DATA STRUCTURES

Data structures are fundamental constructs around which you build your application. A data structure determines the way data is stored, and organized in your computer. Whenever data exists it must have some kind of data structure in order to be stored in a computer.

Contiguous or linked data structures

Data structures can be classified as either contiguous or linked, depending whether they are based on arrays or pointers(references):

Contiguous-allocated structures, are made of single slabs of memory, some of these data structures are arrays, matrices, heaps, and hash tables.

Linked data structures, are composed as distinct chunks of memory linked together by pointers (references). Some of this data structures are lists, trees, and graph adjacency lists.

Comparison

Some advantages of linked lists over static arrays are:

Overflow is more difficult to occur on a linked structures than it is in an array. It only happens when the memory is actually full.
Insertions and deletions are simpler than for contiguous data structures such as arrays.
Linked list don’t need to know size on initialization

Advantages of arrays:

Linked structures require allocating extra space for storing pointers.
Arrays allow efficient access to any item.

ARRAY

Arrays are the fundamental contiguously allocated data structure. They have a fixed size and each element can be efficiently located by its index. Imagine an array is like a street full of houses, one right next to each other, each house can be easily located by its address (index).

The following is an example of usage of Java's implementation of ArrayList, which is an Array that is resized when needed

					
               
    /*
    * We can determine at the moment of instantiation the capacity
    * of the	ArrayList this gives a little boost in performance, 
    * instead of making the array resize constantly
    */
      List<String> exampleList = new ArrayList<>(100);
       
    /*
     * Don't confuse capacity with size, the following 
     * statement outputs 0, as currently the ArrayList 
     * size is 0, but it's capacity is 100
     */
      System.out.println(exampleList.size());
   
      exampleList.add("first");
      exampleList.add("second");
      exampleList.add("third");
      
      System.out.println(exampleList.size());//prints 3

SET

A Set is a Collection that cannot contain duplicate elements.

In Java the Set interface contains methods inherited from Collection and adds the restriction that duplicate elements are prohibited. Java also adds a stronger contract on the behavior of the equals() and hashCode() methods, allowing Set instances to be compared meaningfully even if their implementation types differ. Some methods declared by Set are:

add( ) Adds an object to the collection
clear( ) Removes all objects from the collection
contains( ) Returns true if a specified object is an element within the collection
isEmpty( ) Returns true if the collection has no elements
iterator( ) Returns an Iterator object for the collection which may be used to retrieve an object
remove( ) Removes a specified object from the collection
size( ) Returns the number of elements in the collection

Let's use an example of Java's implementation of set to understand how it works. First, we need to define the DataType and override the equals() and hashCode() methods:

					
   public class DataType {
   	private String name;
   	private int number;
   	
   	public DataType(String name, int number){
   		this.name = name;
   		this.number = number;
   	}
   
   	@Override
   	public int hashCode() {
   		final int prime = 31;
   		int result = 1;
   		result = prime * result + ((name == null) ? 0 :
   		 name.hashCode());
   		return result;
   	}
   
   	    
      /*
      * We override method equals so that objects 
      * with same name can't be added to the set, 
      * but objects with same numbers can be added
      */
   	@Override
   	public boolean equals(Object obj) {
   		if (obj == null)
   			return false;
   		
   		if (this.getClass() != obj.getClass())
   			return false;
   		
   		DataType other = (DataType) obj;
   		if (other.name == null) 
   				return false;
   				
   		return this.name.equals(other.name);
   	}
   
   	@Override
   	public String toString() {
   		return "DataType: name=" + name + ", number=" + number;
   	}
   }

Now we can proceed to use the class

					
   Set<DataType> example = new HashSet<>();
   	
   DataType data1 = new DataType("first", 1);
       	
   //notice name repeated it's not valid
   DataType data2 = new DataType("first", 1);
       
   //notice different name but same number it's valid
   DataType data3 = new DataType("second", 1);
   	
   example.add(data1);
   example.add(data2);
   example.add(data3);
   
   for (DataType x : example){
   	System.out.println(x);
   }

the output is:

					
   DataType: name=first, number=1
   DataType: name=second, number=1

Multiset

A multiset is similar to a set but allows repeated values. For Java, third-party libraries provide multiset functionality

Apache Commons Collections provides the Bag and SortedBag interfaces, with implementing classes like HashBag and TreeBag.
Google Guava provides the Multiset interface, with implementing classes like HashMultiset and TreeMultiset.

This data structure is perfect for when we need to perform statistical data that needs no sorting, for example calculating the average or Standard Deviation of a multiset.

STACKS AND QUEUES

Arrays, Linked list, Trees are best use to represent real objects, Stacks & Queues are best to complete tasks, they are like a tool to complete and then discard.

They are useful to manage data in more a particular way than arrays and lists.

Queue are first in, first out (FIFO)
Stack are last in, first out (LIFO)

When to use stacks and queues:

Use a queue when you want to get things out in the order that you put them in.
Use a stack when you want to get things out in the reverse order than you put them in.
Use a list when you want to get anything out, regardless of when you put them in (and when you don't want them to automatically be removed).

