Develop Reference

HashMap key generation based on hashing

My intention was to make a caching service for a database results, that I can paginate differently based on client's requests.
So, upon the (search) request I am making a key that is composed of parameters, which are in form of two Map<String, String[]> and a:
public class DocMaintainer {
public Manipulator creator;
public Manipulator lastChange;
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
DocMaintainer that = (DocMaintainer) o;
return Objects.equals(creator, that.creator) &&
Objects.equals(lastChange, that.lastChange);
}
#Override
public int hashCode() {
return Objects.hash(creator, lastChange);
}
}
public class Manipulator {
public Date fromDate;
public Date toDate;
public String userId;
public String system;
public Manipulator() {
this.userId = "";
this.system = "";
this._fromJoda = new DateTime(Long.MIN_VALUE);
this._toJoda = new DateTime(Long.MAX_VALUE - DateTimeConstants.MILLIS_PER_WEEK);
}
private DateTime _fromJoda;
private DateTime _toJoda;
public DateTime get_fromJoda() {
_fromJoda = fromDate != null ? new DateTime(fromDate) : _fromJoda;
return _fromJoda;
}
public DateTime get_toJoda() {
_toJoda = toDate != null ? new DateTime(toDate) : _toJoda;
try {
_toJoda = _toJoda.plusDays(1);
} catch (Exception e) {
System.out.println(e);
}
return _toJoda;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Manipulator that = (Manipulator) o;
return Objects.equals(fromDate, that.fromDate) &&
Objects.equals(toDate, that.toDate) &&
Objects.equals(userId, that.userId) &&
Objects.equals(system, that.system);
}
#Override
public int hashCode() {
return Objects.hash(fromDate, toDate, userId, system);
}
}
As you can see I intended to use hashing to create a "key":
public class SearchKey {
public int conjunctionHash;
public int disjunctionHash;
public int maintainerHash;
public SearchKey(int conjunctionHash, int disjunctionHash, int maintainerHash) {
this.conjunctionHash = conjunctionHash;
this.disjunctionHash = disjunctionHash;
this.maintainerHash = maintainerHash;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
SearchKey searchKey = (SearchKey) o;
return conjunctionHash == searchKey.conjunctionHash &&
disjunctionHash == searchKey.disjunctionHash &&
maintainerHash == searchKey.maintainerHash;
}
#Override
public int hashCode() {
return Objects.hash(conjunctionHash, disjunctionHash, maintainerHash);
}
}
a key-object is used directly as a caching key in a singleton service:
#Named
#Singleton
public class SearchCacheSrv {
private Map<SearchKey, ValidMainteinersList<FindDTO>> cache = new HashMap<>();
public ValidMainteinersList<FindDTO> getCached(SearchKey searchKey) {
if (cache.containsKey(searchKey))
return cache.get(searchKey);
else
return new ValidMainteinersList<FindDTO>();
}
public SearchKey makeAkey(Map<String, String[]> conjunction,
Map<String, String[]> disjunction,
DocMaintainer maintainer) {
return new SearchKey(conjunction.hashCode(), disjunction.hashCode(), maintainer.hashCode());
}
public ValidMainteinersList<FindDTO> cache(SearchKey searchKey, ValidMainteinersList<FindDTO> findDTOS) {
return cache.put(searchKey, findDTOS);
}
public void clearCache() {
cache.clear();
}
}
Unfortunately this is not behaving the way I expected and I'm getting different hashes/keys generated for the same parameters.
Naturally question is why?

The problem here is that the hashCode of an array does not depend on the contents, but on the reference. That means that if you have two conjunction / disjunction keys that are equal, but the contained arrays are not the same objects, then the hashcode of the keys will be different.
The solution that probably takes the least effort is replacing the arrays with ArrayLists, which do base their hashCode on the content.

I actually don't see the point of passing conjunction.hashCode(), ... to your SearchKey constructor; I never had to do it this way, but it could be my mistake.
Try passing actual values to your SearchKey class, not hashCodes, so the hashCode method always returns a consistent value.

