
this.id = modelList.get(0).getId();
}
private final String toJson() {
return String.format("{\"ID\":%s,\"UserID\":%s}", id, formatUserIdList());
}
private String formatUserIdList() {
StringBuilder builder = new StringBuilder("[");
Iterator<SpareReturnModel> modelIterator = modelList.iterator();
while (modelIterator.hasNext()) {
builder.append(modelIterator.next().getUserId());
if (modelIterator.hasNext()) {
builder.append(",");
}
}
builder.append("]");
return builder.toString();
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
JsonSpareRuturnModel that = (JsonSpareRuturnModel) o;
return id != null ? id.equals(that.id) : that.id == null;
}
#Override
public int hashCode() {
return id != null ? id.hashCode() : 0;
}
#Override
public int compareTo(JsonSpareRuturnModel other) {
return this.id.compareTo(other.id);
}
#Override
public String toString() {
return toJson();
}
if you need to group by user id you need to sort your models according to id's
and place them to json format model:
public class Main {
public static void main(String[] args) {
List<SpareReturnModel> models = new ArrayList<>(Arrays.asList(
new SpareReturnModel(1, "330"),
new SpareReturnModel(2, "303"),
new SpareReturnModel(3, "090"),
new SpareReturnModel(1, "302"),
new SpareReturnModel(2, "306")
));
Map<Integer, List<SpareReturnModel>> groupById = new HashMap<>();
for (SpareReturnModel model : models) {
List<SpareReturnModel> listById = groupById.get(model.getId());
if (listById == null) {
groupById.put(model.getId(), new ArrayList<>(Arrays.asList(model)));
} else {
listById.add(model);
}
}
List<JsonSpareRuturnModel> jsonList = new ArrayList<>();
for (Map.Entry<Integer, List<SpareReturnModel>> pair : groupById.entrySet()) {
jsonList.add(new JsonSpareRuturnModel(pair.getValue()));
}
System.out.println(jsonList);
final String expected = "[{\"ID\":1,\"UserID\":[330,302]}, {\"ID\":2,\"UserID\":[303,306]}, {\"ID\":3,\"UserID\":[090]}]";
System.out.println(jsonList.toString().equals(expected));
}
}

Find if an object is already present in a HashMap

I am trying to create a HashMap, that adds objects to a line, if they are not already present in this line. This is how I check it:
if (!waiting.containsKey(p)) {
waiting.put(current, p);
current++;
}
Where p is our object, which is stored with an Integer. However, when I run this code. It will store the same object several times under different integers, how can this be prevented?

thats because you call containsKey with the object and not the key:
parameter must be an Integer key
Integer lKey = 0;
if(!waiting.containsKey(lKey)){
waiting.put(current, p);
current++;
}
if your object has an identifier use this identifier for the map.
if(!waiting.containsKey(p.getId())){
waiting.put(p.getId(), p);
current++;
}
otherwise use containsValue():
if(!waiting.containsValue(p)){
waiting.put(current, p);
current++;
}
but then you have to overwrite the equals method.

If you want to use an object as a key, you can override the equals() and hashCode() methods to return and compare the id of the object.
Driver.java
import java.util.HashMap;
import java.util.Map;
public class Driver {
public static void main(String[] args) {
Map<MyObject, Integer> map = new HashMap<MyObject, Integer>();
map.put(new MyObject(1000L, "One"), 1);
map.put(new MyObject(1001L, "Two"), 2);
map.put(new MyObject(1002L, "Three"), 3);
Long id = 1001L;
System.out.println(contains(map, id)); // true
System.out.println(get(map, id)); // 2
}
public static <T, U> boolean contains(Map<T, U> map, T obj) {
return map.containsKey(obj);
}
public static boolean contains(Map<MyObject, Integer> map, Long id) {
return contains(map, new MyObject(id, ""));
}
public static <T, U> U get(Map<T, U> map, T obj) {
return map.get(obj);
}
public static Integer get(Map<MyObject, Integer> map, Long id) {
return get(map, new MyObject(id, ""));
}
}
MyObject.java
public class MyObject {
private Long id;
private String name;
protected Long getId() {
return id;
}
protected void setId(Long id) {
this.id = id;
}
protected String getName() {
return name;
}
protected void setName(String name) {
this.name = name;
}
public MyObject(Long id, String name) {
this.id = id;
this.name = name;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null) return false;
if (getClass() != obj.getClass()) return false;
MyObject other = (MyObject) obj;
if (id == null) {
if (other.id != null) return false;
} else if (!id.equals(other.id)) return false;
return true;
}
#Override
public String toString() {
return "MyObject { id : " + id + ", name : " + name + "}";
}
}

