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Java Sorting

Sorting Algorithms in Java Merge Sort in Java Quick Sort in Java Bubble Sort in Java Insertion Sort in Java Selection Sort in Java Heap Sort in Java Radix Sort in Java Topological Sort in Java Bucket Sort in Java Counting Sort in Java

Java OOPs Concepts

OOPs - Object Oriented Programming Objects and Classes in Java Methods in Java Java Naming Conventions Constructors in Java Java this keyword Java static keyword Inheritance in Java Aggregation in Java Java super keyword Constructor Chaining and Constructor Overloading Java Polymorphism Static and Dynamic Binding in Java Java Abstraction Abstract class in Java Interface in Java Difference between Abstract class and Interface Java final keyword Packages in Java Access Modifiers in Java Java Wrapper classes Java Numbers Java Characters Java Integer Java Boolean Java Arrays Java Command Line Arguments Java strictfp Keyword Java Math

Java Strings

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Java Exceptions

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Automatic Resource Management in Java Java Garbage Collection Java finalize() Java gc() Difference between final, finally and finalize

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Java Serialization Java transient

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Java JDBC

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How to

How to convert String to String array in Java How to find length of integer in Java How to get Day Name from Date in Java How to open the Java control panel How to resolve Illegal state exceptions in Java How to reverse a linked list in java How to encrypt password in Java How to calculate time complexity of any program in Java How to check version of java in Linux How to enable java in chrome How to run Java program How to set path in Java How to check the Java version in cmd How to install Java in Windows 10 How to run Java program in cmd How to add double quotes in a string in Java How to convert list to String in Java How to call a method in Java How to Set Environment Variables for Java How to sort an array in Java How to Create Immutable Classes in Java How to iterate HashMap in Java How to write Java program How to create an array in Java How to create a package in Java How to generate random numbers in Java How to input String in Java How to Create Singleton Class in Java How to create thread in Java How to find length of String in Java How to sort a string in Java How to use scanner in Java How to achieve multiple inheritance in Java How to find the length of an Array in Java How to Read Java JSON file How to run Java program in Eclipse How to call a function in Java How to create array of objects in Java How to create custom exception in Java How to import packages in Java How to run applet Program in Java How to take Array Input in Java How to achieve abstraction in Java How to call static method in Java How to compare characters in Java How to compare dates in Java How to create a linked list in Java How to download Eclipse for Java How to get ASCII value of char in Java How to get the current date and time in Java How to handle NullPointerException in Java How to initialize string array in Java How to Install Java on MAC How to run java program in ubuntu How to find characters with the maximum number of times in a string java How to Set Java_home in Linux How to Split the String in Java with Delimiter How to take Multiple String Input in Java using Scanner class How to uninstall the Java in Ubuntu How to Update Java How to add Elements in Array in Java How to avoid deadlock in java How to Split String by Comma in Java How to remove special characters from String in Java How to remove last character from String in Java How to Read XML Files in Java How to download and install Eclipse in Windows How to Create an API in Java How to Round Double Float up to Two Decimal Places in Java How to create a mirror image of a 2D array in Java How to set timer in Java

Java 8 Features

Java 8 Features Lambda Expressions in Java Functional Interface in Java Streams in Java Java Base64 Encoding and Decoding Type Annotations in Java Java Stringjoiner Class Parallel Arrays Sort in Java Bifunction in java 8 Java 8 filters list Java 8 Consumer Interface in Java

Java 9 Features

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Java 12

What’s new in Java 12

Java 13

Java 13 New Features

Java 14

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Java 15

What’s New in Java 15

Java 16

Java 16

Java 17

What is new in Java 17

Java Servlet Tutorial

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Java JSP Tutorial

Java JSP Tutorial

Hiberate Tutorial

Hibernate Tutorial

Spring Tutorial

Spring Tutorial

Spring MVC Tutorial

Spring MVC Tutorial

Jenkins Tutorial

Jenkins Tutorial Jenkins java net socket connection time out

Java Math Methods

Math.abs() Math.acos() Math.addExact() Math.asin() Math.atan () Math.atan2() Math.cbrt() Math.ceil() Math.copysign() Math.cos() Math.cosh() Math.decrementExact() Math.exp() Math.expm1() Math.floor() Math.floorDiv() Math.floorMod() Math.fma() Math.getExponent() Math.hypot() Math.IEEEremainder() Math.incrementExact() Math.log() Math.log10() Math.log1p() Math.max() Math.min() Math.multiplyExact() Math.multiplyFull() Math.negateExact() Math.nextAfter() Math.nextDown() Math.nextUp() Math.pow() Math.random() Math.rint() Math.round() Math.scalb() Math.signum() Math.sin() Math.sinh() Math.sqrt() Math.subtractExact() Math.tan() Math.tanh() Math.toDegrees() Math.toIntExact() Math.toRadians() Math.ulp()

