In this article I’ll demonstrate how to check a String whether it is null or empty or blank using Apex.
we can use the following methods to check a whether String is null or empty or blank:
IsBlank – It Returns true if the specified String is white space, empty (”) or null, otherwise it returns false.
IsNotBlank – It Returns true if the specified String is not white space, not empty (”) and not null, otherwise it returns false.
IsEmpty – It Returns true if the specified String is empty (”) or null, otherwise it returns false.
IsNotEmpty – Returns true if the specified String is not empty (”) and not null, otherwise it returns false.
Sample Code:

public with sharing class CheckString
{
    public String CheckIt {get;set;}
     
    public CheckString()
    {
        CheckIt = 'Biswajeet';
    }
     
    //Check IsBlank
    public Boolean CheckisBlank()
    {
        if(String.isBlank(CheckIt))
        {
            //String is not white space or not empty ('') or not null
            return false;        
        }
        else
        {
            //String is white space or empty ('') or null
            return true;
        }   
    }     
     
    //Check IsNotBlank
    public Boolean CheckIsNotBlank()
    {
        if(String.isNotBlank(CheckIt))
        {
            //String is not whitespace and not empty ('') and not null
            return true;        
        }
        else
        {
            //String is whitespace or empty ('') or null
            return false;
        }   
    }
 
    //Check IsEmpty
    public Boolean CheckIsEmpty()
    {
        if(String.isEmpty(CheckIt))
        {
            //String is not empty ('') or not null
            return false;        
        }
        else
        {
            //String is empty ('') or null
            return true;
        }   
    }  
     
    //Check IsNotEmpty
    public Boolean CheckisNotEmpty()
    {
        if(String.isNotEmpty(CheckIt))
        {
            //String is not empty ('') and not null
            return true;        
        }
        else
        {
            //String is empty ('') or blank or null
            return false;
        }   
    }     
}