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HashMap containsKey() returns false although hashCode() and equals() are true


What issues should be considered when overriding equals and hashCode in Java?What issues should be considered when overriding equals and hashCode in Java?Java HashMap with overridden hashCode() and equals() returns no dataJava - HashMap and HashSet not backed by Object.hashCode()?java: LinkedHashMap containsKey=true but get returns nullHashMap with incorrect equals and HashCode implementationNumber of calls of hashCode() and equals() in case of HashSet.contains() if hashcode returns a constant valueBoolean hashCode return valueoverriding equals issue in hashmapDifferent behaviors for HashMap and Hashtable when Equals overridden to always return FalseWhy is 'equals' in Java true with different hashcodes?













6















I have a HashMap<Vertex, Integer> called vertexIndexes. If I iterate through it with this code:



public boolean search(String vertexName)
 for (Vertex name: vertexIndexes.keySet()) 
 String key = name.toString();
 String value = vertexIndexes.get(name).toString(); 
 System.out.println(key + " " + value + " "+ (name.hashCode() == vertexName.hashCode()) + " " + name.equals(vertexName)); 
 
...



it produces this output:



Diessen 0 false false
Herrsching 5 false false
Schondorf 2 false false
Greifenberg 3 false false
Stegen 4 false false
Utting 1 false false
Andechs 6 false false
Fischen 7 true true


So you can see, that the Vertex Fischen is present and the hashCode and equals methods work fine. But if I run



vertexIndexes.containsKey("Fischen")


it returns false.



Why is that? I lose my mind over it.






java hashmap







share|improve this question



















  • 12





    So the keys are instances of Vertex, not of String? But the key you are checking for is a String. A String will never be equal to anything but another String.

    – khelwood
    yesterday












  • As you can see in my output (..true true..): I implemented the toString() and equals method of Vertex to correctly compare it to a String.

    – gutenmorgenuhu
    yesterday






  • 9





    toString() is irrelevant, and even if your Vertex instance claims to be equal to a String, the String instance will not say it is equal to a Vertex.

    – khelwood
    yesterday












  • @khelwood unless in a very alternative implementation of an equals method :)

    – Stultuske
    yesterday






  • 1





    Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?

    – Lino
    yesterday















6















I have a HashMap<Vertex, Integer> called vertexIndexes. If I iterate through it with this code:



public boolean search(String vertexName)
 for (Vertex name: vertexIndexes.keySet()) 
 String key = name.toString();
 String value = vertexIndexes.get(name).toString(); 
 System.out.println(key + " " + value + " "+ (name.hashCode() == vertexName.hashCode()) + " " + name.equals(vertexName)); 
 
...



it produces this output:



Diessen 0 false false
Herrsching 5 false false
Schondorf 2 false false
Greifenberg 3 false false
Stegen 4 false false
Utting 1 false false
Andechs 6 false false
Fischen 7 true true


So you can see, that the Vertex Fischen is present and the hashCode and equals methods work fine. But if I run



vertexIndexes.containsKey("Fischen")


it returns false.



Why is that? I lose my mind over it.






java hashmap







share|improve this question



















  • 12





    So the keys are instances of Vertex, not of String? But the key you are checking for is a String. A String will never be equal to anything but another String.

    – khelwood
    yesterday












  • As you can see in my output (..true true..): I implemented the toString() and equals method of Vertex to correctly compare it to a String.

    – gutenmorgenuhu
    yesterday






  • 9





    toString() is irrelevant, and even if your Vertex instance claims to be equal to a String, the String instance will not say it is equal to a Vertex.

    – khelwood
    yesterday












  • @khelwood unless in a very alternative implementation of an equals method :)

    – Stultuske
    yesterday






  • 1





    Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?

    – Lino
    yesterday













6












6








6


0






I have a HashMap<Vertex, Integer> called vertexIndexes. If I iterate through it with this code:



public boolean search(String vertexName)
 for (Vertex name: vertexIndexes.keySet()) 
 String key = name.toString();
 String value = vertexIndexes.get(name).toString(); 
 System.out.println(key + " " + value + " "+ (name.hashCode() == vertexName.hashCode()) + " " + name.equals(vertexName)); 
 
...



it produces this output:



Diessen 0 false false
Herrsching 5 false false
Schondorf 2 false false
Greifenberg 3 false false
Stegen 4 false false
Utting 1 false false
Andechs 6 false false
Fischen 7 true true


So you can see, that the Vertex Fischen is present and the hashCode and equals methods work fine. But if I run



vertexIndexes.containsKey("Fischen")


it returns false.



Why is that? I lose my mind over it.






java hashmap







share|improve this question
















I have a HashMap<Vertex, Integer> called vertexIndexes. If I iterate through it with this code:



public boolean search(String vertexName)
 for (Vertex name: vertexIndexes.keySet()) 
 String key = name.toString();
 String value = vertexIndexes.get(name).toString(); 
 System.out.println(key + " " + value + " "+ (name.hashCode() == vertexName.hashCode()) + " " + name.equals(vertexName)); 
 
...



it produces this output:



Diessen 0 false false
Herrsching 5 false false
Schondorf 2 false false
Greifenberg 3 false false
Stegen 4 false false
Utting 1 false false
Andechs 6 false false
Fischen 7 true true


So you can see, that the Vertex Fischen is present and the hashCode and equals methods work fine. But if I run



vertexIndexes.containsKey("Fischen")


it returns false.



Why is that? I lose my mind over it.






java hashmap




java hashmap






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited yesterday









Eran

290k37478563




290k37478563










asked yesterday









gutenmorgenuhugutenmorgenuhu

1,67011229




1,67011229







  • 12





    So the keys are instances of Vertex, not of String? But the key you are checking for is a String. A String will never be equal to anything but another String.

