Questions
12 of 17
What is the significance of Equals() and GetHashCode() in C#/Java when using objects as keys?
Difficulty: 3/10
Equality and Hash Codes for Keys
When objects are used as Hash Table keys, equality and hashing must obey a consistent contract. In Java, if two objects are equal according to equals(), they must return the same hashCode(). In C#, Equals() and GetHashCode() must similarly agree: objects considered equal must have identical hash codes.
javascript
- 1
Equal keys must produce equal hash codes.
- 2
Unequal keys may still produce the same hash code because collisions are allowed.
- 3
The fields used for equality and hashing should remain stable while the key is stored.
- 4
Breaking the equality/hash contract can cause failed lookups or duplicate logical keys.
- 5
When overriding equality, the corresponding hash implementation must also be updated.
Follow-up Questions
- What happens if equals() is overridden without hashCode()?
- Why are mutable fields dangerous in keys?
- Can unequal objects have the same hash code?