We Keep Coding

Comparing two instances of class having big number of attributes

I have a class having more than 30 attributes.
I want to override the equals method in order to compare two instance of my class.
However I want to avoid re-write all the 30 attributes in my method as this
#Override
public boolean equals(java.lang.Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
Address address = (Address) o;
return Objects.equals(this.attr1, address.attr1) &&
Objects.equals(this.attr2, address.attr2) &&
Objects.equals(this.attr3, address.attr3) &&
......
Objects.equals(this.attr30, address.attr30);
}
Have you a more simple and proper way ?

Well, this is basically the type of boilerplate code that is necessary. Luckily, there are lots of developers just as annoyed of writing such code as you are. For reasons like that, Project Lombok was founded.
Please see this link.
As an example, see the following two code snippets extracted from the page that I linked above:
Lombok
import lombok.EqualsAndHashCode;
#EqualsAndHashCode(exclude={"id", "shape"})
public class EqualsAndHashCodeExample {
private transient int transientVar = 10;
private String name;
private double score;
private Shape shape = new Square(5, 10);
private String[] tags;
private int id;
public String getName() {
return this.name;
}
#EqualsAndHashCode(callSuper=true)
public static class Square extends Shape {
private final int width, height;
public Square(int width, int height) {
this.width = width;
this.height = height;
}
}
}
Vanilla Java
import java.util.Arrays;
public class EqualsAndHashCodeExample {
private transient int transientVar = 10;
private String name;
private double score;
private Shape shape = new Square(5, 10);
private String[] tags;
private int id;
public String getName() {
return this.name;
}
#Override public boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof EqualsAndHashCodeExample)) return false;
EqualsAndHashCodeExample other = (EqualsAndHashCodeExample) o;
if (!other.canEqual((Object)this)) return false;
if (this.getName() == null ? other.getName() != null : !this.getName().equals(other.getName())) return false;
if (Double.compare(this.score, other.score) != 0) return false;
if (!Arrays.deepEquals(this.tags, other.tags)) return false;
return true;
}
#Override public int hashCode() {
final int PRIME = 59;
int result = 1;
final long temp1 = Double.doubleToLongBits(this.score);
result = (result*PRIME) + (this.name == null ? 43 : this.name.hashCode());
result = (result*PRIME) + (int)(temp1 ^ (temp1 >>> 32));
result = (result*PRIME) + Arrays.deepHashCode(this.tags);
return result;
}
protected boolean canEqual(Object other) {
return other instanceof EqualsAndHashCodeExample;
}
public static class Square extends Shape {
private final int width, height;
public Square(int width, int height) {
this.width = width;
this.height = height;
}
#Override public boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof Square)) return false;
Square other = (Square) o;
if (!other.canEqual((Object)this)) return false;
if (!super.equals(o)) return false;
if (this.width != other.width) return false;
if (this.height != other.height) return false;
return true;
}
#Override public int hashCode() {
final int PRIME = 59;
int result = 1;
result = (result*PRIME) + super.hashCode();
result = (result*PRIME) + this.width;
result = (result*PRIME) + this.height;
return result;
}
protected boolean canEqual(Object other) {
return other instanceof Square;
}
}
}
If you like this approach, I'd recommend checking out Project Lombok in its entirety. It really helps cleaning up your code!
!!! Beware !!!
In order to be able to actually use methods generated by Lombok, you need to install the Lombok plugin into your IDE! Otherwise, your IDE won't know of the automatically generated methods.

You can use the Field class in the java.lang.reflect package like so:
#Override
public boolean equals(Object o) {
//instanceof check, null check, etc
Field[] fields = Address.class.getDeclaredFields();
for (Field field : fields) {
try {
field.setAccessible(true);
if (!field.get(this).equals(field.get((Address) o))) {
return false;
} //end if
} catch (IllegalAccessException e) {
//handle exception
} //end try catch
} //end for
return true;
} //equals

You have this method in apache commons library that uses reflection for compare them
org.apache.commons.lang.builder.EqualsBuilder.reflectionEquals(Object, Object)
Example
import org.apache.commons.lang.builder.EqualsBuilder;
public class MyObject {
...
#Override
public boolean equals(Object obj) {
return EqualsBuilder.reflectionEquals(this, obj);
}
}

You can use lombok project to auto-generate hashCode and equals methods at build time .

You can use Unitils http://www.unitils.org/cookbook.html
import static org.unitils.reflectionassert.ReflectionAssert.*;
// Exact field-by-field comparison
assertReflectionEquals(new Person("John", "Doe", new Address("New street", 5, "Brussels")),
new Person("John", "Doe", new Address("New street", 5, "Brussels"));

