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Store java.util.Date as key in a Map based on reference equality instead of "value" equality


Store java.util.Date as key in a Map based on reference equality instead of "value" equality

By : Marco
Date : November 22 2020, 03:03 PM
Any of those help You should use java.util.IdentityHashMap for this task. That way you can have equals, but different objects in your Map.
EDIT: Your example:
code :
Date x = new Date();
Date y = new Date();

Map<Date,Integer> hm = new IndentityHashMap<Date,Integer>();
hm.put(x,1);
hm.put(y,3);
assert hm.size() == 2: hm.size();


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In which circumstances Java reference equality could be different to equals() equality for an object of a type which has

In which circumstances Java reference equality could be different to equals() equality for an object of a type which has


By : Jithendralal M C
Date : March 29 2020, 07:55 AM
will help you No. That's exactly the definition of Object.equals().
...this method returns true if and only if x and y refer to the same object (x == y has the value true) ...
code :
public boolean equals( Object o ) { 
   return this == o;
}
Does Hashcode equality imply refer reference based equality?

Does Hashcode equality imply refer reference based equality?


By : user3010894
Date : March 29 2020, 07:55 AM
wish help you to fix your issue Even though failure to override hashCode() will only break usage of your class in HashSet, HashMap, and other hashCode dependent structures, you should still override hashCode() to maintain the contract described by Object.
The general strategy of most hashCode() implementations is to combine the hash codes of the fields used to determine equality. In your case, a reasonable hashCode() may look something like this:
code :
public int hashCode(){
    return this.getName().hashCode() ^ this.getId() ^ this.getPassword().hashCode();
}
DrRacket: How can I store an "equality detector" in a compound variable?

DrRacket: How can I store an "equality detector" in a compound variable?


By : Ameen A J
Date : March 29 2020, 07:55 AM
it helps some times If you want to access parts of the world, you need to pass the world along:
code :
(define (get-commands w loo0)
   (local [(define (fn-for-loo todo rsf)
              (cond [(empty? todo) rsf]
                    [else (fn-for-obj (first todo)
                                      (rest todo)
                                      rsf)]))

           (define (fn-for-obj obj todo rsf)
              (if ((act-trigger (obj-act obj)) w)
                  (fn-for-loo todo (cons (act-command (obj-act obj)) rsf))
                  (fn-for-loo todo rsf)))]
      (fn-for-loo loo0 empty)))

(define OBJ1
  (make-obj 1
            (make-act (lambda (w) (= (world-var1 w) (world-var2 w)))
                      "start-rain")))

(define OBJ2
  (make-obj 2
            (make-act (lambda (w) (= (world-var1 w) (world-var3 w)))
                      "go-home")))
(define w0 (make-world 1 1 1 (list OBJ1 OBJ2)))

> (get-commands w0 (world-loo w0))
'("go-home" "start-rain")
(define (get-commands w loo0)
  (foldl (lambda (o rs) (if ((act-trigger (obj-act o)) w)
                            (cons (act-command (obj-act o)) rs)
                            rs))
         '()
         loo0))
(define (get-commands w loo0)
  (foldr (lambda (o rs) (if ((act-trigger (obj-act o)) w)
                            (cons (act-command (obj-act o)) rs)
                            rs))
         '()
         loo0))
(define (get-commands w loo0)
  (let ((triggered (filter (lambda (o) ((act-trigger (obj-act o)) w)) loo0)))
    (map (lambda (o) (act-command (obj-act o))) triggered)))
Whats the shortest way to definitely rewrite "loose equality logic" to "strict equality logic" ie: `

Whats the shortest way to definitely rewrite "loose equality logic" to "strict equality logic" ie: `


By : user3177066
Date : March 29 2020, 07:55 AM
To fix the issue you can do No, there is no automated way of rewriting all of the == in your code to === so that it works for every possible case.
You will have to bite the bullet and change them all to === and run your unit tests to discover what broke (you do have unit tests, right? Right!?). Or go through them one by one, making sure you understand the potential values so that the correct choice can be made. Or keep them as == and deal with the outliers.
When is it appropriate to use reference equality vs object equality in Java?

When is it appropriate to use reference equality vs object equality in Java?


By : Spark
Date : March 29 2020, 07:55 AM
Hope that helps That's not really "instead of" the equality check, but you could do the reference check before you do the equality check for performance and null-safety reasons.
In effect, this is what happens when you call Objects.equals(a,b) instead of a.equals(b).
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