Java String Methods

toCharArray() copyValueOf() endsWith() equals() equalsIgnoreCase() format() getBytes() getChars() hashCode() indexOf() intern() isEmpty() join() lastIndexOf() length() replace() replaceAll() replaceFirst() split() startsWith() subSequence() substring() toLowerCase() toUpperCase() trim() valueOf()

Java Conversion

Java Convert String to int Java Convert int to String Java Convert String to long Java Convert long to String Java Convert String to float Java Convert float to String Java Convert String to double Java Convert double to String Java Convert String to Date Java Convert Date to String Java Convert String to Object Java Convert Object to String Java Convert String to char Java Convert char to String Java Convert int to long Java Convert long to int

Misc

List all files in a Directory in Java Nonagonal number in Java Null Pointer Exception in Java Package naming convention in Java Packages Program in java Pangram Program in Java PriorityBlockingQueue Class in Java Race Condition in Java Repdigit Numbers in Java Reserved Keywords in Java Rotate matrix by 90 degrees in Java Segment Tree in Java Sierpinski Number in Java Sort Dates in Java Thread Safety and How to Achieve it in Java Normal and Trace of a Matrix in Java Transient variable in Java Tribonacci series in Java Web Crawler in Java Zigzag Traversal of Binary Tree in Java Java Full Stack Java IO filenotfoundexception Java Output Formatting Java Plot Java Pop Java program to print matrix in Z form Java Set to List Java Switch string Java Thread Lifecycle: States and Stages Java Viva Questions and Answers Lazy Propagation in Segment Tree in Java Level order Traversal of a Binary Tree in Java Method chaining in Java Narcissistic Number in Java Java Case Keyword Java Try-Catch Block Java Class KeywordJava concurrency interview questions Java Continue Keyword Java CountDownLatch Java Default Keyword Java Double Keyword Java Editors Java Else Keyword Java Enum Keyword Java Extends keywordJava Finally Keyword Java Font Example of Static import in Java How Many Ways to Create Objects in Java Implementing Queue Using Array in Java Instanceof operator in Java Interchange Diagonal elements in Java Java Arrays Fill Java Char Keyword Convert list to array Java Convert milliseconds to date in Java Copy data/content from one file to another in java CRC Program in Java Creating a Jar file in Java Crown Pattern in Java Difference Between Access Specifiers and Modifiers in Java Difference Between Java and PHP Difference Between replace() and replaceall() in Java Duodecimal in Java Finding middle node of a linked list in Java Group by in Java 8 Hollow Diamond Pattern in Java How annotations work in Java Java Boyer Moore Catalan number in Java Cosmic Superclass in Java Difference between this and super in Java Fall through in Java Figurate Number in Java Java Anon Proxy Java RMI Lazy loading in Java Magnanimous Number in java Shallow copy in Java String to JSON in Java Tetranacci Number in Java Vigesimal in Java Why main method is static in Java Add Time in Java Bellman Ford Algorithm in Java Best Java Libraries Blockchain in Java Blocking Queue in Java Example Bully Algorithm code in Java Contextual keywords in Java Convert JSON File to String in Java Determine the Upper Bound of a Two-Dimensional Array in Java Java.net.ConnectionException Java.net.SocketException Web Service Response Time Calculation in Java Functional Interfaces in Java Singleton class in Java Awesome explanation of Strings in Java Object class in Java Static class in Java All the important string methods in Java String Handling Method in Java What are Array strings in Java Advantages and Disadvantages of Strings in Java Big Decimal class in Java Class definition in Java Replace character in string Java String Coding Interview Questions in Java What is String in Java? 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Rehashing in Java

The most crucial idea mostly in the data structure is hashing, which is utilized to change a particular key into some other value. The hash function can be used to create a new value.

In this article, we will comprehend the hashing, load factor, and rehashing concepts in Java.

Rehashing

Rehashing is a method for maintaining the complexity of the get & put operations while dynamically increasing the size of a Map, Array, or Hashtable O(1).

Rehashing, which is the process of recalculating the hash code of the already stored entries and shifting them to a greater size hash map when the map's element counts hit the maximum threshold value, is another definition for the term.

Rehashing, to put it simply, is the opposite of the hashing process. The performance is kept. The load factor is crucial while rehashing.

Load Factor

When to expand the HashMap or Hashtable capacity to keep the get() & put() operations of complexity O is determined by the load factor (1). HashMap's load factor is set by default to 0.75 (or 75% of the map's size). The load factor determines "when and how to increase the number of buckets that store the key-value pair," in other words.

