Develop Reference

Java Reflection traverse into nested object and list and update the fields

I have the following complex POJO class Invoice
public class Measure {
private String id;
private Float value;
public String getId() { return id; }
public void setId(String id) { this.id = id; }
public Float getValue() { return value; }
public void setValue(Float value) { this.value = value; }
}
public class LineItem {
private Integer lineNumber;
private Measure shipped;
private List<LineItem> lineItems;
public Integer getLineNumber() { return lineNumber; }
public void setLineNumber(Integer lineNumber) { this.lineNumber = lineNumber; }
public Measure getShipped() { return shipped; }
public void setShipped(Measure shipped) { this.shipped = shipped; }
public List<LineItem> getLineItems() { return lineItems; }
public void setLineItems(List<LineItem> lineItems) { this.lineItems = lineItems; }
}
public class Invoice {
private String originUid;
private String vehicleUid;
private List<LineItem> lineItems;
public String getOriginUid() { return originUid; }
public void setOriginUid(String originUid) { this.originUid = originUid; }
public String getVehicleUid() { return vehicleUid; }
public void setVehicleUid(String vehicleUid) { this.vehicleUid = vehicleUid; }
public List<LineItem> getLineItems() { return lineItems; }
public void setLineItems(List<LineItem> lineItems) { this.lineItems = lineItems; }
}
Now I want to traverse deep into every single field including nested objects in the Invoice object and update them using Reflection.
I can call updateIncomingObject() recursively. But I need to know how to get the nested objects out of the field as shown in the commented section.
public Object updateIncomingObject(Object incomingObject) {
Field[] incoming = incomingObject.getClass().getDeclaredFields();
for (Field incomingField : incoming) {
incomingField.setAccessible(true);
if (incomingField.getType().isArray()) {
// for (each occurrence in thisArray ???) {
// Object result = updateIncomingObject(occurrence);
// thisArray.set(index,result);
// }
// incomingField.set(incomingObject, thisArray);
}
else if (!incomingField.getType().getName().startsWith("java.lang")) {
// Object objInstance = incomingField.???;
// Object result = updateIncomingObject(objInstance);
// incomingField.set(incomingObject, result);
}
else {
if (incomingField.getType().equals(String.class) && incomingField.get(incomingObject) != null) {
String trimmed = incomingField.get(incomingObject).toString().trim();
incomingField.set(incomingObject, trimmed);
}
}
}
return incomingObject;
}
How do I turn field into object instance?

To access the objects of an array (fields defined with []) you can do the following:
if (incomingField.getType().isArray()) {
Object[] thisArray = (Object[]) incomingField.get(incomingObject);
for (int k = 0; k < thisArray.length; k++) {
Object occurrence = thisArray[k];
Object result = updateIncomingObject(occurrence);
thisArray[k] = result;
}
incomingField.set(incomingObject, thisArray);
}
To access the objects of a List you can do the following:
if (List.class.isAssignableFrom(incomingField.getType())) {
List<?> thisList = (List<?>) incomingField.get(incomingObject);
for (int k = 0; k < thisList.size(); k++) {
Object occurrence = thisList.get(k);
Object result = updateIncomingObject(occurrence);
thisList.set(k, occurrence);
}
}
Note: If you call updateIncomingObject when iterating on arrays/lists and the object in the array/list is an array (Object[]) your method won't do anything, because the number of declared fields for an array (Object[]) class is 0.
I hope this helps you.

