I'm trying to make a generic tuple class. It stores its elements as an ArrayList. Of course, this class should override hashcode and equals methods.
How could I make hashcode method for this class? You see, in the code, I am having trouble.
Also, for the equals method, why does the compiler force me to use the '?'. Why couldn't I just use the T?
public static class Tuple<T> {
ArrayList<T> tuple = new ArrayList<>();
public Tuple(ArrayList<T> items) {
for (T item : items) {
tuple.add(item);
}
}
#Override
public int hashCode() {
T sum = ???;
for (T item : tuple) {
sum += item.hashCode();
}
return sum;
}
#Override
public boolean equals(Object o) {
if (o instanceof Tuple<?>) {
Tuple<?> tup= (Tuple<?>) o;
if (tup.tuple.size() != this.tuple.size()) {
return false;
}
for (int i = 0; i < this.tuple.size(); i++) {
if (this.tuple.get(i) != tup.tuple.get(i)) {
return false;
}
}
return true;
} else {
return false;
}
}
}
As mentioned in the comments, we should delegate the hashCode and the equals methods to the ArrayList<T> tuple instance variable. For the hashCode it's trivial. For the equals it's just a little more complicated than that because we don't want our custom Tuple to be equals with an ArrayList. So here it is:
public class Tuple<T> {
// I made this private because I'm pedantric ;)
private final ArrayList<T> tuple = new ArrayList<>();
// this does the same as your code, it's just easier to read
public Tuple(ArrayList<T> items) {
tuple.addAll(items);
}
#Override
public int hashCode() {
return tuple.hashCode();
}
// generated by eclipse
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Tuple other = (Tuple) obj;
if (tuple == null) {
if (other.tuple != null)
return false;
} else if (!tuple.equals(other.tuple))
return false;
return true;
}
}
If you want to deal with the case when the tuple can be null, then you can use a slightly more complex hashCode:
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((tuple == null) ? 0 : tuple.hashCode());
return tuple.hashCode();
}
In general, I don't like to write these methods myself. Usually, I make my IDE to generate the stuff. All I need to take care of is to re-generate it when I add new fields. Apache HashCodeBuilder and EqualsBuilder are also great alternatives.
Related
This is my code:
private class UniqueClassByTwoIntProperties {
private final int propertyOne;
private final int propertyTwo;
UniqueCase(final int propertyOne, final int propertyTwo) {
this.propertyOne= propertyOne;
this.propertyTwo= propertyTwo;
}
#Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (this == obj) {
return true;
}
if (!(obj instanceof UniqueCase)) {
return false;
}
UniqueClassByTwoIntProperties unique = (UniqueClassByTwoIntProperties) obj;
return unique.claimApplicationId == claimApplicationId && unique.claimCoverageId == claimCoverageId;
}
#Override
public int hashCode() {
return Objects.hash(propertyOne, propertyTwo);
}
}
I am looping through a list with objects, where I want to get the unique's in the following way:
myList.stream()
.map(row -> new UniqueClassByTwoIntProperties(row.getOne(), row.getTwo()))
.collect(Collectors.toSet());
I was wondering if there was a build-in class/method in Java. I have looked into dictionaries and MultiMapValues, but it was a bit hacky.
You can simply do it as:
Set<UniqueClassByTwoIntProperties> uniques = new HashSet<>(myList);
Aside: Make sure the hashCode and equals of the objects are implemented correctly for the object to be comparable.
If you just want the List of uniques then you can use Stream.distinct as :
List<UniqueClassByTwoIntProperties> uniques = myList.stream()
.distinct() // for unique objects
.collect(Collectors.toList());
I need to write abstract class, which looks like this.
public abstract class Value {
public abstract String toString();
public abstract Value add(Value v);
public abstract Value sub(Value v);
public abstract boolean eq(Value v);
public abstract boolean lte(Value v);
public abstract boolean gte(Value v);
public abstract boolean neq(Value v);
public abstract boolean equals(Object other);
public abstract int hashCode();
public abstract Value create(String s);
}
Now I need to make few classe, which inherit from that one. I started from Int class and implemented it like this:
public class Int extends Value {
int val;
public String toString() {
String toStr = Integer.toString(val);
return toStr;
}
public Int add(Value v) {
Int result = new Int();
if(v instanceof Int) {
Int temp = (Int) v;
result.val = val + temp.val;
}
return result;
}
public Int sub(Value v) {
Int result = new Int();
if(v instanceof Int) {
Int temp = (Int) v;
result.val = val - temp.val;
}
return result;
}
public boolean eq(Value o) {
if(this == o) return true;
if(this == null) return false;
if(getClass() != o.getClass()) return false;
Int other = (Int) o;
return toString() == other.toString();
}
public boolean lte(Value v) {
if(v instanceof Int) {
Int temp = (Int) v;
return this.val < temp.val;
}
return false;
}
public boolean gte(Value v) {
if(v instanceof Int) {
Int temp = (Int) v;
return this.val > temp.val;
}
return false;
}
public boolean neq(Value v) {
if(v instanceof Int) {
Int temp = (Int) v;
return !eq(temp);
}
return true;
}
public boolean equals(Object o) {
if(this == o) return true;
if(this == null) return false;
if(getClass() != o.getClass()) return false;
Int other = (Int) o;
return toString() == other.toString();
}
public int hashCode() {
Integer hash = val;
return hash.hashCode();
}
public Int create(String s) {
val = Integer.parseInt(s);
return this;
}
}
Everything is compiling and working, but I have no clue if my hashcode() function and equals() are good. Furthermore i want to use create() to make objects like this:
getInstance().create("1234");
Is my method also sufficient?