Larger load factor: Higher lookups but less space used.

Smaller load factor: larger space usage compared to the number of pieces needed.

How is the load factor calculated?

The following formula can be used to determine the load factor:

Starting capacity of the HashMap * HashMap load factor

For instance:

HashMap's initial storage size is 16

HashMap's default load factor is 0.75.

16*0.75 equals 12, according to the formula.

It indicates that the HashMap's 12th key-value pair will maintain its 16-bit size. The HashMap will expand from its default size of 24 = 16 buckets to 25 = 32 buckets as soon as the 13th element (key-value pair) is added.

Load factor example

Let's use an illustration to better grasp the load factor.

We are aware that HashMap's default bucket size is 16. After adding the first element, we will determine whether or not we need to extend HashMap's capacity. The formula: can be used to determine it.

HashMap Size (m) / Amount of Buckets (n)

In this instance, our bucket size is 16, and the size of a hashMap is 1. Enough that, 1/16 = 0.0625. Compare the calculated number towards the default load factor now (0.75).

0.0625 < 0.75

The amount is less than the load factor's pre-set value. Therefore, there is no need to expand the HashMap. Consequently, we are not required to expand the HashMap's size to include the 12th entry since

12/16 = 0.75

The resultant number, which is 0.75, is the same as the standard load factor.

The size of the hash map increases as we add the thirteenth entry because:

13/16 = 0.8125

The obtained number in this case is higher than the load factor's default value.

0.8125 > 0.75

The size of the hashMap must be increased in order to add the thirteenth member.

It is suggested to keep your load factor around 0.75 if you want to maintain the O(1) complexity of your get() and put() operations.

Why is rehashing necessary?

When the load factor rises, rehashing is necessary. When we add key-value pairs to the map, both the load factor and the time complexity rise. HashMap typically has an O time complexity (1). We employ the rehashing strategy to lessen HashMap's load factor and temporal complexity.

Why is Rehashing required?

Rehashing becomes necessary when the load factor rises. When we insert a key-value pair into the map, the load factor and time complexity both rise. Typically, HashMap's time complexity is O (1). We employ the rehashing strategy to lessen HashMap's temporal complexity and load factor.

For the following situations, rehashing is recommended:

  • halfway through the table.
  • whenever the load has reached a particular point (load > 1/2).
  • a failed insertion.
  • Pick a load factor threshold based on heuristics and evaluated when it is crossed.

Rehashing in Practice

To carry out the rehashing process, the following actions must be taken:

  • For each new element that is added to the Map, check the load factor.
  • Implementing rehashing becomes necessary if the load factor exceeds its default value of 0.75.
  • new double-sized bucketArray to be created
  • The insert() method should be called for each element after iterating through the preceding array's (old bucketArray) elements. All of the larger-sized entries in the newly generated bucketArray are added using the insert() method.

Given that there will be a cost associated with rehashing N elements:

O(1)*N + O(N) = O(N)

Rehashing Java Program

Let's use a Java application to implement the rehashing technique.