Method that adds elements to set always throwing custom exception

Ok, so I've got a method which adds elements to a list but it is always throwing my custom exception, no matter what, even when there are no elements in the Set I made.
private Set<Plan> planSet = new HashSet<Plan>();
public Plan createPlan(String name) throws DuplicatePlan{
Plan plan = new Plan(name);
if(!planSet.contains(plan)){
planSet.add(plan);
} else {
throw(new DuplicatePlan("Error, duplicate plan"));
}
return plan;
}
I'm thinking that my equals() and hashCode() methods are causing this. Currently I'm using the default overridden Eclipse hashCode() and equals(), this is what I've got there:
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
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;
}
Plan other = (Plan) obj;
if (name == null) {
if (other.name != null){
return false;
}
} else if (!name.equals(other.name)){
return false;
}
return true;
}
This is what Plan does:
private String name;
private Set<Tables> tablesSet;
public Plan(String name){
this.name = name ;
}
Here's what's supposed to happen if a user sets the same name in the TextField:
newPlan.setOnAction(new EventHandler<ActionEvent>(){
#Override
public void handle(ActionEvent action){
if(!newPlan.getText().isEmpty()){
try {
String name = planName.getText();
plan.createPLan(name);
esquema = esquemas.createPlan(planName.getText());
optionsPlans.getItems().add(plan.getName());
} catch (DuplicatePlan e) {
dialog.errorDialog(planError, duplicate);
}
} else {
dialog.errorDialog(empty, emptySpace);
}
}
});

Had to use Answer because it was too long for comment.
This here looks suspicious to me:
String name = planName.getText();
plan.createPLan(name);
esquema = esquemas.createPlan(planName.getText());
I.e. what's up with createPLan and createPlan? Copy & paste error? Or are you calling the same method twice (which would explain the behavior)?

ConcurrentHashMap - Odd behaviour

Can anyone let me know what goes wrong in this piece of code? I'm pulling my hair out!
There isn't any problem if I use HashMap instead of ConcurrentHashMap. The code is compiled with JDK 5.0
public class MapTest {
public Map<DummyKey, DummyValue> testMap = new ConcurrentHashMap<DummyKey, DummyValue>();
public MapTest() {
DummyKey k1 = new DummyKey("A");
DummyValue v1 = new DummyValue("1");
DummyKey k2 = new DummyKey("B");
DummyValue v2 = new DummyValue("2");
testMap.put(k1, v1);
testMap.put(k2, v2);
}
public void printMap() {
for(DummyKey key : testMap.keySet()){
System.out.println(key.getKeyName());
DummyValue val = testMap.get(key);
System.out.println(val.getValue());
}
}
public static void main(String[] args){
MapTest main = new MapTest();
main.printMap();
}
private static class DummyKey {
private String keyName = "";
public DummyKey(String keyName){
this.keyName = keyName;
}
public String getKeyName() {
return keyName;
}
#Override
public int hashCode() {
return keyName.hashCode();
}
#Override
public boolean equals(Object o) {
return keyName.equals(o);
}
}
private static class DummyValue {
private String value = "";
public DummyValue(String value){
this.value = value;
}
public String getValue() {
return value;
}
}
}
This is the output:
B
Exception in thread "main" java.lang.NullPointerException
at test.MapTest.printMap(MapTest.java:27)
at test.MapTest.main(MapTest.java:34)

DummyKey.equals method implementation is incorrect, due to that testMap.get(key) always returns null. Try this
public boolean equals(Object o) {
if (o instanceof DummyKey) {
DummyKey other = (DummyKey) o;
return keyName == null ? other.keyName == null : keyName.equals(other.keyName);
}
return false;
}
hashCode also needs a little change to be consistent with equals
public int hashCode() {
return keyName == null ? 0 : keyName.hashCode();
}

The problem comes from your equals in DummyKey.
When you call DummyValue val = testMap.get(key);, the hashcode function finds a match (both keyname of k1 and key are the same and so are their hashcode). Yet equals returns false because k1.keyname is equal to "A" which is not equal to key itself, which is actually of type DummyValue: you are not comparing properly!
Therefore, you need to modify your equals function:
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
DummyKey other = (DummyKey) obj;
if (keyName == null) {
if (other.keyName != null)
return false;
} else if (!keyName.equals(other.keyName))
return false;
return true;
}

Please note that if you change hashCode(), then you must change equals() as well. Otherwise, you will run into problems. If equals() returns true for two items, then their hashCode() value must be equal! The opposite is not required but preferable for better hashing performance. Here is an implementation of equals() and hashCode().
HINT: if you are using eclipse, you can utilize its source generation capability to create the correct hashCode() and equals() method for you. The only thing you need to do is to pick the instance variables that identify the object. To do so in eclipse, while your source code is open, go to the tabs in the top and choose "source", then choose "Generate hashCode() and equals()..."
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((keyName == null) ? 0 : keyName.hashCode());
return result;
}
Override
public boolean equals(Object other) {
if(this == other) return true; //for optimization
if(! other instanceof this) return false; //also covers for when other == null
return this.keyName == null ? other.keyName == null : this.keyName.equals(other.keyName);
}