Can't Marshal java.lang.String

Here is my dilemma:
I have a dto class for marshaling back and forth from/to XML.
Here is the trick: Because of the number of dto classes our project deals with that are collections with a plural outter tag, I decided to create a delegate collection that allows me to take one of these classes and effortlessly turn them into a Collection and get the convenience that comes with it (iteration, add, etc.).
In our project we have marshaling tests to flush out annotation errors and such.
Below is my trouble code.
Problem:
Depending on the marshaler, if I extend this QuickCollection I get the below error.
When the object is unmarshaled to xml using CXF as a response to a webservice request, it fails. Exact error:
com.sun.istack.SAXException2: unable to marshal type "java.lang.String" as an element because it is missing an #XmlRootElement annotation
When it's marshaled/unmarshaled with JAXB in test it's fine.
When This same QuickCollection is used to marshal in results from 3rd parties using spring RestOperations and works fine
the mind screw:
When I remove the inheritance and manage the collection as a private member it all just works!
This makes not a stitch of sense to me as I am literally returning the exact data type in both situations.
Below is all relevant code.
This is the Inherited delegate class.
public class QuickCollection<T> implements Collection<T> {
// to be set if needed after instantiation. To behave like a normal collection, we set it to something safe
protected Collection<T> delegate = Collections.emptySet();
public QuickCollection() {
}
public QuickCollection(Collection<T> delegate) {
this.delegate = delegate;
}
#Override
public int size() {
return delegate.size();
}
#Override
public boolean isEmpty() {
return delegate.isEmpty();
}
#Override
public boolean contains(Object o) {
return delegate.contains(o);
}
#Override
public Iterator<T> iterator() {
return delegate.iterator();
}
#Override
public Object[] toArray() {
return delegate.toArray();
}
#Override
public <T> T[] toArray(T[] a) {
return delegate.toArray(a);
}
#Override
public boolean add(T t) {
return delegate.add(t);
}
#Override
public boolean remove(Object o) {
return delegate.remove(o);
}
#Override
public boolean containsAll(Collection<?> c) {
return delegate.containsAll(c);
}
#Override
public boolean addAll(Collection<? extends T> c) {
return delegate.addAll(c);
}
#Override
public boolean removeAll(Collection<?> c) {
return delegate.removeAll(c);
}
#Override
public boolean retainAll(Collection<?> c) {
return delegate.retainAll(c);
}
#Override
public void clear() {
delegate.clear();
}
#Override
public String toString() {
return "" + delegate.toString();
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
QuickCollection that = (QuickCollection) o;
if (delegate != null ? !delegate.equals(that.delegate) : that.delegate != null) return false;
return true;
}
#Override
public int hashCode() {
return delegate != null ? delegate.hashCode() : 0;
}
}
Here is the child DTO class
#XmlAccessorType(XmlAccessType.PROPERTY)
#XmlType(name = "BuddyCodes")
#XmlRootElement(name = "BuddyCodes")
public class BuddyCodes extends QuickCollection<String> implements Xml {
private Long accountId;
private Date expirationDate;
public BuddyCodes() {
super.delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Set<String> codes, Date expirationDate) {
super(codes);
this.accountId = accountId;
this.expirationDate = expirationDate;
super.delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Date expirationDate) {
this.accountId = accountId;
this.expirationDate = expirationDate;
super.delegate = new HashSet<String>();
}
#Override
public String toXml() {
String retVal;
try {
retVal = StringUtils.toXml(this);
}
catch (JAXBException e) {
retVal = e.toString();
}
return retVal;
}
public Long getAccountId() {
return accountId;
}
public void setAccountId(Long accountId) {
this.accountId = accountId;
}
public Set<String> getCodes() {
return (Set<String>) super.delegate;
}
#XmlElement(name = "code")