    – khelwood
    yesterday












  • As you can see in my output (..true true..): I implemented the toString() and equals method of Vertex to correctly compare it to a String.

    – gutenmorgenuhu
    yesterday






  • 9





    toString() is irrelevant, and even if your Vertex instance claims to be equal to a String, the String instance will not say it is equal to a Vertex.

    – khelwood
    yesterday












  • @khelwood unless in a very alternative implementation of an equals method :)

    – Stultuske
    yesterday






  • 1





    Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?

    – Lino
    yesterday












  • 12





    So the keys are instances of Vertex, not of String? But the key you are checking for is a String. A String will never be equal to anything but another String.

    – khelwood
    yesterday












  • As you can see in my output (..true true..): I implemented the toString() and equals method of Vertex to correctly compare it to a String.

    – gutenmorgenuhu
    yesterday






  • 9





    toString() is irrelevant, and even if your Vertex instance claims to be equal to a String, the String instance will not say it is equal to a Vertex.

    – khelwood
    yesterday












  • @khelwood unless in a very alternative implementation of an equals method :)

    – Stultuske
    yesterday






  • 1





    Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?

    – Lino
    yesterday







12




12





So the keys are instances of Vertex, not of String? But the key you are checking for is a String. A String will never be equal to anything but another String.

– khelwood
yesterday






So the keys are instances of Vertex, not of String? But the key you are checking for is a String. A String will never be equal to anything but another String.

– khelwood
yesterday














As you can see in my output (..true true..): I implemented the toString() and equals method of Vertex to correctly compare it to a String.

– gutenmorgenuhu
yesterday





As you can see in my output (..true true..): I implemented the toString() and equals method of Vertex to correctly compare it to a String.

– gutenmorgenuhu
yesterday




9




9





toString() is irrelevant, and even if your Vertex instance claims to be equal to a String, the String instance will not say it is equal to a Vertex.

– khelwood
yesterday






toString() is irrelevant, and even if your Vertex instance claims to be equal to a String, the String instance will not say it is equal to a Vertex.

– khelwood
yesterday














@khelwood unless in a very alternative implementation of an equals method :)

– Stultuske
yesterday





@khelwood unless in a very alternative implementation of an equals method :)

– Stultuske
yesterday




1




1





Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?

– Lino
yesterday





Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?

– Lino
yesterday












1 Answer
1






active

oldest

votes


















39














name.equals(vertexName) compares a Vertex to a String. While your Vertex class equals method might return true when you pass a String to it, String's equals will never return true when you pass a Vertex to it.



HashMap probably tests whether vertexName.equals(name), which returns false.



Change



vertexIndexes.containsKey("Fischen")


to



vertexIndexes.containsKey(new Vertex("Fischen"))


or change the key of your Map to String.



BTW, you could have avoided that issue in the first place if you followed the contract of the equals method that appears in the Javadoc of the Object class:




• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.




Your equals implementation is not symmetric.






share|improve this answer
























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    1 Answer
    1






    active

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    active

    oldest

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    active

    oldest

    votes









    39














    name.equals(vertexName) compares a Vertex to a String. While your Vertex class equals method might return true when you pass a String to it, String's equals will never return true when you pass a Vertex to it.



    HashMap probably tests whether vertexName.equals(name), which returns false.



    Change



    vertexIndexes.containsKey("Fischen")


    to



    vertexIndexes.containsKey(new Vertex("Fischen"))


    or change the key of your Map to String.



    BTW, you could have avoided that issue in the first place if you followed the contract of the equals method that appears in the Javadoc of the Object class:




    • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.




    Your equals implementation is not symmetric.






    share|improve this answer





























      39














      name.equals(vertexName) compares a Vertex to a String. While your Vertex class equals method might return true when you pass a String to it, String's equals will never return true when you pass a Vertex to it.



      HashMap probably tests whether vertexName.equals(name), which returns false.



      Change



      vertexIndexes.containsKey("Fischen")


      to



      vertexIndexes.containsKey(new Vertex("Fischen"))


      or change the key of your Map to String.



      BTW, you could have avoided that issue in the first place if you followed the contract of the equals method that appears in the Javadoc of the Object class:




      • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.




      Your equals implementation is not symmetric.






      share|improve this answer



























        39












        39








        39







        name.equals(vertexName) compares a Vertex to a String. While your Vertex class equals method might return true when you pass a String to it, String's equals will never return true when you pass a Vertex to it.



        HashMap probably tests whether vertexName.equals(name), which returns false.



        Change



        vertexIndexes.containsKey("Fischen")


        to



        vertexIndexes.containsKey(new Vertex("Fischen"))


        or change the key of your Map to String.



        BTW, you could have avoided that issue in the first place if you followed the contract of the equals method that appears in the Javadoc of the Object class:




        • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.




        Your equals implementation is not symmetric.






        share|improve this answer















        name.equals(vertexName) compares a Vertex to a String. While your Vertex class equals method might return true when you pass a String to it, String's equals will never return true when you pass a Vertex to it.



        HashMap probably tests whether vertexName.equals(name), which returns false.



        Change



        vertexIndexes.containsKey("Fischen")


        to



        vertexIndexes.containsKey(new Vertex("Fischen"))


        or change the key of your Map to String.



        BTW, you could have avoided that issue in the first place if you followed the contract of the equals method that appears in the Javadoc of the Object class:




        • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.




        Your equals implementation is not symmetric.







        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited yesterday

























        answered yesterday









        EranEran

        290k37478563




        290k37478563





























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