As others have pointed, the problem lies in the way you override hashcode and equals.
Two options : 1) Just remove the hashcode and equals and it works fine
2) I let eclipse generate the source for hashcode and equals and it works fine. This is what my eclipse belted out for me :
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result
+ ((keyName == null) ? 0 : keyName.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;
DummyKey other = (DummyKey) obj;
if (keyName == null) {
if (other.keyName != null)
return false;
} else if (!keyName.equals(other.keyName))
return false;
return true;
}

Calling equals method of superclass in Java

I have an issue where I am trying to use test the equals method of a superclass using objects of a subclass:
My superclass is:
public class Excluded_DateTime implements Serializable {
private Date fromDate;
private Time fromTime;
private Date toDate;
private Time toTime;
private Valid active;
And the subclass differs by having an identifier as a key:
public class Classifier_Excluded_DateTime implements Serializable {
private Integer classifierExcludedDateTimeNo;
private Excluded_DateTime classifierExcludedDateTime;
So I want to test the equality of Classifier_Excluded_DateTime objects without using the field classifierExcludedDateTimeNo.
But what I am finding is that the equals method of the superclass is never called.
The NetBeans generated equals and hashCode methods of the superclass are as follows:
#Override
public int hashCode() {
int hash = 7;
hash = 23 * hash + (this.fromDate != null ? this.fromDate.hashCode() : 0);
hash = 23 * hash + (this.fromTime != null ? this.fromTime.hashCode() : 0);
hash = 23 * hash + (this.toDate != null ? this.toDate.hashCode() : 0);
hash = 23 * hash + (this.toTime != null ? this.toTime.hashCode() : 0);
hash = 23 * hash + (this.active != null ? this.active.hashCode() : 0);
return hash;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final Excluded_DateTime other = (Excluded_DateTime) obj;
if (this.fromDate != other.fromDate && (this.fromDate == null || !this.fromDate.equals(other.fromDate))) {
return false;
}
if (this.fromTime != other.fromTime && (this.fromTime == null || !this.fromTime.equals(other.fromTime))) {
return false;
}
if (this.toDate != other.toDate && (this.toDate == null || !this.toDate.equals(other.toDate))) {
return false;
}
if (this.toTime != other.toTime && (this.toTime == null || !this.toTime.equals(other.toTime))) {
return false;
}
if (this.active != other.active) {
return false;
}
return true;
}
And those of the subclass as follows:
#Override
public int hashCode() {
int hash = 7;
hash = 79 * hash + (this.getClassifierExcludedDateTimeNo() != null ? this.getClassifierExcludedDateTimeNo().hashCode() : 0);
return hash;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
if (! super.equals(obj)) {
return false;
}
final Classifier_Excluded_DateTime other = (Classifier_Excluded_DateTime) obj;
if (this.getClassifierExcludedDateTimeNo() != other.getClassifierExcludedDateTimeNo() && (this.getClassifierExcludedDateTimeNo() == null || !this.classifierExcludedDateTimeNo.equals(other.ClassifierExcludedDateTimeNo))) {
return false;
}
return true;
}
Can I do what I'm trying to do by amending these methods of either class?
I am trying to test whether two instance of Classifier_Excluded_DateTime are the same without including the field classifierExcludedDateTimeNo in the comparison.

You are not declaring Classifier_Excluded_DateTime as a subclass of Excluded_DateTime. To do that, you would need to say:
public class Classifier_Excluded_DateTime extends Excluded_DateTime {
Right now, it seems more like you are treating Excluded_DateTime as a kind of delegate of Classifier_Excluded_DateTime, as you have a member instance variable inside of Classifier_Excluded_DateTime that is of type Excluded_DateTime (the place where you say private Excluded_DateTime classifierExcludedDateTime;). I don't think that's your intent, since you are talking about subclasses and superclasses in your post.
(Update: removed error where I thought there was not an explicit super.equals() call).