add description to each enum value for swagger-UI documentation

How can I add some description to each enum value for swagger-UI documentation?
My EnumClass:
#ApiModel
public enum EnumCarrierSelectionSubstitutionInformation {
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_1(1), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_2(2), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_3(3), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_4(4), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_5(5), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_6(6), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_7(7);
private int numVal;
EnumCarrierSelectionSubstitutionInformation(int numVal) {
this.numVal = numVal;
}
public int getNumVal() {
return numVal;
}
}
The model
private EnumCarrierSelectionSubstitutionInformation carrierSelectionSubstitutionInformation;
// getter setter......
I would like to add some description to CARRIER_SELECTION_SUBSTITUTION_INFORMATION_1.
I tried
#ApiModelProperty(value = "blabla2")
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_1(1), //
but that is not working.
Swagger-UI output:
carrierSelectionSubstitutionInformation (string, optional) = ['CARRIER_SELECTION_SUBSTITUTION_INFORMATION_1', 'CARRIER_SELECTION_SUBSTITUTION_INFORMATION_2', 'CARRIER_SELECTION_SUBSTITUTION_INFORMATION_3', 'CARRIER_SELECTION_SUBSTITUTION_INFORMATION_4', 'CARRIER_SELECTION_SUBSTITUTION_INFORMATION_5', 'CARRIER_SELECTION_SUBSTITUTION_INFORMATION_6', 'CARRIER_SELECTION_SUBSTITUTION_INFORMATION_7']
string
Enum: "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_1", "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_2", "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_3", "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_4", "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_5", "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_6", "CARRIER_SELECTION_SUBSTITUTION_INFORMATION_7"
How can I do that?

public class Enum {
public enum EnumCarrierSelectionSubstitutionInformation {
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_1(1,"ONE"), //
CARRIER_SELECTION_SUBSTITUTION_INFORMATION_2(2,"TWO"),
private final int value;
private final String description;
EnumCarrierSelectionSubstitutionInformation(int value, String desc){
this.value = value;
this.description = desc;
}
public int value(){
return this.value;
}
public String description(){
return this.description;
}
}
public static void main(String[] args){
for (EnumCarrierSelectionSubstitutionInformation elem: EnumCarrierSelectionSubstitutionInformation.values()){
System.out.println(elem + "value is "+ new Integer(elem.value()) + " desc is " + elem.description());
}
}
}

