Develop Reference

Java HashSet contains returns false, even with overridden equals() and hashCode()

I initialize the HashSet like this:
private HashSet<Rule> ruleTable = new HashSet<Rule>();
The equals() and hashCode() methods of my TcpRule object (sub-class of abstract class Rule) look like this:
#Override
public int hashCode() {
// Ignore source Port for now
return (this.getSrcPool() + ":" + this.getDstPool() + ":" + this.getProtocol() + ":" + this.dstTcp).hashCode();
}
#Override
public boolean equals(Object obj) {
if (!(obj instanceof TcpRule))
return false;
if (obj == this)
return true;
TcpRule r = (TcpRule) obj;
return (this.getSrcPool().equals(r.getSrcPool()) && this.getDstPool().equals(r.getDstPool()) && this.getProtocol().equals(r.getProtocol()) && this.getSrcTcp() == r.getSrcTcp() && this.getDstTcp() == r.getDstTcp());
}
I have even written a simple unit test, which does not give any error:
#Test
public void equalsTest() {
Pool srcPool = new Pool("PROXY");
Pool dstPool = new Pool("WEB");
int srcTcp = 54321;
int dstTcp = 80;
TcpRule r1 = new TcpRule(srcPool, dstPool, srcTcp, dstTcp);
TcpRule r2 = r1;
assert r1.equals(r2);
TcpRule r3 = new TcpRule(srcPool, dstPool, srcTcp, dstTcp);
TcpRule r4 = new TcpRule(srcPool, dstPool, srcTcp, dstTcp);
assert r3.equals(r4);
}
#Test
public void hashCodeTest() {
Pool srcPool = new Pool("PROXY");
Pool dstPool = new Pool("WEB");
int srcTcp = 54321;
int dstTcp = 80;
TcpRule r1 = new TcpRule(srcPool, dstPool, srcTcp, dstTcp);
TcpRule r2 = new TcpRule(srcPool, dstPool, srcTcp, dstTcp);
assert r1.hashCode() == r2.hashCode();
HashSet<Rule> rules = new HashSet<Rule>();
rules.add(r1);
assert rules.contains(r1);
assert rules.contains(r2);
}
In my application, I have an add() method where I simply add a Rule object to the HashSet:
#Override
public void add(Rule rule) {
ruleTable.add(rule);
}
In another method, I check if a rule exists in the HashSet:
#Override
public boolean isPermittedTcp(IpAddress sourceAddress, IpAddress destinationAddress, short srcTcp, short dstTcp) {
Pool sourcePool = poolService.getPool(new Host(sourceAddress));
Pool destinationPool = poolService.getPool(new Host(destinationAddress));
Rule r = new TcpRule(sourcePool, destinationPool, srcTcp, dstTcp);
log.info("Checking: " + r.toString());
log.info("Hash-Code: " + r.hashCode());
log.info("Hashes in ruleTable:");
for(Rule rT : ruleTable) {
log.info("" + rT.hashCode());
}
if(ruleTable.contains(r)) {
log.info("Hash found!");
} else {
log.info("Hash not found!");
}
return ruleTable.contains(r);
}
The log messages indicate that the hash of the Rule object (r.hashCode()) is -1313430269, and that one hash in the HashSet (rT.hashCode() in the loop) is also -1313430269.
But ruleTable.contains(r) always returns false. What am I doing wrong?
I have found similar questions on StackOverflow, but these mostly involve the equals() or hashCode() methods not being (correctly) overridden. I think I have implemented this two methods correctly.

Your problem is that hashCode() and equals() do not agree.
Your hashCode() implementation is based on the toString() of the pool, but your equals() uses .equals() of the pool class.
Change your .equals() to compare the strings used to generate the hash code.

There are some possibilities:
Rule is mutable, after adding a rule to the set some key (w.r.t. hash or equals) field was changed;
If two objects are equal they should have the same hashCode;
Bug, like a comparison in equals using == i.o. equals.
Here I would guess you have two Pool instances without equals on pool name or hashCode on pool name.

You have an extra condition in equals this.getSrcTcp() == r.getSrcTcp() which is not part of hash code - maybe thats the issue, hashcode is same, but equals is false. Check if this field is different in the values you are comparing.
Inspite of comments, I think the reason this does not work is because the equals & hashCode implementations do not use the same fields.
Code to simulate the problem:
import java.util.HashSet;
/**
* #author u332046
*
*/
public class HashCodeCollisionTest {
public static class KeyDemo {
String id;
#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;
KeyDemo other = (KeyDemo) obj;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
return true;*/
return false;
}
public KeyDemo(String id) {
super();
this.id = id;
}
}
static HashSet<KeyDemo> set = new HashSet<>();
public static void main(String[] args) {
set.add(new KeyDemo("hi"));
set.add(new KeyDemo("hello"));
System.out.println(set.contains(new KeyDemo("hi")));
}
}
This prints false. Uncomment the equals code and it prints true