The following is an implementation of Queues in Java, based on the course by R.Sedgewick in coursera

					
       
   /*This implementation uses a singly-linked list 
    * with a static nested class for linked-list nodes. 
    * All operations take constant O(1) time in the worst case.
    */
   public class Queue<Generic> implements Iterable<Generic> {
      	private Node<Generic> firstNode; // beginning of queue
      	private Node<Generic> lastNode; // end of queue
      	private int size; // number of elements on queue
      
      	private static class Node<Item> {
      		private Item item;
      		private Node<Item> next;
      	}
      
      	public Queue() {
      		firstNode = null;
      		lastNode = null;
      		size = 0;
      	}
      
      	public boolean isEmpty() {
      		return firstNode == null;
      	}
      
      	public int size() {
      		return size;
      	}
      
      	public Generic peek() {
      		if (isEmpty())
      			throw new NoSuchElementException("Queue underflow");
      		return firstNode.item;
      	}
      
      	public void enqueue(Generic item) {
      		Node<Generic> oldlast = lastNode;
      		lastNode = new Node<Generic>();
      		lastNode.item = item;
      		lastNode.next = null;
      		if (isEmpty())
      			firstNode = lastNode;
      		else
      			oldlast.next = lastNode;
      		size++;
      	}
      
      	public Generic dequeue() {
      		if (isEmpty())
      			throw new NoSuchElementException("Queue underflow");
      		Generic item = firstNode.item;
      		firstNode = firstNode.next;
      
      		if (isEmpty())
      			lastNode = null;// to avoid loitering
      		size--;
      		return item;
      	}
      
      	public String toString() {
      		StringBuilder s = new StringBuilder();
      		for (Generic item : this)
      			s.append(item + " ");
      		return s.toString();
      	}
      
      	public Iterator<Generic> iterator() {
      		return new ListIterator<Generic>(firstNode);
      	}
      
      	private class ListIterator<Item> implements Iterator <Item> {
      		private Node <Item> current;
      
      		public ListIterator(Node<Item> first) {
      			current = first;
      		}
      
      		public boolean hasNext() {
      			return current != null;
      		}
      
      		public void remove() {
      			throw new UnsupportedOperationException();
      		}
      
      		public Item next() {
      			if (!hasNext())
      				throw new NoSuchElementException();
      			Item item = current.item;
      			current = current.next;
      			return item;
      		}
      	}
      
      	public static void main(String[] args) {
      		Queue<String> q = new Queue<>();
      		q.enqueue("FIRST IN");
      		q.enqueue(" 2nd ");
      		q.enqueue(" 3rd ");
      
      		System.out.println(q.dequeue() + " first out ==> FIFO");
      	}
   }

DICTIONARIES

A Dictionary is a data structure that maps a key to a value.This is useful in cases where you want to be able to access data via a particular key rather than an integer index.

In Java, Dictionaries are implemented as a Map: The Map interface maps unique keys to values. A key is an object that you use to retrieve a value at a later date.

Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key.

Following is a simple Map Implementation as an array. Firstly, we create a class to help store the key and it's value in an object:

					
   public class Entry<K, V> {
      private final K key;
      private V value;
      
      public Entry(K key, V value) {
      	this.key = key;
      	this.value = value;
      }
      
      public K getKey() {
      	return key;
      }
      
      public V getValue() {
      	return value;
      }
      
      public void setValue(V value) {
      	this.value = value;
      }
   }

Then the implementation of map:

					
   public class Map<K, V> {
      private int size;
      private int CAPACITY = 16;
      private Entry<K, V>[] entriesArray = new Entry[CAPACITY];
      
      public void put(K key, V value) {
      	boolean insert = true;
      	for (int i = 0; i < size; i++) {
      		if (entriesArray[i].getKey().equals(key)) {
      			entriesArray[i].setValue(value);
      			insert = false;
      		}
      	}
      	if (insert) {
      		growArray();
      		entriesArray[size++] = new Entry<K, V>(key, value);
      	}
      }
      
      private void growArray() {
      	if (size == entriesArray.length) {
      		int newSize = entriesArray.length * 2;
      		entriesArray = Arrays.copyOf(entriesArray, newSize);
      	}
      }
      
      public V get(K key) {
      	for (int i = 0; i < size; i++) {
      		if (entriesArray[i] != null) {
      			if (entriesArray[i].getKey().equals(key)) {
      				return entriesArray[i].getValue();
      			}
      		}
      	}
      	return null;
      }
      
      public void remove(K key) {
      	for (int i = 0; i < size; i++) {
      		if (entriesArray[i].getKey().equals(key)) {
      			entriesArray[i] = null;
      			size--;
      			condenseArrayElements(i);
      		}
      	}
      }
          
       //Moves backwards elements from start arg
       private void condenseArrayElements(int start){ 
        for (int i = start; i < size; i++) {
      	  entriesArray[i] = entriesArray[i+1];
        }
       }
       
       public int size(){ return size; }
      
        public Set<K> keySet(){
        Set<K> set = new HashSet<K>();
        for (int i = 0; i < size; i++) {
         set.add(entriesArray[i].getKey());
        }
        return set;
       }
      
        public static void main(String[] args) {
      	  Map<String, Integer> mapExample = new Map<>();
      	   mapExample.put("Key 1", 100);
      	  System.out.println(mapExample.get("Key 1"));
      	  
      	  mapExample.put("Key 2", 200);
      	  mapExample.put("Woaah", 100000);
      	  System.out.println(mapExample.get("Key 2"));
      	    
      	  System.out.println("keySet: " + mapExample.keySet());
      	 
      	  System.out.print("Values: ");
      	  for (String key : mapExample.keySet()){
      	   System.out.print(mapExample.get(key) + " ");
      	  }
      	    mapExample.remove("Key 2");
      	  System.out.println("\nkeySet: " + mapExample.keySet());
      	 }
   }