Jackson: Parsing an object with an array of objects

I'm working on a project where I'm integrating with a 3rd party service that returns a JSON response. I am using Jackson to deserialize the response into a Java POJO. The response is an object that has a few simple fields that Jackson is able to easily parse. However the response also contains an array with a single entry that is also an object. When Jackson attempts to deserialize it I get the top level object containing a list with the single entry in the list, however all of the fields for the single entry in the list are null. Any idea what I'm doing wrong here?
UPDATE: after setting FAIL_ON_UNKNOWN_PROPERTIES to true, I am getting the following stack trace. Still not sure why it is having issues with the wrapped "RecurringDetail" object in the array.
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "RecurringDetail" (class com.adyen.www.models.RecurringDetail), not marked as ignorable (19 known properties: "variant", "contractTypes", "tokenDetails", "aliasType", "name", "creationDate", "firstPspReference", "elv", "card", "additionalData", "shopperName", "socialSecurityNumber", "billingAddress", "bank", "recurringDetailReference", "paymentMethodVariant", "alias", "acquirer", "acquirerAccount"])
at [Source: response.json; line: 5, column: 33] (through reference chain: com.adyen.www.models.RecurringDetailsResult["details"]->java.util.ArrayList[0]->com.adyen.www.models.RecurringDetail["RecurringDetail"])
at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:62)
at com.fasterxml.jackson.databind.DeserializationContext.handleUnknownProperty(DeserializationContext.java:834)
at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:1094)
at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1470)
at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1448)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:282)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:140)
at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:287)
at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:259)
at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:26)
at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:499)
at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:101)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:276)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:140)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3798)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2759)
JSON
{
"creationDate": "2017-01-26T23:11:20+01:00",
"details": [
{
"RecurringDetail": {
"acquirer": "TestPmmAcquirer",
"acquirerAccount": "TestPmmAcquirerAccount",
"additionalData": {
"cardBin": "440000"
},
"alias": "B133243153928547",
"aliasType": "Default",
"card": {
"expiryMonth": "8",
"expiryYear": "2018",
"holderName": "Steve HAll",
"number": "0008"
},
"contractTypes": [
"RECURRING"
],
"creationDate": "2017-01-26T23:11:20+01:00",
"firstPspReference": "8524854686798738",
"paymentMethodVariant": "visadebit",
"recurringDetailReference": "8414854686802111",
"variant": "visa"
}
}
],
"invalidOneclickContracts": "false",
"lastKnownShopperEmail": "someones#email.com",
"shopperReference": "xggZcGauSSG5jP+akIlijQ=="
}
Unit Test
public class RecurringDetailResultTest {
public static ObjectMapper mapper = new ObjectMapper()
{
private static final long serialVersionUID = -174113593500315394L;
{
configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
configure(DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS, true);
setSerializationInclusion(JsonInclude.Include.NON_NULL);
}
};
#Test
public void testParseRecurringDetailResulte() throws IOException {
RecurringDetailsResult result = mapper.readValue(new File("response.json"), new TypeReference<RecurringDetailsResult>(){});
if (result.getDetails() != null && !result.getDetails().isEmpty()) {
RecurringDetail detail = result.getDetails().get(0);
if (StringUtils.isEmpty(detail.getRecurringDetailReference())) {
fail("Recurring detail does not contain any information.");
}
} else {
fail("No result details returned.");
}
}
}
Model (Root object)
public class RecurringDetailsResult
implements java.io.Serializable {
private static final long serialVersionUID = 5297684963950973136L;
private Date creationDate;
private String shopperReference;
private List<RecurringDetail> details;
private String lastKnownShopperEmail;
#JsonGetter("creationDate")
public Date getCreationDate ( ) {
return this.creationDate;
}
#JsonSetter("creationDate")
public void setCreationDate (Date value) {
this.creationDate = value;
}
#JsonGetter("shopperReference")
public String getShopperReference ( ) {
return this.shopperReference;
}
#JsonSetter("shopperReference")
public void setShopperReference (String value) {
this.shopperReference = value;
}
#JsonGetter("details")
public List<RecurringDetail> getDetails ( ) {
return this.details;
}
#JsonSetter("details")
public void setDetails (List<RecurringDetail> value) {
this.details = value;
}
#JsonGetter("lastKnownShopperEmail")
public String getLastKnownShopperEmail ( ) {
return this.lastKnownShopperEmail;
}
#JsonSetter("lastKnownShopperEmail")
public void setLastKnownShopperEmail (String value) {
this.lastKnownShopperEmail = value;
}
}
Model (the object in the array, all of the fields of this guy are null when Jackson deserializes the JSON)
public class RecurringDetail
implements java.io.Serializable {
private static final long serialVersionUID = 5302883242997268343L;
private String name;
private Date creationDate;
private Card card;
private ELV elv;
private Address billingAddress;
private String additionalData;
private Name shopperName;
private String socialSecurityNumber;
private String recurringDetailReference;
private BankAccount bank;
private String alias;
private String aliasType;
private TokenDetails tokenDetails;
private String variant;
private String paymentMethodVariant;
private String firstPspReference;
private List<String> contractTypes;
private String acquirer;
private String acquirerAccount;
#JsonGetter("name")
public String getName ( ) {
return this.name;
}
#JsonSetter("name")
public void setName (String value) {
this.name = value;
}
#JsonGetter("creationDate")
public Date getCreationDate ( ) {
return this.creationDate;
}
#JsonSetter("creationDate")
public void setCreationDate (Date value) {
this.creationDate = value;
}
#JsonGetter("card")
public Card getCard ( ) {
return this.card;
}
#JsonSetter("card")
public void setCard (Card value) {
this.card = value;
}
#JsonGetter("elv")
public ELV getElv ( ) {
return this.elv;
}
#JsonSetter("elv")
public void setElv (ELV value) {
this.elv = value;
}
#JsonGetter("billingAddress")
public Address getBillingAddress ( ) {
return this.billingAddress;
}
#JsonSetter("billingAddress")
public void setBillingAddress (Address value) {
this.billingAddress = value;
}
#JsonGetter("additionalData")
public String getAdditionalData ( ) {
return this.additionalData;
}
#JsonSetter("additionalData")
public void setAdditionalData (String value) {
this.additionalData = value;
}
#JsonGetter("shopperName")
public Name getShopperName ( ) {
return this.shopperName;
}
#JsonSetter("shopperName")
public void setShopperName (Name value) {
this.shopperName = value;
}
#JsonGetter("socialSecurityNumber")
public String getSocialSecurityNumber ( ) {
return this.socialSecurityNumber;
}
#JsonSetter("socialSecurityNumber")
public void setSocialSecurityNumber (String value) {
this.socialSecurityNumber = value;
}
#JsonGetter("recurringDetailReference")
public String getRecurringDetailReference ( ) {
return this.recurringDetailReference;
}
#JsonSetter("recurringDetailReference")
public void setRecurringDetailReference (String value) {
this.recurringDetailReference = value;
}
#JsonGetter("bank")
public BankAccount getBank ( ) {
return this.bank;
}
#JsonSetter("bank")
public void setBank (BankAccount value) {
this.bank = value;
}
#JsonGetter("alias")
public String getAlias ( ) {
return this.alias;
}
#JsonSetter("alias")
public void setAlias (String value) {
this.alias = value;
}
#JsonGetter("aliasType")
public String getAliasType ( ) {
return this.aliasType;
}
#JsonSetter("aliasType")
public void setAliasType (String value) {
this.aliasType = value;
}
#JsonGetter("tokenDetails")
public TokenDetails getTokenDetails ( ) {
return this.tokenDetails;
}
#JsonSetter("tokenDetails")
public void setTokenDetails (TokenDetails value) {
this.tokenDetails = value;
}
#JsonGetter("variant")
public String getVariant ( ) {
return this.variant;
}
#JsonSetter("variant")
public void setVariant (String value) {
this.variant = value;
}
#JsonGetter("paymentMethodVariant")
public String getPaymentMethodVariant ( ) {
return this.paymentMethodVariant;
}
#JsonSetter("paymentMethodVariant")
public void setPaymentMethodVariant (String value) {
this.paymentMethodVariant = value;
}
#JsonGetter("firstPspReference")
public String getFirstPspReference ( ) {
return this.firstPspReference;
}
#JsonSetter("firstPspReference")
public void setFirstPspReference (String value) {
this.firstPspReference = value;
}
#JsonGetter("contractTypes")
public List<String> getContractTypes ( ) {
return this.contractTypes;
}
#JsonSetter("contractTypes")
public void setContractTypes (List<String> value) {
this.contractTypes = value;
}
#JsonGetter("acquirer")
public String getAcquirer ( ) {
return this.acquirer;
}
#JsonSetter("acquirer")
public void setAcquirer (String value) {
this.acquirer = value;
}
#JsonGetter("acquirerAccount")
public String getAcquirerAccount ( ) {
return this.acquirerAccount;
}
#JsonSetter("acquirerAccount")
public void setAcquirerAccount (String value) {
this.acquirerAccount = value;
}
}