Comparing objects within a set

Right now I have this Java code
import java.util.HashSet;
import java.util.Objects;
import java.util.Set;
public class Dummy {
private String value;
public Dummy(final String value) {
this.value = value;
}
public boolean equals(final Object that) {
return that instanceof Dummy && Objects.equals(value, ((Dummy) that).value);
}
public int hashcode() {
return Objects.hash(value);
}
public static void main(final String... args) {
final Set<Dummy> dummies = new HashSet<>();
dummies.add(new Dummy("toto"));
System.out.println(dummies.contains(new Dummy("toto")));
}
}
The output is "false", and I'm supposed to change it to "true" by changing only one character, but I have absolutely no idea how to do that... Any ideas? Thanks. :)

hashcode() is not a Object's method but hashCode() is.
public int hashcode() {
return Objects.hash(value);
}
should be
public int hashCode() {
return Objects.hash(value);
}

In my case I had lots of objects who all need a equals and hash method. I have used lombok to reduce the work and code.
#Data
#AllArgsConstructor
#EqualsAndHashCode
public class JsonPictureStuff {
private String type;
private String url;
private String width;
private String height;
}
The annotations instruct lombok to add the code for you.

your set is not working because you are implementing equals and hashcode in a not proper way...
specially because you are not even considering the string field
you are avoiding the annotation Override that will hint you about wrongly named methods like hashcode and hashCode
you can do something like
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((value == null) ? 0 : value.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;
Dummy other = (Dummy ) obj;
if (value == null) {
if (other.value != null)
return false;
} else if (!value.equals(other.value))
return false;
return true;
}
in ide like eclipse you can do right click and auto generate those methods so you can save the implementation

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 + "}";
}
}

Linkedlist implementation in Java Sorting not working

I created my own linkedlist. I wanted to sort my linkedlist using Collections.sort method.
So I extends MyLinkedList class to java.util.LinkedList. I also created Comparator and Comparable implementation. But both are not working. Please find below code.
// Linked List implementation.
package com.java.dsa;
class Node<E> {
E data;
Node<E> nextLink;
public Node(E data) {
this.data = data;
}
}
public class MyLinkedList<E> extends java.util.LinkedList<E>{
private static final long serialVersionUID = 1L;
private Node<E> firstNodePointer;
private Node<E> nodePointer;
public boolean isEmpty() {
return nodePointer == null;
}
public boolean add(E data) {
super.add(data);
Node<E> node = new Node<E>(data);
if (firstNodePointer == null) {
firstNodePointer = node;
nodePointer = node;
}else{
nodePointer.nextLink = node;
}
nodePointer = node;
return true;
}
public boolean remove(Object data){
super.remove(data);
Node<E> counterNodePointer = firstNodePointer;
Node<E> tempNodePointer = firstNodePointer;
while (counterNodePointer != null && !counterNodePointer.data.equals(data)) {
tempNodePointer = counterNodePointer;
counterNodePointer = counterNodePointer.nextLink;
}
if(tempNodePointer.equals(firstNodePointer)){
firstNodePointer = firstNodePointer.nextLink;
return true;
}
else if(counterNodePointer != null && tempNodePointer != null){
tempNodePointer.nextLink = counterNodePointer.nextLink;
return true;
}
return false;
}
public void printList() {
Node<E> counterNodePointer = firstNodePointer;
while (counterNodePointer != null) {
System.out.println(counterNodePointer.data);
counterNodePointer = counterNodePointer.nextLink;
}
}
}
// Test Linkedlist
package com.java.dsa;
import java.util.Collections;
import java.util.Comparator;
//Employee Class
class Employee implements Comparable<Employee> {
private String name;
private int id;
public Employee(String name, int id) {
super();
this.name = name;
this.id = id;
}
public String getName() {
return name;
}
public int getId() {
return id;
}
#Override
public String toString() {
return this.name + " " + this.id;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
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;
Employee other = (Employee) obj;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
#Override
public int compareTo(Employee employee) {
return this.id - employee.id;
}
}
class EmployeeSort implements Comparator<Employee> {
#Override
public int compare(Employee emp1, Employee emp2) {
if (emp2.getId() - emp1.getId() > 0)
return 1;
else
return -1;
}
}
public class TestLinkedList {
public static void main(String[] args) {
MyLinkedList<Employee> myList = new MyLinkedList<Employee>();
for (int i = 10; i > 0; i--) {
Employee emp = new Employee("Sohan "+i, i);
myList.add(emp);
}
myList.printList();
Collections.sort(myList, new EmployeeSort());
myList.printList();
}
}

Actually it works. It's just that your internal data structure is not updated by Collections.sort(), and since you base your assertion that the program doesn't work on the output of printList(), and this relies on that data structure, you see the order of elements untouched. Use this method instead:
public void printParentDataStructure() {
for ( E e : this ) System.out.println( e );
}
and see that your comparator perfectly does its job. So your problem is that you have two data structures and don't keep them in sync. Your next question may be "And how can I keep them sync'ed?" - Well, essentially you should override each and every method, and call super() like you do in add() and remove(). Don't do that! It'd be a complete nonsense.
It's clear that you want to implement a linked list for learning the data strcuture, but maybe you should first better understand the basic principles of OOP programming.

java.util.LinkedList is not a class designed for subclassing and your code is probably just breaking its internals and invariants.
If you want to implement a linked list on your own, but want to save yourself the effort of implementing the full List interface, then use AbstractList as your base class. This class's express purpose is exactly what you are trying to do.

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.