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What exactly is Hash Code in Java?

Updated to 1 day ago

Table of contents

Preface

1. What is Hashcode in Java?

2. What is a Hash table?

1. Overview

2. The composition of hash table

1. Hash function

2. Array

3. How to deal with the problem that different keys have the same index (collision)

3. What is Hashing?

4. What is HashMap in Java?

5. HashCode() usage example

Summarize

Preface

As beginners in Java, we always encounter some conceptual problems. I recently encountered a problem: What is hashcode? What is its use? After reviewing many blog posts, I have a preliminary understanding of hashcode and write down the knowledge I have learned here to share with you. Welcome to follow me and grow and make progress with me.

The links that are mainly referenced in this article are

What is a hash code in Java?

1. What is Hashcode in Java?

The hash code is an integer that is associated with every object in Java. The hash code is used to implement hashing in hash tables. Hash code is data structured by HashMap. Let us explain the meaning of hash tables, hashing and HashMap one by one. We will also introduce how the hashCode() method is used in Java.

2. What is a Hash table?

1. Overview

A Hash table is a data structure that stores key-value pairs. (key-value)

The key is sent to a hash function, which performs a series of arithmetic operations based on the key. The result is the index of the key-value pair in the hash table, which is usually called hash value or hash.

2. The composition of hash table

The basic hash table consists of two aspects

1. Hash function

Hash function determines what the index of our key-value pairs is.

An effective hash function determines the quality of the hash table. Hash table should have the following two properties:

  • We cannot get the corresponding key from the Hash value
  • Different keys should correspond to different Hash values

2. Array

Array is the way to store all key-value pairs in a hash table. The size of the Array should be set according to the amount of data we expect to get.

3. How to deal with the problem that different keys have the same index (collision)

  • Linear Probing: If a key-value pair is mapped to an already occupied index by hash, it will search linearly to find the next empty position in the table.
  • Chaining: hash table is actually an array of elements as linked lists. All keys mapped to the same index will be stored in the corresponding position as linked list nodes.
  • Resize: As the name suggests, when the hash table reaches a threshold that sets the percentage of the hash table occupied, the hash table will increase its size. So the hash table will have more space to place the index conflict problem.

3. What is Hashing?

Hashing describes the process of converting a given key into a value as we mentioned earlier. What we mentioned before is hash function to convert key into index is hashing. A simple hashing method is actually like this:

index = key MOD tableSize

The above is actually a modulus operation. Of course, the premise for hashing is that the key is in a numerical form. The benefit of doing this is the same as the principle of modulus calculations in computers: avoid crossing the boundaries.

In fact, hashing is also often used in digital encoding. The form of storing passwords is actually their hashes. In this way, even if the source database is leaked, the real password will not be leaked due to the one-way attribute of hashing (the key cannot be pushed out from the hash value).

4. What is HashMap in Java?

Personally, I understand that HashMap and hashtable are similar. But it seems that the Internet says that there is a little difference between HashMap and hashtable in Java, and the inheritance is different. Personally, HashTable is more like a theory. The HashMap class in Java implements the map interface by applying a hash table.

import ; // import HashMap class
import ; // import Map Interface
class MyClass {
    public static void main( String args[] ) {
    HashMap<Integer, String> shapes = new HashMap<Integer,String>(); // Create an ArrayList object with string data type
    }
}

There are the following useful methods in HashMap:

  • (6,"hexagon") is used to add elements
  • (3) Used to obtain the value corresponding to the key
  • () is used to delete all elements
  • (1) Used to delete the element where a specific key is located
  • () is used to check whether the hashmap is empty
  • () is used to return the size of HashMap

The following is a specific application of HashMap


import ; //importing the HashMap class
import ; // importing the Map interface
class MyClass {
  public static void main( String args[] ) {
    HashMap<Integer,String> shapes = new HashMap<Integer,String>(); //create a HashMap with Integer keys and String Values
    (4,"square");       // add square at key 4
    (3,"triangle");     // add triangle at key 3
    (6,"hexagon");      // add hexagon at key 6
    (8,"octagon");     // add octagon at key 8
    (5,"pentagon");   // add pentagon at key 5

    (shapes); // print keys and mapped values

    (3);          // removing value at key 3
    (5);          // removing value at key 5
    ("Updated map:");  
    (shapes); // prints keys and mapped values
    (4,"rhombus");     // replaces value at key 4 with rhombus
    ("Updated map:"); 
    (shapes); // prints keys and mapped values
    ("Size of map is "+());
  
    ();             // removes all key/value pairs in the HashMap
    ("Updated map:"); 
    (shapes); // prints keys and mapped values
    
  }
}

5. HashCode() usage example

In java,hashCode()This method is defined in the default parent class Object of all classes, so all custom classes actually inherit it. It returns the value of the hash code corresponding to the object.

Generally, it returns the same integer value for the same object. In special cases, ifequals()is modified, then even for different objects, the hash code value is the same because the equals() value is the same.

The following is an example

class Hash{
    public static void main(String[] args){
        String a = "200";
        String b = "200";

        if((b)){
            ("Equal variables:");
            (() + "\n" + ());
        }

        String c = "10";
        String d = "50";

        if(!(d)){
            ("\nUn-equal variables:");
            (() + "\n" + ());
        }
    }
}

Summarize

The above is a sharing of the knowledge about hashcode in Java. If you like it, please like it and leave a comment! Image processing reading notes will be updated in the future, so everyone can continue to pay attention!