The easiest thing I can think to do is to make your java objects look like your Json. Because this Json has a wrapped inner object but not the outer object you would have to have a similar wrapping in java. It isn't elegant but works.
public class RecurringDetailsResult implements java.io.Serializable {
private Date creationDate;
private String shopperReference;
private List<RecurringDetailWrapper> details;
private String lastKnownShopperEmail;
// getters and setters here. No need for any #JsonGetter or #JsonSetter annotations
}
#JsonRootName("RecurringDetail")
public class RecurringDetailWrapper {
#JsonProperty("RecurringDetail")
RecurringDetail recurringDetail;
public RecurringDetail getRecurringDetail() {
return recurringDetail;
}
public void setRecurringDetail(RecurringDetail recurringDetail) {
this.recurringDetail = recurringDetail;
}
}
public class RecurringDetail implements java.io.Serializable {
private static final long serialVersionUID = 5302883242997268343L;
private String name;
private Date creationDate;
private Card card;
private AdditionalData additionalData;
private String socialSecurityNumber;
private String recurringDetailReference;
private String alias;
private String aliasType;
private String variant;
private String paymentMethodVariant;
private String firstPspReference;
private List<String> contractTypes;
private String acquirer;
private String acquirerAccount;
public class AdditionalData {
String cardBin;
public String getCardBin() {
return cardBin;
}
public void setCardBin(String cardBin) {
this.cardBin = cardBin;
}
}
// getters and setters here. No need for any #JsonGetter or #JsonSetter annotations
}
Then in your unit test:
#Test
public void testParseRecurringDetailResulte() throws IOException {
InputStream inputStream = this.getClass().getClassLoader().getResourceAsStream("ID41901838.json");
ObjectReader objectReader = mapper.readerFor(RecurringDetailsResult.class);
RecurringDetailsResult result = objectReader.readValue(inputStream);
if (result.getDetails() != null && !result.getDetails().isEmpty()) {
RecurringDetailWrapper detail = result.getDetails().get(0);
if (StringUtils.isEmpty(detail.getRecurringDetail().getRecurringDetailReference())) {
fail("Recurring detail does not contain any information.");
}
} else {
fail("No result details returned.");
}
}
I posted the full working code here:
https://github.com/teacurran/java-experiments/tree/master/stackoverflow-sandbox/src/main/java/com/wirelust/stackoverflowsandbox/ID41901838

