Collections

• Not fixed in size, that is growable in nature.
• Can hold heterogeneous objects as well as homogeneous objects.
• Every collection class is based on some standard data structure.
• Collections are more performance expensive.
• Collection can hold only objects, but not primitive.
• Collection is a group of individual objects as a single entity
• Collection framework defines several classes and interfaces that can be used to represent collection and perform operations on it.
• Key interfaces in collection framework are
• Collection
• Defines the most common methods Of collection framework like contains, empty, add, remove, et cetera
• Collection interface is considered as root interface of collection framework.
• There is no concrete class which implements collection interface directly.
• Collection versus Collections
• Collection is an interface. Collections is a class.
• Collection is a root interface of collection framework. Collections is a utility class Which defines several utility methods for collection object like sort.
• Collection interface can be used to represent a group of individual objects as a single entity.
• List interface
• Duplicate are allowed
• Insertion order is preserved
• Implementation classes are
• ArrayList
• LinkedList
• Vector
• Stack
• Vector and stack are also called as legacy classes as they came in 1.0 version.
• List is a child interface of collection
• Set interface
• Duplicates are not allowed
• Insertion order is not maintained
• Implementation classes are
• Hash set
• Linked hash set
• Set a child interface of collection
• Sorted set interface
• Sorted set is the child interface of Set interface
• If we want to represent a group of individual objects as a single entity, where duplicates are not allowed, but all objects are sorted According to some sorting order, then we should go for sorted set.
• Navigable Set
• It is a child interface of sorted set
• Implementation class is
• TreeSet
• Define methods for navigation purposes
• Queue
• Represents a group of objects prior to processing In first in first out manner.
• Implementation classes are
• Priority Queue
• Blocking Queue
• Linked blocking Queue
• Priority blocking queue
• Map Interface
• All the before interfaces, are meant for representing a group of individual objects.
• If we want to represent a group of objects as key value pairs, then we should go for a map interface.
• It is not a child interface of collection
• Duplicate keys are not allowed
• Duplicate values are allowed
• Implementation classes are
• Hashmap
• Linked Hashmap
• Weak Hashmap
• Identity Hashmap
• Hash table
• Properties
• Has table is the child of Dictionary.
• Hashtable is a legacy class
• Sorted map interface
• When we want to represent a group of values As key value pairs according to sorted value of keys.
• It is the child interface of map interface.
• Navigable map
• Child interface of sorted map.
• Defines navigation functions for sorted map
• Implementation classes are
• Treemap
• ArrayList
• ArrayList Is implemented with concept of dynamic array.
• In ArrayList Addition and removal of element takes O(n) notation And is expensive. Searching is, however, not expensive.
• LinkedList
• In linked list, the implementation is with the concept of double Link list.
• In link, list, addition and removal is in expensive, but searching for an element or traversing a node is expensive.
• Hashmap
• There are unique keys
• o(1) Operation for search, insert, and delete
• Maps, larger values to smaller values
• Should uniformly distribute large keys into a hash table slot
• Algorithm used
• For large keys, use modular algorithm
• For string keys Use weight Sum
• For objects Use weighted sum of elements
• The default, bucket size of Java hash map is 16
• Once the link list length, which is eight, then it is converted to a self balancing binary tree.
• Equals and hash code contract
• If two objects are equal according to the equals(object o) Method, then the hash code for both the object must be same(integer value)
• It is not necessary that if you have same hash code for two objects means those two objects are equal. This is collision.better has function prevents this.
• Whenever it is invoked on same object More than once during the execution of a Java application, the hash code method Must consistently return same integer.
• Example
• https://github.com/gauravmatta/javacodes/blob/415216abca2877eeac69b322cb2910a57a8502e7/PojoClasses/src/javaimplant/equalshashcode/StudentComparer.java