How does indexOf() know how to get the index in the ArrayList of the object passed into it? [duplicate]

Lets say I have a class
public class Data{
public int k;
public int l;
public Data(int k, int l){
this.k = k;
this.l = l;
}
public boolean equals(Date m){
if(this.k == m.k && this.l = m.l)
return true;
return false;
}
}
And I add a few Data objects to a ArrayList:
ArrayList<Data> holder = new ArrayList<Data>;
Data one = new Data(0,0);
Data two = new Data(0,4);
Data three = new Data(0,5);
Why does indexOf not find this?:
holder.indexOf(new Data(0,4)); //returns -1
Is indexOf any better than going through the whole array list myself? Or am I missing something.

The indexOf() method does go through the entire list. Here's an excerpt from Java 7 source code:
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
It'd be better to let Java go through it than write it yourself. Just make sure that your equals method is sufficient at finding the object you want. You'll also want to override hashCode() as well.
I won't write your equals method out, but I would recommend that you at least:
Check for null
Test if the instances you're comparing are the same
You don't need to do if(boolean_expr) { return true; }; just return the boolean expression.
Make sure you're actually overriding your equals method - the signature of that requires an Object parameter, not Date.

The signature of your equals method is wrong. You are not overriding the equals in Object, but just overloading it.
To override the behavior of equals method in Object, your signature must exactly match with the one in Object. Try this:
public boolean equals(Object o) {
if(!(o instanceof Data)) return false;
Data other = (Data) o;
return (this.k == other.k && this.l == other.l);
}
In addition, as others suggested, it is a good idea to override hashCode method also for your object to work correctly in map based collections.

The answer from Makoto is right. The same i would say to. But you have some mistakes in your code above.
You wrote "public boolean equals(Date m){". I think, you meant Data instead of Date.
You wrote "if(this.k == m.k && this.l = m.l)". The second condition in if query have to be "==".
To your question:
Makoto's answer is one solution.
My solution is to use the help of eclipse to auto generate hashcode and equals methods. Like this:
public class Data {
// your class code here
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + k;
result = prime * result + l;
return result;
}
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (!(obj instanceof Data)) {
return false;
}
Data other = (Data) obj;
if (k != other.k) {
return false;
}
if (l != other.l) {
return false;
}
return true;
}
}

By convention you want to override hashcode also when you override equals
You will most probably find that the indexOf uses the hashcode method to match the object not the equals
If you use eclise to edit you code - eclipse will generate a good equals and hashcode method for you from the "source" menu.

How do I change what it means for objects to be duplicate in a set?

I want to store a set of Edges:
class Edge {
int u;
int v;
char symbol;
}
The problem is that it's possible for two Edge objects to have the same u, v and symbol, but they can both be stored in a HashSet because they're not the same object even though I want them to be considered the same object. How can I store only one object that has a unique (u, v, symbol) in a Set?

You need to override the following two methods equals and hashcode.
public boolean equals(Object obj) {
if (obj == null) return false;
if (!(obj instanceof Edge)) return false;
// return true if they are the same, otherwise false
}
public int hashCode() {
// return an int that represents similarity
// Example: name.hashCode(), if they are the same with the same name
}

Depends on what kind of set you want to use; The below applies for HashSet for instance, but not for any subclass of SortedSet
By overriding equals() and hashCode():
class Edge {
int u;
int v;
char symbol;
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + symbol;
result = prime * result + u;
result = prime * result + v;
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Edge other = (Edge) obj;
return symbol == other.symbol && u == other.u && v == other.v;
}
}

You have to override equals(). Like this:
public boolean equals(Object obj) {
//do the comparison here; remember to cast obj to Edge
}

HashMap in java cannot hash MyObject

I have defined a simple private class named SetOb which contains an int and a Set data structure. I have a HashMap in the 'main' method with SetOb as Key and Integer as value. Now as you can see in the main method, when I feed the HashMap with a SetOb instance and then look for an instance with exactly the same value, it returns 'null'. This has happened with me quite a few times before when I use my own defined data structures like SetOb as Key in HashMap. Can someone please point me what am I missing ?
Please note that in the constructor of SetOb class, I copy the Set passed as argument.
public class Solution {
public static Solution sample = new Solution();
private class SetOb {
public int last;
public Set<Integer> st;
public SetOb(int l , Set<Integer> si ){
last = l;
st = new HashSet<Integer>(si);
}
}
public static void main(String[] args) {
Map<SetOb, Integer> m = new HashMap< SetOb, Integer>();
Set<Integer> a = new HashSet<Integer>();
for(int i =0; i<10; i++){
a.add(i);
}
SetOb x = sample.new SetOb(100, a);
SetOb y = sample.new SetOb(100, a);
m.put(x,500);
Integer val = m.get(y);
if(val!= null) System.out.println("Success: " + val);
else System.out.println("Failure");
}
}

