LeadTriggerHandlerFinal.cls

public class LeadTriggerHandler extends TriggerHandler {
    
    List<Lead> newLeads;

    public LeadTriggerHandler(){
        newLeads = (List<Lead>) Trigger.new;
    }
    
    public override void beforeInsert() {
        Set<String> companyNames = new Set<String>(); 					// Will hold the set of Lead Company Names
        Map<String, ID> accountNamesToId = new Map<String, ID>();		// Will hold the set of Account Names mapped to the Account ID
        List<Case> newCases = new List<Case>(); 						// Will hold the new cases to create
        for(Lead l : newLeads){											// Look at each Lead individually
            companyNames.add(l.Company);								// Add the Lead Company to our set
        }   
        
        // Get all existing Accounts with the same names as the Leads being inserted
        List<Account> existingAccounts = [SELECT Name FROM Account WHERE Name IN :companyNames];
        for(Account a : existingAccounts) {								// Look at each Account individually
            accountNamesToId.put(a.Name, a.Id);							// Add all Account names to a set and map it to the Account ID
        }
    
        for(Lead l : newLeads) {										// Go through list of Leads again
            if(accountNamesToId.keySet().contains(l.Company)) {			// If the Lead Company name is in the list of existing Account names
                Case c = new Case();									// Create a new case
                c.AccountId = accountNamesToId.get(l.Company);			// Get the Account ID that corresponds to this Account Name
                c.Subject = 'Account already exists for ' + l.Company;	// Set the subject
                c.Status = 'New';										// Set the status
                newCases.add(c);										// Add our Case to the list of Cases to create
            }   
        }
        insert newCases;												// Insert our Cases into Salesforce
    }
}