RehashingExpl.java

import java.util.ArrayList;  
class Map<K1, V1>   
{  
class MapNode<K1, V1>   
{  
K1 k;  
V1 v;  
MapNode<K1, V1> next;  
// the MapNode class's function Object() { [native code] } being created  
public MapNode(K1 k, V1 v)  
{  
this.k = k;  
this.v = v;  
next = null;  
}  
}  
// constructing an array of buckets for key-value pairings
ArrayList<MapNode<K1, V1> > buckets;  
// The variable n indicates how many pairs are kept 
// in the bucket array.  
int s;  
// variable that indicates the bucket's size Range - b
int numberBuckets;  
// establishing the standard loadFactor  
final double DEFAULT_LOAD_FACTOR = 0.75;  
// creating the constructor for the Map class  
public Map()  
{  
// describing the bucket's dimensions    
numberBuckets = 6;  
buckets = new ArrayList<>(numberBuckets);  
// loop runs for the specified number of bucket 
// sizes and initializes nodes to null.  
for (int j = 0; j < numberBuckets; j++)   
{  
// initial bucket contains nothing  
buckets.add(null);  
}  
// outputs all of the initial set values. 
System.out.println(" HashMap has created ");  
System.out.println(" Pairs in the map, number: " + s);  
System.out.println(" Map Size is: " + numberBuckets);  
System.out.println(" The Default Load Factor : " + DEFAULT_LOAD_FACTOR + "\n");  
}  
private int getBucketInd(K1 k)  
{  
// using the Object class hCode() function to obtain the hash code of the relevant key
int hCode = k.hashCode();  
// The array index is calculated, and the same value is returned.  
return (hCode % numberBuckets);  
}  
// constructing a function that adds items to an array 
public void insert(K1 k, V1 v)  
{  
//determining the index where it should be inserted  
int bucketInd = getBucketInd(k);  
// the index's leading node  
MapNode<K1, V1> head = buckets.get(bucketInd);  
// In order to determine whether the key already 
// exists, the first loop through all of 
// the nodes existing at that index.
// The loop iterates over each node displayed 
// at a certain index and determines whether or 
// not the key has previously been used.
while (head != null)   
{  
// This value is updated even if the key has already been supplied. 
if (head.k.equals(k))   
{  
head.v = v;  
return;  
}  
head = head.next;  
}  
// establishing a new key-value pair node 
MapNode<K1, V1> newElementNode = new MapNode<K1, V1>(k, v);  
// the index's head node 
head = buckets.get(bucketInd);  
// Making the new node the head 
// and the previous head its 
// next allows for the insertion 
// of the new node.  
newElementNode.next = head;  
buckets.set(bucketInd, newElementNode);  
System.out.println("Pair(" + k + ", " + v + ") has inserted successfully.\n");  
// as fresh K-V pairs are added
// to the map, the size increases.
s++;  
// the load factor is calculated  
double loadFactor = (1.0 * s) / numberBuckets;  
System.out.println(" Present Load factor = " + loadFactor);  
// Rehashing is carried out if the computed load factor is higher than the default load factor of 0.75. 
if (loadFactor > DEFAULT_LOAD_FACTOR)   
{  
System.out.println(DEFAULT_LOAD_FACTOR + " is lesser than " + loadFactor);  
System.out.println(" Rehashing will be done as a result.\n ");  
// invoking the rehash() method 
rehash();  
System.out.println(" New size of the map: " + numberBuckets + "\n");  
}  
System.out.println(" Pairs in the map, number: " + s);  
System.out.println(" Map size is : " + numberBuckets + "\n");  
}  
// Method to perform rehashing  
private void rehash()  
{  
System.out.println("\n **** Rehashing has Started **** \n");  
// The current wish list is only meant to be temporary.  
ArrayList<MapNode<K1, V1> > temp = buckets;  
// making a new bucket list that is twice as 
// long as the last one, or (2 * numberBuckets). 
buckets = new ArrayList<MapNode<K1, V1> >(2 * numberBuckets);  
for (int j = 0; j < 2 * numberBuckets; j++)   
{  
// all items were initially set to "null"  
buckets.add(null);  
}  
// Now that size has been reduced to zero, 
// we loop through each node in the first
// bucket list (temp) and add it to the new list.
s = 0;  
numberBuckets = numberBuckets * 2;  
for (int j = 0; j < temp.size(); j++)   
{  
// at that index, the chain's head  
MapNode<K1, V1> head = temp.get(j);  
while (head != null)   
{  
K1 k = head.k;  
V1 v = head.v;  
// since the new list is now a bucketArray, 
// inserting each temp node one at a time.  
insert(k, v);  
head = head.next;  
}  
}  
System.out.println("\n **** Rehashing is Done **** \n");  
}  
// how to print the key-value pairs from the map  
public void printMap()  
{  
// Temporary bucket list is created   
ArrayList<MapNode<K1, V1> > temp = buckets;  
System.out.println(" existing hashmap: ");  
// loop across the Map repeatedly  
for (int j = 0; j < temp.size(); j++)   
{  
// obtaining the temporary bucket list's head index 
MapNode<K1, V1> head = temp.get(j);  
while (head != null)   
{  
System.out.println(" key = " + head.k + ", val = " + head.v);  
head = head.next;  
}  
}  
System.out.println();  
}  
}  
// main class  
public class RehashingExpl   
{  
public static void main(String args[])  
{  
// setting up a Map class instance  
Map<Integer, String> m = new Map<Integer, String>();  
// incorporating items into the Map  
m.insert(1, " Deekshitha ");  
m.printMap();  
m.insert(2, " Gopi ");  
m.printMap();  
m.insert(3, " Ramu ");  
m.printMap();  
m.insert(4, " Ramesh ");  
m.printMap();  
m.insert(5, " Suresh ");  
m.printMap();  
m.insert(6, " Mahesh ");  
m.printMap();  
m.insert(7, " Madhavi ");  
m.printMap();  
m.insert(8, " Mukesh ");   
m.printMap();  
m.insert(9, " Monitha ");   
m.printMap();  
m.insert(10, " Deepa ");   
m.printMap();  
}  
}   

Output:

Rehashing in Java
Rehashing in Java Rehashing in Java Rehashing in Java Rehashing in Java Rehashing in Java



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