#Hashing in Java

NANDYALA ANUROOP — Thu, 25 Nov 2021 12:58:07 +0000

Hashing

*In hashing there is a hash function that maps keys to some values. But these hashing function may lead to collision that is two or more keys are mapped to same value. Chain hashing avoids collision. The idea is to make each cell of hash table point to a linked list of records that have same hash function value.
Let’s create a hash function, such that our hash table has ‘N’ number of buckets.
To insert a node into the hash table, we need to find the hash index for the given key. And it could be calculated using the hash function.
Example: hashIndex = key % noOfBuckets
Insert: Move to the bucket corresponds to the above calculated hash index and insert the new node at the end of the list.
Delete: To delete a node from hash table, calculate the hash index for the key, move to the bucket corresponds to the calculated hash index, search the list in the current bucket to find and remove the node with the given key (if found). *

Methods to implement Hashing in Java

Method-1:

with help of HashTable (A synchronized implementation of hashing)
import java.util.*;
class GFG {
public static void main(String args[])
{

    // Create a HashTable to store
    // String values corresponding to integer keys
    Hashtable<Integer, String>
        hm = new Hashtable<Integer, String>();

    // Input the values
    hm.put(1, "Geeks");
    hm.put(12, "forGeeks");
    hm.put(15, "A computer");
    hm.put(3, "Portal");

    // Printing the Hashtable
    System.out.println(hm);
}

}
Output:

{15=A computer, 3=Portal, 12=forGeeks, 1=Geeks}

Method-2:

With the help of HashMap (A non-synchronized faster implementation of hashing)

// Java program to create HashMap from an array
// by taking the elements as Keys and
// the frequencies as the Values

import java.util.*;

class GFG {

// Function to create HashMap from array
static void createHashMap(int arr[])
{
    // Creates an empty HashMap
    HashMap<Integer, Integer> hmap = new HashMap<Integer, Integer>();

    // Traverse through the given array
    for (int i = 0; i < arr.length; i++) {

        // Get if the element is present
        Integer c = hmap.get(arr[i]);

        // If this is first occurrence of element
        // Insert the element
        if (hmap.get(arr[i]) == null) {
            hmap.put(arr[i], 1);
        }

        // If elements already exists in hash map
        // Increment the count of element by 1
        else {
            hmap.put(arr[i], ++c);
        }
    }

    // Print HashMap
    System.out.println(hmap);
}

// Driver method to test above method
public static void main(String[] args)
{
    int arr[] = { 10, 34, 5, 10, 3, 5, 10 };
    createHashMap(arr);
}

}
Output:

{34=1, 3=1, 5=2, 10=3}

Method-3:

With the help of LinkedHashMap (Similar to HashMap, but keeps order of elements)

// Java program to demonstrate working of LinkedHashMap
import java.util.*;

public class BasicLinkedHashMap
{
public static void main(String a[])
{
LinkedHashMap lhm =
new LinkedHashMap();
lhm.put("one", "practice.geeksforgeeks.org");
lhm.put("two", "code.geeksforgeeks.org");
lhm.put("four", "quiz.geeksforgeeks.org");

    // It prints the elements in same order 
    // as they were inserted    
    System.out.println(lhm);

    System.out.println("Getting value for key 'one': "
                                   + lhm.get("one"));
    System.out.println("Size of the map: " + lhm.size());
    System.out.println("Is map empty? " + lhm.isEmpty());
    System.out.println("Contains key 'two'? "+ 
                              lhm.containsKey("two"));
    System.out.println("Contains value 'practice.geeks"
    +"forgeeks.org'? "+ lhm.containsValue("practice"+
    ".geeksforgeeks.org"));
    System.out.println("delete element 'one': " + 
                       lhm.remove("one"));
    System.out.println(lhm);
}

}
Output:

{one=practice.geeksforgeeks.org, two=code.geeksforgeeks.org, four=quiz.geeksforgeeks.org}
Getting value for key 'one': practice.geeksforgeeks.org
Size of the map: 3
Is map empty? false
Contains key 'two'? true
Contains value 'practice.geeksforgeeks.org'? true
delete element 'one': practice.geeksforgeeks.org
{two=code.geeksforgeeks.org, four=quiz.geeksforgeeks.org}

Decision making and looping

NANDYALA ANUROOP — Tue, 11 May 2021 06:35:01 +0000

A normal program is not a sequential execution of expressions or statements one after the other. It will have certain conditions to be checked or it will have certain number of iterations.

When we have to execute particular set of instructions multiple times or known number of times (iterations), then we use FOR loops or DO/WHILE loops.

If… Else Statements

For Loops

While and Do/While Loops

    The general syntax for while and do/while loops are given below :

If… Else Statements

These are the decision making statements. It is used for checking certain condition to decide which block of code to be executed. syntax for if..else statement is

if (condition / s){
expressions / statements;
} else {
expressions / statements;
}

if (condition / s){
expressions / statements;
} else if {
expressions / statements;
} else {
expressions / statements;
}

example :
if (intDivisor == 0) {
printf ("Warning!: Divisor is Zero!!\n Please re-enter the divisor :");
scanf ("%d", &intDivisor);
}

intResult=intDividend / intDivisor;

If statement can have expressions to be executed when the condition is false. Those expressions are added in the else part of the ‘if’ statement.

if (condition / s) {
expressions / statements;
} else {
expressions / statements;
}

Example :
if (intVal >= 0)
printf ("You entered a Positive Number!");
else
printf ("You entered a Negative Number!");

We can have if statement within another if statement, i.e.; we can have nested if statements.
e.g.: –
if (intVal >= 0)
if (intVal == 0)
printf ("You entered a Zero!");
else
printf ("You entered a Positive Number!");
else
printf ("You entered a Negative Number!");

For Loops:

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

Syntax:

for (intial_value; condition; increment_factor) {
statement/s;
}

Example:

void main () {
int intNum;
printf ("\n First 15 natural numbers are: \n");
for (intNum = 0; intNum < 15; intNum++) {
printf ("%d ", intNum);
}
}

While loop:

A while loop in C programming repeatedly executes a target statement as long as a given condition is true.
Syntax:
while (condition/s){
Expression / statements;
}

Example:

void main() {
int intNum = 0;
printf("\n Example of WHILE Loop\n");
printf("First 15 natural numbers are: \n");
while (intNum < 15){
printf("%d ", intNum);
intNum++;

Do/While Loops:

The body of do...while loop is executed once. · If the test expression is true, the body of the loop is executed again and the test expression is evaluated
Syntax:

do{
Expression / statements;
} while (condition/s);

Example:
{
printf("\n Example of DO/WHILE Loop\n");
printf("First 15 natural numbers in descending order is: \n");
while (intNum >=0){
printf("%d ", intNum--);

}

DEV Community: NANDYALA ANUROOP

#Hashing in Java

Hashing

Methods to implement Hashing in Java

Method-1:

Method-2:

Method-3:

Decision making and looping

If… Else Statements

For Loops:

While loop:

Do/While Loops: