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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

1

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

edited Mar 26 at 10:12

Eran

291k37481564

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

• 
  
  
  

  
  13
  

  
  
  
  
  
  

  
  
  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
  Mar 26 at 10:03
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Mar 26 at 10:05
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 26 at 10:06
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @khelwood unless in a very alternative implementation of an equals method :)
  
  – Stultuske
  Mar 26 at 10:08
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?
  
  – Lino
  Mar 26 at 10:10
  

  
  
  
  

 | 
show 1 more comment

6

1

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

edited Mar 26 at 10:12

Eran

291k37481564

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

• 
  
  
  

  
  13
  

  
  
  
  
  
  

  
  
  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
  Mar 26 at 10:03
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Mar 26 at 10:05
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 26 at 10:06
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @khelwood unless in a very alternative implementation of an equals method :)
  
  – Stultuske
  Mar 26 at 10:08
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Possible duplicate of What issues should be considered when overriding equals and hashCode in Java?
  
  – Lino
  Mar 26 at 10:10
  

  
  
  
  

 | 
show 1 more comment

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

edited Mar 26 at 10:12

Eran

291k37481564

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

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)); 
 
...

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

vertexIndexes.containsKey("Fischen")

share|improve this question

edited Mar 26 at 10:12

Eran

291k37481564

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

share|improve this question

edited Mar 26 at 10:12

Eran

291k37481564

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

edited Mar 26 at 10:12

Eran

291k37481564

edited Mar 26 at 10:12

Eran

291k37481564

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

asked Mar 26 at 10:01

gutenmorgenuhugutenmorgenuhu

1,67311229

1 Answer
1

active

oldest

votes

41

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

edited Mar 26 at 10:24

answered Mar 26 at 10:05

EranEran

291k37481564

add a comment | 

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41

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

edited Mar 26 at 10:24

answered Mar 26 at 10:05

EranEran

291k37481564

add a comment | 

41

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

edited Mar 26 at 10:24

answered Mar 26 at 10:05

EranEran

291k37481564

add a comment | 

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

edited Mar 26 at 10:24

answered Mar 26 at 10:05

EranEran

291k37481564

vertexIndexes.containsKey("Fischen")

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

share|improve this answer

edited Mar 26 at 10:24

answered Mar 26 at 10:05

EranEran

291k37481564

answered Mar 26 at 10:05

EranEran

291k37481564

answered Mar 26 at 10:05

EranEran

291k37481564