Your x and y are not the same object instances hence contains is not able to match y against x, which ends up not finding the matching key/value in the Map.
If you want the match to succeed, please implement(override) hasCode & equals method in SetOb which will compare the field values.
Sample methods(Eclipse generated) as below:
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + last;
result = prime * result + ((st == null) ? 0 : st.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;
SetOb other = (SetOb) obj;
if (last != other.last)
return false;
if (st == null) {
if (other.st != null)
return false;
} else if (!st.equals(other.st))
return false;
return true;
}

The default implementation of hashCode uses object identity to determine the hash code. You will need to implement hashCode (and equals) in your private class if you want value identity. For instance:
private class SetOb {
public int last;
public Set<Integer> st;
public SetOb(int l , Set<Integer> si ){
last = l;
st = new HashSet<Integer>(si);
}
#Override
public boolean equals(Object other) {
if (other.class == SetOb.class) {
SetOb otherSetOb = (SetOb) other;
return otherSetOb.last == last && otherSetOb.st.equals(st);
}
return false;
}
#Override
public int hashCode() {
return 37 * last + st.hashCode();
}
}

SetOb needs to override the hashCode() and thus the equals() methods.
Hash-based collections use these methods to store (hashCode()) and retrieve (hashCode()) and equals()) your objects.

Java: Problems with TreeSet

I have a class Odp. I want to use TreeSet to keep a sorted collection of Odp objects. However, I've been having problems.
public class OdpStorage {
private TreeSet<Odp> collection = new TreeSet<Odp>();
public addOdp(Odp o) {
return collection.add(o);
}
public int size() {
return collection.size();
}
}
collection.add(Odp o) is supposed to do nothing if it's already in the tree, right? Somehow, this unit test fails:
OdpStorage ts = new OdpStorage();
Odp ftw = new Odp("LOL");
Odp ktr = new Odp("OMG");
ts.addOdp(ftw);
ts.addOdp(ftw); //should do nothing
ts.addOdp(ftw); //should do nothing
ts.addOdp(ftw); //should do nothing
ts.addOdp(ktr);
assertEquals(2, ts.size());
The assertion fails. It expects 2, but the return value is 5. Why? Could the odp.equals() function be messed up?
Similarly, calling collection.contains(o) fails, even when the there is an object in the set X for which o.equals(X) returns true.
The .equals() function of Odp: (generated by Eclipse)
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (!(obj instanceof Odp))
return false;
Gene other = (Odp) obj;
if (sequence == null) {
if (other.sequence != null)
return false;
} else if (!sequence.equals(other.sequence))
return false;
return true;
}
compareTo:
/**
* this = g0
* if they are equal, g1 is presumed to come first
*
* #return -1 if g0 comes before g1; 1 if g0 comes after g1
*/
#Override
public int compareTo(Odp g1) {
if (sequence.length() < g1.getSeq().length()) {
return -1;
}
else if (sequence.length() > g1.getSeq().length()) {
return 1;
}
if (sequence.compareTo(g1.getSeq()) < 0) {
return -1;
}
return 1;
}
hashCode() is not overridden. Problem?
UPDATE
hashCode() is as follows:
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result
+ ((sequence == null) ? 0 : sequence.hashCode());
return result;
}
But that still doesn't solve the problem.

Your compareTo implementation never returns 0. It should return 0 when the object instances are equal.

It appears that your collection.add(o) is failing to find the object in the backing TreeMap. Does your Odp implement Comparable or are you setting a default Comparable on your TreeSet whose compare method you have implemented? If so, you will need to ensure that your compareTo (for the Comparable), or your Comparator compare method will return 0 if the objects passed in are equals.
EDIT (in response to your comment to the original post):
It is recommended that you override HashCode() whenever you override equals()
EDIT2 in response to your compareTo implementation:
If g0 and g1 are equal, you should return 0. This is the root of the problem.

Mate cleanup your equals, its got too many if/elses. replace it with a nice do/while with lots of condition tests. If all the tests pass then reutrn true...Yes its got "goto" statements but its very easy to read and even easier to insert new conditions as necessary without lots of nesting. Nesting if/elses is evil. Using "elses" is evil and almost always never needed.
#Override
public boolean equals(final Object object) {
boolean equals = false;
do {
if (this == object) {
equals = true;
break;
}
if (false == super.equals(object)) {
break;
}
final DocumentView view = Unsafe.cast(object);
if (false == this.document.equals(view.document)) {
break;
}
if (this.revision != view.revision) {
break;
}
if (false == this.user.equals(view.user)) {
break;
}
if (false == this.timestamp.equals(view.timestamp)) {
break;
}
equals = true;
} while (false);
return equals;
}