Everything is compiling and working, but I have no clue if my hashcode() function and equals() are good.
Your equals() should compare int val and not result of toString() of compared objects (this.val == other.val).
Your hashCode() looks good, though I would add #Override to it (same with equals()).
Furthermore i want to use create() to make objects like this: getInstance().create("1234");
Looking at its implementation, it looks fine (i.e. would work according to your needs):
public Int create(String s) {
val = Integer.parseInt(s);
return this;
}
though I don't think you really want to use it with getInstance(). Simply Int.create() would be enough:
public static Int create(String s) {
val = Integer.parseInt(s);
return new Int(val);
}
Note that you would need a private constructor.
Also, as someone noted in the comments, consider using generics instead of inheritance.
The hashCode() method is fine (although I'd add an #Override annotation, just to make the code easier to maintain and avoid mistakes), but the equals(Object) definitely isn't.
Following the logic you have in place, == isn't the right way to compare strings. You should use equals instead (see, e.g., How do I compare strings in Java?). In addition, as Joakim Danielson noted in the comments, this can never be null - you should check if o is null instead:
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null) {
return false;
}
if(getClass() != o.getClass()) {
return false;
}
Int other = (Int) o;
return toString().equals(other.toString()); // Here!
}
But in all fairness, there's no reason to use toString - you could just compare the internal val:
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null) {
return false;
}
if(getClass() != o.getClass()) {
return false;
}
Int other = (Int) o;
return val == other.val; // Here!
}
First when you override Methods please do it with #Override Annotation. Then i would implement your equals method in another way. Just do return this.val == other.val instead of doing this.toString() == other.toString(). Your toString() method implementation is ok. Your hashCode is good as well. But please remove that create method. Use a constructor instead.
Can I implement equals() method using eq() like this ?
public boolean equals(Object o) {
Value compare = (Value) o;
return eq(compare);
}
I am trying to retrieve some values from a Hash Map, before returning the value I am checking if the key is present in the map or not, and this check always fails which results in null value. I have overridden hash Code and equals method as well. Could someone tell me what I am doing wrong here?
class fields:
private static final List<String> DZ=new ArrayList<String>();
private static final Map<Participant,List<String>> subDz=new HashMap<Participant,List<String>>();
Method where I am putting into the map:
public static synchronized void handleSubs(String[] subData,String dz){
int[] lowdims=new int[subData.length];
int[] highdims=new int[subData.length];
try {
for (int i=1;i<subData.length;i++){
if (!subData[i].equals("") && !subData[i].equals("\n")){
if (i%2==0){
highdims[i]=Integer.parseInt(subData[i].trim());
}
else {
lowdims[i]=Integer.parseInt(subData[i].trim());
}
}
}
if (!DZ.isEmpty()){
DZ.clear();
}
DZ.add(dz);
allSubDZs.add(dz);
int[] newlow=removeZeroes(lowdims);
int[] newhigh=removeZeroes(highdims);
allSubs.add(new Participant(newlow,newhigh));
subDz.put(new Participant(newlow,newhigh),DZ );
}
Method where I am retrieving the values:
public static List<String> getSubDz(Participant sub){
if (subDz.containsKey(sub)){
return subDz.get(sub);
}
else{
logger.info("Subscription DZ not available");
return null;
}
}
The if check in the getSubDz always fails, even though I put the key in it.
hashCode and equals methods:
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((DZ == null) ? 0 : DZ.hashCode());
return result;
}
#Override
public boolean equals(final Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (this.getClass() != obj.getClass()) {
return false;
}
final SubscriptionHandler other=(SubscriptionHandler)obj;
if (DZ == null) {
if (other.DZ != null) {
return false;
}
} else if (!DZ.equals(other.DZ)) {
return false;
}
return true;
You need equals and hashcode on the key class. This would be the class Participant in your case.
I want specific object with all it's values by using it's unique id of object from object list.
I have tried but i am getting index -1 while running below code.