It seems your current structure will work for the response JSON as below (an extra named layer is removed)
{
"creationDate": "2017-01-26T23:11:20+01:00",
"details": [
{
"acquirer": "TestPmmAcquirer",
"acquirerAccount": "TestPmmAcquirerAccount",
"additionalData": {
"cardBin": "440000"
},
"alias": "B133243153928547",
"aliasType": "Default",
"card": {
"expiryMonth": "8",
"expiryYear": "2018",
"holderName": "Steve HAll",
"number": "0008"
},
"contractTypes": [
"RECURRING"
],
"creationDate": "2017-01-26T23:11:20+01:00",
"firstPspReference": "8524854686798738",
"paymentMethodVariant": "visadebit",
"recurringDetailReference": "8414854686802111",
"variant": "visa"
}
],
"invalidOneclickContracts": "false",
"lastKnownShopperEmail": "someones#email.com",
"shopperReference": "xggZcGauSSG5jP+akIlijQ=="
}
For named object maybe you can try something like
public class RecurringDetailsResult
implements java.io.Serializable {
private static final long serialVersionUID = 5297684963950973136L;
private Date creationDate;
private String shopperReference;
private List<Map<String,RecurringDetail>> details;
private String lastKnownShopperEmail;
#JsonGetter("creationDate")
public Date getCreationDate ( ) {
return this.creationDate;
}
#JsonSetter("creationDate")
public void setCreationDate (Date value) {
this.creationDate = value;
}
#JsonGetter("shopperReference")
public String getShopperReference ( ) {
return this.shopperReference;
}
#JsonSetter("shopperReference")
public void setShopperReference (String value) {
this.shopperReference = value;
}
#JsonGetter("details")
public List<Map<String, RecurringDetail>> getDetails ( ) {
return this.details;
}
#JsonSetter("details")
public void setDetails (List<Map<String, RecurringDetail>> value) {
this.details = value;
}
#JsonGetter("lastKnownShopperEmail")
public String getLastKnownShopperEmail ( ) {
return this.lastKnownShopperEmail;
}
#JsonSetter("lastKnownShopperEmail")
public void setLastKnownShopperEmail (String value) {
this.lastKnownShopperEmail = value;
}
}

Searching enum entries in groovy or java

I have the following enum created in groovy:
enum Status {
FAILED(0, "Failed"),
SUCCESSFUL(1, "Successful")
private final int key
private final String val
public Status(int key, String val) {
this.val = val
this.key = key
}
String toString() { return val }
}
I would like to write a function to search the entries of my enum class that returns true of Status.contains("Failed"). Is it possible to do this?