public void setCodes(Set<String> codes) {
super.delegate = codes;
}
public Date getExpirationDate() {
return expirationDate;
}
public void setExpirationDate(Date expirationDate) {
this.expirationDate = expirationDate;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
BuddyCodes that = (BuddyCodes) o;
if (accountId != null ? !accountId.equals(that.accountId) : that.accountId != null) return false;
if (delegate != null ? !super.delegate.equals(that.delegate) : that.delegate != null) return false;
if (expirationDate != null ? !expirationDate.equals(that.expirationDate) : that.expirationDate != null)
return false;
return true;
}
#Override
public int hashCode() {
int result = accountId != null ? accountId.hashCode() : 0;
result = 31 * result + (expirationDate != null ? expirationDate.hashCode() : 0);
result = 31 * result + (super.delegate != null ? super.delegate.hashCode() : 0);
return result;
}
#Override
public String toString() {
return "BuddyCodes{" +
"accountId=" + accountId +
"codes=" + super.delegate +
", expirationDate=" + expirationDate +
'}';
}
}
And it doesn't work. I get the error.
Now, here is the child class after removing the inheritance and it works!!!
import javax.xml.bind.JAXBException;
import javax.xml.bind.annotation.*;
import java.util.Collection;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
/**
* #author christian.bongiorno
* Date: 10/3/11
* Time: 6:11 PM
*/
#XmlAccessorType(XmlAccessType.PROPERTY)
#XmlType(name = "BuddyCodes")
#XmlRootElement(name = "BuddyCodes")
public class BuddyCodes implements Xml {
private Long accountId;
private Date expirationDate;
private Set<String> delegate;
public BuddyCodes() {
delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Set<String> codes, Date expirationDate) {
this.accountId = accountId;
this.expirationDate = expirationDate;
delegate = new HashSet<String>();
}
public BuddyCodes(Long accountId, Date expirationDate) {
this.accountId = accountId;
this.expirationDate = expirationDate;
delegate = new HashSet<String>();
}
#Override
public String toXml() {
String retVal;
try {
retVal = StringUtils.toXml(this);
}
catch (JAXBException e) {
retVal = e.toString();
}
return retVal;
}
public Long getAccountId() {
return accountId;
}
public void setAccountId(Long accountId) {
this.accountId = accountId;
}
public Set<String> getCodes() {
return delegate;
}
#XmlElement(name = "code")
public void setCodes(Set<String> codes) {
delegate = codes;
}
public Date getExpirationDate() {
return expirationDate;
}
public void setExpirationDate(Date expirationDate) {
this.expirationDate = expirationDate;
}
public boolean add(String s) {
return delegate.add(s);
}
public int size() {
return delegate.size();
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
BuddyCodes that = (BuddyCodes) o;
if (accountId != null ? !accountId.equals(that.accountId) : that.accountId != null) return false;
if (delegate != null ? !delegate.equals(that.delegate) : that.delegate != null) return false;
if (expirationDate != null ? !expirationDate.equals(that.expirationDate) : that.expirationDate != null)
return false;
return true;
}
#Override
public int hashCode() {
int result = accountId != null ? accountId.hashCode() : 0;
result = 31 * result + (expirationDate != null ? expirationDate.hashCode() : 0);
result = 31 * result + (delegate != null ? delegate.hashCode() : 0);
return result;
}
}
Why does the inheritance matter at all???
I haven't figured this out but, I have another DTO in a similar layout (BuddyTypes BuddyType). BuddyType has 2 members: Long and String. Both are annoted as XmlElement. This one works just fine.
It seems the problem that the members of the set making up the delegate are not annotated in my problem case and I don't know how to annotate a parent member. As an inherited class, it wouldn't make sense to have some sort of default name/annotation. But, I tried this madness and the annotation is ignored -- I have seen parent member annotations ignored before so this isn't new.
I don't know if it's possible, but I need to annotate a parent member.

