Many collections in Java use the hashCode() method to efficiently store and retrieve elements. The most common collections that use hashCode() are:
HashMap: HashMap is a hash table-based implementation of the Map interface. It uses the hashCode() method of keys to store and retrieve values in the underlying hash table.
HashSet: HashSet is a hash table-based implementation of the Set interface. It uses the hashCode() method of elements to store and retrieve elements in the underlying hash table.
Hashtable: Hashtable is a synchronized hash table-based implementation of the Map interface. It also uses the hashCode() method of keys to store and retrieve values in the underlying hash table.
IdentityHashMap: IdentityHashMap is a hash table-based implementation of the Map interface that uses reference equality instead of object equality when comparing keys. It also uses the hashCode() method of keys to store and retrieve values in the underlying hash table.
LinkedHashMap: LinkedHashMap is a hash table-based implementation of the Map interface that maintains the order of insertion of elements. It also uses the hashCode() method of keys to store and retrieve values in the underlying hash table.
WeakHashMap: WeakHashMap is a hash table-based implementation of the Map interface that uses weak references to keys. It also uses the hashCode() method of keys to store and retrieve values in the underlying hash table.
In general, any collection that stores its elements in a hash table or a hash-based data structure is likely to use the hashCode() method to determine the position of an element in the table.
If we don’t override hashcode() while using an object in hashing collection, what will be the impact?
If we don’t override the hashCode() method of an object while using it in a hash-based collection, such as HashMap or HashSet, the default implementation of the hashCode() method inherited from the Object class will be used. The default implementation of hashCode() returns a unique hash code for each object based on its memory address.
The problem with relying on the default implementation of hashCode() is that it does not take into account the contents of the object, which means that two objects with the same contents may have different hash codes. This can lead to incorrect behavior when using hash-based collections, such as:
Duplicate elements: If two objects with the same contents have different hash codes, they will be considered different by the hash-based collection, even though they should be considered the same. This can lead to duplicate elements in the collection.
Inefficient lookups: If the hash codes of elements in the collection are not evenly distributed, the hash-based collection may become unbalanced, with some buckets having too many elements and others having too few. This can lead to inefficient lookups, as the collection may need to traverse many elements in a bucket to find the one being looked up.
Therefore, it is important to override the hashCode() method when using an object in a hash-based collection, and ensure that it returns a unique hash code based on the contents of the object, in order to avoid these problems and ensure the correct functioning of the collection.
Do I need to override object’s equals() and hashcode() method for its use in a TreeMap?
Yes, you need to override an object’s equals() and hashCode() methods when using it in a TreeMap. This is because TreeMap uses the natural ordering of elements, which is determined by the compareTo() method of the objects, to maintain a sorted order of elements.
When adding elements to a TreeMap, the keys (which are the elements themselves) are compared using their natural ordering. If two keys are equal according to their compareTo() method, then TreeMap will consider them equal and will only keep one of them.
The equals() and hashCode() methods are used by TreeMap to determine if the two keys are equal. If these methods are not overridden, the default implementation inherited from the Object class will be used, which only considers two objects equal if they are the same object in memory.
Therefore, if you want to use an object as a key in a TreeMap, you should override its equals() and hashCode() methods to ensure that two objects with the same content are considered equal by the TreeMap. This will ensure that TreeMap maintains a consistent ordering of elements and that duplicate elements are not added.