How to define multiple equals() function for a class

I want to override "public boolean equals(Object obj)" function, for name and age, in my class named MyObject whose structure is given below
public class MyObject{
private String name;
private int age;
}
How can i ?
#balusC :
What about this ?
vo = new MyObject() {
public boolean equals(Object obj) {
return ((MyObject)obj).name().equals(this.getName());
}
vo = new MyObject() {
public boolean equals(Object obj) {
return ((MyObject)obj).age() == (this.getAge());

Your question is a bit vague, but if the sole purpose is to have different sorting algorithms depending on what property you'd like to use, then rather use a Comparator.
public class Person {
private String name;
private int age;
public static Comparator COMPARE_BY_NAME = new Comparator<Person>() {
public int compare(Person one, Person other) {
return one.name.compareTo(other.name);
}
}
public static Comparator COMPARE_BY_AGE = new Comparator<Person>() {
public int compare(Person one, Person other) {
return one.age > other.age ? 1
: one.age < other.age ? -1
: 0; // Maybe compare by name here? I.e. if same age, then order by name instead.
}
}
// Add/generate getters/setters/equals()/hashCode()/toString()
}
which you can use as follows:
List<Person> persons = createItSomehow();
Collections.sort(persons, Person.COMPARE_BY_NAME);
System.out.println(persons); // Ordered by name.
Collections.sort(persons, Person.COMPARE_BY_AGE);
System.out.println(persons); // Ordered by age.
As to the actual equals() implementation, I'd rather let it return true when the both Person objects are techically or naturally identical. You can use either a DB-generated PK for this to compare on technical identity:
public class Person {
private Long id;
public boolean equals(Object object) {
return (object instanceof Person) && (id != null)
? id.equals(((Person) object).id)
: (object == this);
}
}
or just compare every property to compare on natural identity:
public class Person {
private String name;
private int age;
public boolean equals(Object object) {
// Basic checks.
if (object == this) return true;
if (object == null || getClass() != object.getClass()) return false;
// Property checks.
Person other = (Person) object;
if (name == null ? other.name != null : !name.equals(other.name)) return false;
if (age != other.age) return false;
// All passed.
return true;
}
}
Don't forget to override hashCode() as well when you override equals().
See also:
Object ordering
Sorting an ArrayList of objects
Overriding equals() and hashCode()

I'm not exactly sure what you're aiming at with this. The general expectation of equals() is that it returns false for null and objects of other classes and performs value equality on the relevant fields of the class in question.
While you can certainly handle String and Integer in the following way:
public boolean equals(Object o) {
if (o == null) return false;
if (o instanceof String) return name.equals(o);
if (o instanceof Integer) return ((Integer)o) == age;
...
}
this breaks the contract for equals so you can't do it (except not without things going wrong in very weird ways).
equals is an equivalence relation, so it has to be reflexive, symmetric and transitive. The symmetric part here is key, since if a.equals(b) then b.equals(a). Both String and Integer won't do that for you.
If you want just helper functions that check whether the name or the age is equals to a given name/age, then you can do that without using equals():
public boolean equalsName(String name) { return name.equals(this.name); }
public boolean equalsAge(int age) { return age == this.age; }

Just keep it short and simple (aka KISS principle): write setters and getters. Something like in the following example:
public class Person {
private String name;
private int age;
public String getName() {
return name;
}
public int getAge() {
return age;
}
And then in the method you need to do the check you can write:
Person person = new Person();
if(person.getName().equals("Something")) doThis();
if(person.getAge() == 1337) doThat();

Not sure what you mean by "multiple equals()". If you want compare both your fields, you just need to override the equals method like this,
public boolean equals( Object o )
{
if ( o != null && o instanceof MyObject )
{
MyObject m = (MyObject) o;
if (this.name == null)
return false;
return this.name.eqauls(m.name) && this.age == m.age;
}
return false;
}
/// Compute a hash code for the pair.
public int hashCode()
{
int code = name == null ? 0 : name.hashCode();
return code ^ age;
}
It's a good practice to change hashCode whenever you change equals so HashMap works efficiently with your object.

if you do want to override equals, it should look something like this:
static private <T> boolean checkEquals(T t1, T t2)
{
return (t1 == null) ? (t2 == null) : t1.equals(t2);
}
#Override public boolean equals (Object o)
{
if (o instanceof MyObject)
{
MyObject obj = (MyObject)o;
return checkEquals(this.name, obj.getName())
&& this.age == o.getAge();
}
else
return false;
}
#Override public int hashCode()
{
// implement hashCode
}
You need to override both hashCode() and equals() or neither. And you also should make sure your class is final, otherwise there are potential pitfalls with equals.

public class MyObject {
private String name;
private int age;
#Override
public boolean equals(Object o){
if(o instanceof MyObject){
MyObject otherObject = (MyObject)o;
if(name == null){
return otherObject.name == null && otherObject.age == age;
} else {
return name.equals(otherObject.name) && otherObject.age == age;
}
} else {
return false;
}
}
// When we overriding equals it is a good practice to override hashCode
// for consistecy
#Override
public int hashCode(){
int nameCode = (name == null) ? 0 : name.hashCode();
// See Item 9 in book Effective Java 2nd Edition
return 31 * nameCode + age;
}
}