A bit out of the box: try Simple XML library instead of JAXB. My experience with it is the best.

DropDownChoice set a particular index selected

I have a DropDownChoice :
DropDownChoice timePeriod = new DropDownChoice("timePeriod", Arrays.asList(new TimePeriod(1, "Weekly"), new TimePeriod(2, "Bi-Weekly"), new TimePeriod(3, "Semi-Monthly"), new TimePeriod(4, "Monthly"), new TimePeriod(5, "Yearly")), new IChoiceRenderer() {
private static final long serialVersionUID = 10102L;
#Override
public String getIdValue(Object object, int index) {
return ((TimePeriod) object).getId() + "";
}
#Override
public Object getDisplayValue(Object object) {
return ((TimePeriod) object).getPeriodType();
}
});
timePeriod.setNullValid(false);
My question is:
How to set the selected index to 2 or any other?
How to remove first default blank option?
Thank you.

You should be able to set the selected index by using a PropertyModel instead of hard-coding the values of the dropdown. I can't test at the moment, but it would be something like
String timePeriodValue = "Bi-Weekly";
DropDownChoice timePeriod = new DropDownChoice("timePeriod",
new PropertyModel(this, "timePeriodValue"),
Arrays.asList(/* your options */),
new IChoiceRenderer() {
// ...
// Again, this is pseudocode only
As a bonus, the very act of setting a default value should eliminate the blank option problem.

DropDownChoice has a constructor which accepts the default value.
Or in your code you can add timePeriod.setModel(Model.of(new TimePeriod(2, "Bi-Weekly")))
I guess TimePeriod has proper #equals() and #hashCode() implementations.
About the blank option: see org.apache.wicket.markup.html.form.AbstractSingleSelectChoice.isNullValid()

Lord Torgamus and martin-g thank you both of you. I did a small test. And it is working perfectly. As Lord Torgamus indicated,
#SuppressWarnings({ "unchecked", "rawtypes", "serial" })
public class MyPage extends WebPage {
public MyPage() {
add(new MyForm("form"));
}
private class MyForm extends Form {
public MyForm(String id) {
super(id);
setOutputMarkupPlaceholderTag(true);
setModel(new Model(new FormModel()));
final DropDownChoice timePeriod = new DropDownChoice("timePeriod", new PropertyModel(getModel(), "timePeriod"), Arrays.asList(new TimePeriod(1, "Weekly"), new TimePeriod(2, "Bi-Weekly"), new TimePeriod(3, "Semi-Monthly"), new TimePeriod(4, "Monthly"), new TimePeriod(5, "Yearly")), new IChoiceRenderer() {
private static final long serialVersionUID = 10102L;
#Override
public String getIdValue(Object object, int index) {
return ((TimePeriod) object).getId() + "";
}
#Override
public Object getDisplayValue(Object object) {
return ((TimePeriod) object).getPeriodType();
}
});
timePeriod.setNullValid(false);
add(timePeriod);
timePeriod.setOutputMarkupPlaceholderTag(true);
timePeriod.add(new AjaxFormComponentUpdatingBehavior("onChange") {
#Override
protected void onUpdate(AjaxRequestTarget target) {
MyForm form = (MyForm) timePeriod.getParent();
FormModel formModel = (FormModel) form.getModelObject();
formModel.setTimePeriod(new TimePeriod(4, "Monthly"));
form.setModel(new Model(formModel));
target.addComponent(form);
}
});
}
#Override
public void onSubmit() {
System.out.println(getModelObject());
}
}
private class FormModel implements Serializable {
private TimePeriod timePeriod = new TimePeriod(2, "Bi-Weekly");
public FormModel() {
}
public TimePeriod getTimePeriod() {
return timePeriod;
}
public void setTimePeriod(TimePeriod timePeriod) {
this.timePeriod = timePeriod;
}
#Override
public String toString() {
return "FormModel [timePeriod=" + timePeriod + "]";
}
}
private class TimePeriod implements Serializable {
private int id;
private String periodType;
public TimePeriod(int id, String periodType) {
super();
this.id = id;
this.periodType = periodType;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getPeriodType() {
return periodType;
}
public void setPeriodType(String periodType) {
this.periodType = periodType;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + getOuterType().hashCode();
result = prime * result + id;
result = prime * result + ((periodType == null) ? 0 : periodType.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
TimePeriod other = (TimePeriod) obj;
if (!getOuterType().equals(other.getOuterType()))
return false;
if (id != other.id)
return false;
if (periodType == null) {
if (other.periodType != null)
return false;
} else if (!periodType.equals(other.periodType))
return false;
return true;
}
private LoginPage getOuterType() {
return LoginPage.this;
}
#Override
public String toString() {
return "TimePeriod [id=" + id + ", periodType=" + periodType + "]";
}
}
}
The above code is provided for other users as it might be helpful and I wrote it for testing purpose so all the classes are written in one .java file although it is not advisable.
Thank you.