In Groovy, you could do:
enum Status {
FAILED(0, 'Failed'),
SUCCESSFUL(1, 'Successful')
private final int key
private final String val
public Status(int key, String val) {
this.val = val
this.key = key
}
String toString() { val }
static boolean containsVal(String val) {
Status.values()*.val.contains val
}
}
assert Status.containsVal('Failed')

You can write necessary methods yourself
Here is an example in java:
enum Status {
FAILED(0, "Failed"),
SUCCESSFUL(1, "Successful");
private final int key;
private final String val;
Status(int key, String val) {
this.val = val;
this.key = key;
}
public static Status containsName(String name) {
for (Status status : Status.values()) {
if (status.name().equalsIgnoreCase(name)) {
return status;
}
}
return null;
}
public static Status containsVal(String val) {
for (Status status : Status.values()) {
if (status.val.equalsIgnoreCase(val)) {
return status;
}
}
return null;
}
public String toString() { return val; }
}
Method containsName returns a Status instance if its name equals to an argument. Method containsValue returns a Status instance if its value equals to an argument.
Status failed = Status.containsName("FAILED");
Status successful = Status.containsVal("Successful");

How to customize Writable class in Hadoop?

I'm trying to implement Writable class, but i have no idea on how to implement a writable class if in my class there is nested object, such as list, etc. Could any body help me? thanks
public class StorageClass implements Writable{
public String xStr;
public String yStr;
public List<Field> sStor
//omitted ctors
#override
public void write(DataOutput out) throws IOException{
out.writeChars(xStr);
out.WriteChars(yStr);
//WHAT SHOULD I DO FOR List<Field>
}
#override
public void readFields(DataInput in) throws IOException{
xStr = in.readLine();
yStr = in.readLine();
//WHAT SHOULD I DO FOR List<Field>
}
}
public class SubStorage{
public String x;
public String y;
}
}
Following is the Field class:
public final class Field implements Comparable<Field>, Serializable {
private String name;
private DataType dataType;
private Object value;
private FieldType fieldType;
public Field(){
}
public Field(String name, DataType dataType, FieldType fieldType){
this(name, dataType, null, fieldType);
}
public Field(String name, DataType type, Object value, FieldType fieldType){
this.name = name;
this.dataType = type;
this.value = value;
this.fieldType = fieldType;
}
}
public enum FieldType {
PRI, LOOKUP, SCD, VERSION, OTHER
}
public enum DataType {
UNDEFINED(4) {
public int getSizeInBytes(Object value) {
return STRING.getSizeInBytes(value);
}
},
STRING(4) {
public int getSizeInBytes(Object value) {
if (value == null) {
return 0;
}
return super.getSizeInBytes(value) + (value.toString().length() * 2); // length + chars
}
},
INT(4),
LONG(8),
DOUBLE(8),
DATETIME(8),
BOOLEAN(1),
BYTE(1),
FLOAT(4),
SHORT(2),
CHAR(2),
DATE(8),
TIME(8),
BLOB(0) {
public int getSizeInBytes(Object value) {
if (value == null) {
return 0;
}
return ((byte[])value).length;
}
};
private final int sizeInBytes;
private DataType(int sizeInBytes) {
this.sizeInBytes = sizeInBytes;
}
public int getSizeInBytes(Object value) {
return sizeInBytes;
}
}

Serializing collections is quite simple.
#Override
public void readFields(DataInput in) throws IOException {
int size = in.readInt();
list= new ArrayList<Field>(size);
for(int i = 0; i < size; i++){
Field f = new Field();
f.readFields(in);
list.add(f);
}
}
#Override
public void write(DataOutput out) throws IOException {
out.writeInt(list.size());
for (Field l : list) {
l.write(out);
}
}
Field has to implement Writable as well.

This tutorial explains better : http://www.hadoopmaterial.com/2013/10/custom-hadoop-writable-data-type.html