In Java, a hash code is an integer value that is associated with each object. Its main purpose is to facilitate hashing in hash tables, which are used by data structures like HashMap. The hashCode() method in Java is used to compute hash values of Java objects. It returns an integer whose value represents the hash value of the input object. The hashCode() method is used to generate the hash values of objects, and using these hash values, these objects are stored in Java collections such as HashMap, HashSet, and HashTable.

The hashCode() method is defined in the Object class in Java, and it computes the hash values of given input objects. This method is implemented by default in the Object class and is, therefore, inherited by user-defined classes as well. The hash code value of an object is returned by calling the hashCode() method on that object. The hashCode() method returns the same integer value (when called on the same object during the same instance of a Java application), provided that no data used by the equals() method is modified. When hashCode() is called on two separate objects (which are equal according to the equals() method), it returns the same hash code value. However, if it is called on two unequal objects, it will not necessarily return different integer values.

The hash code is used for bucketing in hash implementations like HashMap, HashTable, HashSet, etc. The value received from hashCode() is used as the bucket number for storing elements of the set/map. This bucket number is the address of the element inside the set/map. When you do a lookup for an element in a HashMap, HashSet, or HashTable, the hash code of the key is computed and based on that bucket number, the elements are searched in the set/map.

In summary, a hash code in Java is an integer value associated with each object that is used to facilitate hashing in hash tables, which are used by data structures like HashMap. The hashCode() method in Java is used to compute hash values of Java objects, and it returns an integer whose value represents the hash value of the input object. The hash code is used for bucketing in hash implementations like HashMap, HashTable, HashSet, etc.