List<JobDataDetail> jobList = getJobList();
JobDataDetail object = jobList.get(jobList.indexOf(new JobDataDetail(jobReferenceId)));
from the class
public class JobDataDetail implements Serializable,Comparable<JobDataDetail> {
public int jobSequence;
public String jobReferenceId;
public String jobAddress;
public String jobScheduledDate;
public JobDataDetail() {
super();
// TODO Auto-generated constructor stub
}
public JobDataDetail(int jobSequence){
super();
this.jobSequence = jobSequence ;
}
public JobDataDetail(String jobReferenceId){
super();
this.jobReferenceId = jobReferenceId;
}
public int getJobSequence() {
return jobSequence;
}
public void setJobSequence(int jobSequence) {
this.jobSequence = jobSequence;
}
public String getJobReferenceId() {
return jobReferenceId;
}
public void setJobReferenceId(String jobReferenceId) {
this.jobReferenceId = jobReferenceId;
}
public String getJobAddress() {
return jobAddress;
}
public void setJobAddress(String jobAddress) {
this.jobAddress = jobAddress;
}
public String getJobScheduledDate() {
return jobScheduledDate;
}
public void setJobScheduledDate(String jobScheduledDate) {
this.jobScheduledDate = jobScheduledDate;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result
+ ((jobReferenceId == null) ? 0 : jobReferenceId.hashCode());
result = prime * result + jobSequence;
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
JobDataDetail other = (JobDataDetail) obj;
if (jobReferenceId == null) {
if (other.jobReferenceId != null)
return false;
} else if (!jobReferenceId.equals(other.jobReferenceId))
return false;
if (jobSequence != other.jobSequence)
return false;
return true;
}
#Override
public int compareTo(JobDataDetail another) {
return this.getJobReferenceId().compareTo(another.getJobReferenceId());
}
}
List.indexOf() uses equals() method to compare objects.
In your case, you are assuming that two objects with same jobReferenceId are equals but your equals() method doesn't say so (because of the jobSequence test at the end of your method).
If you want to get an item from your list by one of its attribute, the easiest way would be using filter expression in Java 8:
JobDataDetail job = jobList.stream()
.filter(j -> j.getAttribute().equals(someValue))
.findFirst();
If Java 8 is not an option, I would go for a classic for loop iterating over the list.
I have removed jobSequence condition check from equals method and it's working.
I have a HashMap that stores an object I created as a key and maps to an ArrayList of similar objects.
However, I am calling the get method, and using jGrasp's debugger I can clearly see that the key I am using in get() exists and indeed maps to an array but the only value I can get is a null value.
Here is where I am getting the null value.
public List<Entry> query(Record query) {
List<Entry> candList;
Entry key = new Entry(makeKey(query));
candList = map.get(key);
return candList;
}
Here is where I am populating the HashMap from a main store.
for(int i = 0; i < main.size(); i++) {
if(main.get(i).isActive()) {
values.clear();
tmp = new Entry(main.get(i).record());
key = new Entry(Record.make(tmp.entity(),tmp.relation(),wild));
if(!map.containsKey(key)) {
for(int v = 0; v < main.size(); v++) {
value = main.get(v);
if(key.entity().equals(value.entity()) && key.relation().equals(value.relation())) {
values.add(value);
}
}
map.put(key,new ArrayList(values));
}
}
}
Entry is a wrapper class that defaults to the equals() method of its inner object, here.
public boolean equals(Object o){
if(o == null){
return false;
}
else if(o instanceof Record){
Record r = (Record) o;
return this.entity.equals(r.entity) && this.relation.equals(r.relation) && this.property.equals(r.property);
}
else return false;
}
I also have a hashcode written for the object here.
int h = 0;
public int hashCode() {
int hash = h;
if(h != 0)
return hash;
String len = entity.concat(relation.concat(property));
for(int i = 0; i < len.length(); i++)
hash = hash * 31 +(int)len.charAt(i);
return hash;
}
For a little clarification, the Entry object holds an object of type Record that contains three immutable Strings, hence where the hashCode equation comes from.
For further clarification someone asked to see the entire Entry class.
private static class Entry {
private static boolean active;
private Record rec;
public Entry(Record r){
this.rec = r;
this.active = true;
}
public String entity() {
return rec.entity;
}
public String relation() {
return rec.relation;
}
public String property() {
return rec.property;
}
public Record record(){
return this.rec;
}
public boolean isActive(){
return this.active;
}
public void deactivate(){
this.active = false;
}
public void activate(){
this.active = true;
}
public boolean equals(Entry e) {
return this.rec.equals(e.record());
}
public int hashCode() {
return this.rec.hashCode();
}
public String toString() {
return rec.toString();
}
}
There are some collisions occurring in my HashMap but I know that's not supposed to be too much of an issue. Any ideas?
public boolean equals(Object o){
if(o == null){
return false;
}
else if(o instanceof Record){
Record r = (Record) o;
return this.entity.equals(r.entity) && this.relation.equals(r.relation) && this.property.equals(r.property);
}
else return false;
}
your Entry equals method may have some problem,what's the definition of relation?
the relation must be overwrite equals() and hashCode()
It's great to put all your code here,what's your main's definition?
and In your code there are many places contains maybe null pointer bug
your hashcode function might have a problem when setting the int to 0 (int h = 0) ... a good explanation can be found in Josh Bloch's Effectiv Java book (item 8).
Here is an example:
#Override
public int hashCode() {
int result = 17;
// this line should change depending on your fields
// let say you have a string property that is not null
result = 31 * result + property.hashCode();
return result;
}
... you can also use a library like Guava
#Override
public int hashCode() {
return Objects.hashCode(this.property1, this.property2);
}