Data structures and Java collections

data structures and java collections

Java collections are Java built-in libraries provided by JDK. You don’t have to implement your own data structures classes and methods. You can directly call the library. Before we list the Java Collections, the original data structures are introduced. There are arrays, linked lists, stacks, binary trees, hash tables, and graphs among others. Some may not be in the library (eg trees and graphs). If you know the differences, you can choose when to use the library and when to implement your own.

Table of Content

  1. Data structures
  2. Java Collections

1. Data structures

With data structure, we find ways to make data access more efficient. When dealing with data structure, we not only focus on one piece of data but also on different sets of data and how they can relate to one another in an organized manner.

Array: An array is an index-based data structure, which means every element is referred to by an index. An array holds the same data type elements.

data structure array diagram

Linked list: A linked list is a sequence of nodes in which each node is connected to the node following it. This forms a chain link of data storage. It consists of data elements and a reference to the next record.

data structure linkedlist diagram

Tree: A tree is a collection of nodes connected by edges. Each node points to several nodes. A tree represents the hierarchical graphic form.

data structure tree diagram

Binary tree: A binary tree has 1 or 2 nodes. It can have a minimum of zero nodes, which occurs when the nodes have NULL values.

data structure binary tree diagram

Binary search tree: A binary search tree (BST) is a binary tree. The left subtree contains nodes whose keys are less than the node’s key value, while the right subtree contains nodes whose keys are greater than or equal to the node’s key value. Moreover, both subtrees are also binary search trees. A binary search tree can retrieve data efficiently.

data structure binary search tree diagram

Matrix: A matrix is a double-dimensioned array. It makes use of two indexes rows and columns to store data.

data structure matrix diagram

Graph: A graph contains a set of nodes and edges. The nodes are also called vertices. Edges are used to connect nodes. Nodes are used to store and retrieve data.

data structure graph diagram

Hash table: A hash table is a data structure that can map keys to values. A hash table uses a hash function to compute a key into an integer (hash value), which indicates the index of the buckets (aka array). From the key, the correct value can be stored and found. The hash table is one of the most used data structures.

Java class HashMap diagram

Stack: a stack is a LIFO data structure in which only the top element can be accessed. The data is added by push and removed by pop on top.

data structure stack diagram

Queue: A queue is a FIFO data structure. In this structure, new elements are inserted at one end and existing elements are removed from the other end.

data structure queue diagram

Priority queue: A priority queue is a queue, with each element having a priority associated with it. The implementation can use ordered array, unordered array, and heap.

Java class PriorityQueue diagram

Max-Heap: A heap is a tree-based data structure in which all the nodes of the tree are in a specific order. Max-heap is a binary tree. It is complete. The data item stored in each node is greater than or equal to the data items stored in its children.

data structure max heap diagram

Min-Heap: Min-heap is a binary tree. It is complete. The data stored in each node is less than the data items stored in its children.

data structure min heap diagram

Trie: A Trie is a tree. In a trie, every node (except the root node) stores one character or a digit. By traversing the trie down from the root node to a particular node n, a common prefix of characters or digits can be formed which is shared by other branches of the trie as well.

data structure trie diagram

Suffix trie: Suffix trie is a trie containing all the suffixes of the given text. Suffix trie allows particularly fast implementations of many important string operations.

data structure suffix trie diagram

2. Java Collections

Java collection framework is the set of collection types that are included as part of core Java. It provides the APIs or methods that you can use directly to operate on the data structures such as Arrays, Linked Lists, stacks, queues, sets, and maps. If you master Java collections, it will save you tons of time and help to solve complex problems.

ArrayList: ArrayList class is a resizable array implementation of the List interface. It implements all optional list operations and permits all elements.

Java class ArrayList diagram

Vector: Vector is very similar to ArrayList but Vector is synchronized and slow. It is a legacy class. ArrayList is preferred.
String: String class is used to create and manipulate strings.

Java class String diagram

LinkedList: The linkedList class is a doubly-linked list implementation of the List and Deque interface. A LinkedList stores its data as a list of elements and every element is linked to its previous and next element.

Java class LinedList diagram

HashMap: A HashMap is a collection class that implements a Map interface. It requires a hash function and uses hashCode() and equals() methods, in order to put and retrieve elements to and from the collection respectively.

Java class HashMap diagram

Hashtable: The Hashtable class is similar to HashMap. It implements a Dictionary. Hashtable provides an Enumeration of its keys. It doesn’t allow null as a key or value. Please note that since HashMap was created later, it is an advanced version and improvement on the Hashtable. Hashtable is synchronized and slower. HashMap is preferred over Hashtable.
TreeMap: TreeMap implements the SortedMap interface. It is sorted in the ascending order of its keys. The complexity of the operations is O(logn).

Java class TreeMap diagram

LinkedHashMap: LinkedHashMap keeps the inserting order. The complexity is the same as HashMap O(1).

Java class LinkedHashMap diagram

HashSet: HashSet class implements Set interface. Duplicate values are not allowed. Its elements are not ordered. NULL elements are allowed in HashSet.

Java class HashSet diagram

TreeSet: TreeSet is implemented using a tree structure. The elements in a TreeSet are sorted. The complexity of operations is O(logn).

Java class TreeSet diagram

LinkedHashSet: LinkedHashSet maintains the insertion order. Elements get sorted in the same sequence in which they have been added to the Set. The complexity is the same as HashSet O(1).

Java class LinkedHashSet diagram

Stack: The stack class extends the Vector class with five operations to support LIFO (Last In First Out). Stack internally has a pointer: TOP, which refers to the top of the Stack element.

Java class Stack diagram

PriorityQueue: PriorityQueue class is the implementation of Queue. The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time.

Java class PriorityQueue diagram

What are the most used data structures in Java?

The most used data structures in Java are ArrayList and HashMap. Others include HastSet and PriorityQueue.

What are data structures in Java Collections?

·         ArrayList
·         LinedList
·         String
·         HashMap
·         TreeMap
·         LinkedHashMap
·         HashSet
·         Treeset
·         LinedHashSet
·         Stack
·         PriorityQueue

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