Full Answer
Do you override gethashcode when working on custom objects?
As you know, GetHashCode returns a semi-unique value that can be used to identify an object instance in a collection. As a good practice, it is recommended to override this method and implement your own. My question is - do you override this method when working on custom objects? If so, what algorithm do you use to generate the unique ID?
What are the rules of gethashcode in Java?
The GetHashCode () method should reflect the Equals logic; the rules are: if two things are equal ( Equals (...) == true) then they must return the same value for GetHashCode () if the GetHashCode () is equal, it is not necessary for them to be the same; this is a collision, and Equals will be called to see if it is a real equality or not.
What happens if gethashcode() is equal to return fooid?
if the GetHashCode () is equal, it is not necessary for them to be the same; this is a collision, and Equals will be called to see if it is a real equality or not. In this case, it looks like " return FooId; " is a suitable GetHashCode () implementation.
Can gethashcode mutate an object?
In particular, note the section "Rule: the integer returned by GetHashCode must never change while the object is contained in a data structure that depends on the hash code remaining stable." He writes that "It is permissible, though dangerous, to make an object whose hash code value can mutate as the fields of the object mutate." – drf
Should you override GetHashCode?
If you're implementing a reference type, you should consider overriding the Equals method if your type looks like a base type, such as Point, String, BigNumber, and so on. Override the GetHashCode method to allow a type to work correctly in a hash table.
Why override GetHashCode when Equals method is overridden?
It is because the framework requires that two objects that are the same must have the same hashcode. If you override the equals method to do a special comparison of two objects and the two objects are considered the same by the method, then the hash code of the two objects must also be the same.
What should GetHashCode return?
GetHashCode returns a value, based on the current instance, that is suited for hashing algorithms and data structures such as a hash table. Two objects that are the same type and are equal must return the same hash code to ensure that instances of the following types work correctly: System. Collections.
What method should be overridden as well to ensure that Foo works correctly in a hash table?
So, if in our class we override equals() we should override hashcode() method also to follow this rule. Both methods, equals() and hashcode() , are used in Hashtable , for example, to store values as key-value pairs.
What happens if we override only equals?
Overriding only equals() method without overriding hashCode() causes the two equal instances to have unequal hash codes, which violates the hashCode contract (mentioned in Javadoc) that clearly says, if two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two ...
What is GetHashCode used for?
The GetHashCode method provides this hash code for algorithms that need quick checks of object equality. For information about how hash codes are used in hash tables and for some additional hash code algorithms, see the Hash Function entry in Wikipedia. Two objects that are equal return hash codes that are equal.
What is GetHashCode in IEqualityComparer?
GetHashCode. This method is used to generate an integer hash code for the single provided value or object. It is essential that the hash codes for two values that are deemed equal by the Equals method are the same. In some cases the method can return the hash code generated by the GetHashCode method of the argument.
Is string GetHashCode unique?
If two string objects are equal, the GetHashCode method returns identical values. However, there is not a unique hash code value for each unique string value. Different strings can return the same hash code. The hash code itself is not guaranteed to be stable.
Why should we override equals method in C#?
For a value type, you should always override Equals, because tests for equality that rely on reflection offer poor performance. You can also override the default implementation of Equals for reference types to test for value equality instead of reference equality and to define the precise meaning of value equality.
Can we override and return same hashCode for two objects?
1) If two objects are equal (i.e. the equals() method returns true), they must have the same hashcode. 2) If the hashCode() method is called multiple times on the same object, it must return the same result every time. 3) Two different objects can have the same hash code.
How do you correctly override the hashCode () and equals () methods?
if two objects are equal, then their hashCode values must be equal as well....For the Best practice use below steps to implement your equals() method:Use this == that to check reference equality.Use instanceof to test for correct argument type.Cast the argument to the correct type.Compare significant fields for equality.
Can two unequal objects have same hashCode?
Unequal objects must have different hash codes – WRONG! Objects with the same hash code must be equal – WRONG!
Why should we override equals method in C#?
For a value type, you should always override Equals, because tests for equality that rely on reflection offer poor performance. You can also override the default implementation of Equals for reference types to test for value equality instead of reference equality and to define the precise meaning of value equality.
Is string GetHashCode unique?
If two string objects are equal, the GetHashCode method returns identical values. However, there is not a unique hash code value for each unique string value. Different strings can return the same hash code. The hash code itself is not guaranteed to be stable.
What is the difference between == and equals in C#?
In C#, the equality operator == checks whether two operands are equal or not, and the Object. Equals() method checks whether the two object instances are equal or not. Internally, == is implemented as the operator overloading method, so the result depends on how that method is overloaded.
What is IEquatable in C#?
IEquatable - Provides an equality check when there is only one way of comparing the objects (implemented inside the class) IEqualityComparer - Allows you to define and use multiple equality checks (implemented outside the class) This protects both the caller and callee from passing incompatible object types.
Introduction
Equals method is intended to return true when another object is supplied which is semantically equal to current instance. GetHashCode method is intended to return an integer value which can be used as a hash code, i.e. key that accompanies the object when object is stored in a hashed data structure.
Default Implementation
Default implementation of Equals method for reference types simply compares object references, rather than their contents. Conversely, default implementation of GetHashCode makes no guarantees that any two instances that return true when compared using their Equal methods will return the same hash code.
Custom Implementation Goals
When reference types have semantic meaning, their implementation should include overrides for GetHashCode and Equals methods.
Custom Implementation Details
In this section we will consider practices to apply when overriding Equals and GetHashCode methods in derivable classes. These are general rules for Equals method, some of them basically implementing equality relation as defined in mathematics:
Final Solution
Main problem that has been identified in the previous section is related to derived classes. Each derived class may or may not add significant properties that it takes into account when comparing with other objects for equality. These are typical calling scenarios for the Equals method:
Conclusion
In this article we have discussed issues that arise when class with custom implementation of GetHashCode and Equals methods is being extended in derived classes. Basic idea behind the solution is to keep it simple, and stick with mathematical principles that define equality relation.