I am new to Apex programming, please help me to write a test class for below controller:
/**
 * Controller used by the "scheduleOppsLwc" Lightning Web Component.

 * @since   2021-10-05
 * @version 2021-10-05
 */
public class ScheduleOppsCC {

    /**
     * On-load method used to determine if the "Schedule Opportunities" quick action
     * is available for use based on Opportunity details/user status.
     * @param   parentOpp   ID of Opportunity from quick action context
     * @return  String with reason for closing action if invalid use, empty string otherwise
     */
    @AuraEnabled(cacheable=true)
    public static String canSchedule(String parentOpp) {
        /* escape String for SOQL injection safety and validate that is for an Opportunity */
        Id parentOppId = Id.valueOf( String.escapeSingleQuotes(parentOpp) );
        if ( Schema.Opportunity.SObjectType != parentOppId.getSObjectType() ) {
            return 'This action is only available for Opportunity records.';
        }

        /* query for the Opportunity and related team members and check that the ID exists */
        List<Opportunity> oppList = [ SELECT Id, OwnerId, Scheduled_as_Child__c, Scheduled_with_Children__c,
                                             Total_SIMs__c, (Select UserId From OpportunityTeamMembers)
                                        FROM Opportunity
                                       WHERE Id = :parentOppId ];
        if ( (oppList == null) || oppList.isEmpty() ) {
            return 'This Opportunity cannot be found.';
        }

        if (oppList[0].Scheduled_with_Children__c) {
            return 'This Opportunity has already been scheduled, please review existing installments.';
        }
        if (oppList[0].Scheduled_as_Child__c) {
            return 'This Opportunity was scheduled as an installment and cannot be scheduled.';
        }
        if ( oppList[0].Total_SIMs__c == 0 ) {
            return 'Please provide line items in order to proceed with initiating the scheduler.';
        }

        /* loop through Opportunity team members to get user IDs and compare against running user */
        Set<Id> teamMemberIds = new Set<Id>();
        if ( (oppList[0].OpportunityTeamMembers != null) && !oppList[0].OpportunityTeamMembers.isEmpty() ) {
            for ( OpportunityTeamMember otm : oppList[0].OpportunityTeamMembers ) {
                teamMemberIds.add(otm.UserId);
            }
        }
        if ( (UserInfo.getUserId() != oppList[0].OwnerId) && !teamMemberIds.contains( UserInfo.getUserId() ) ) {
            return 'Only the Opportunity owner or any team members can schedule installments.';
        }

        /* if here, all checks to prevent scheduling fail, so return an empty string to allow */
        return '';
    }


    /**
     * Creates new child installment Opportunities related to the Opportunity
     * used for the "Schedule Opportunities" quick action and updates the parent
     * as needed to CPQ-related details.
     * @param   parentOpp       ID of Opportunity from quick action context
     * @param   parentOppJson   JSON string-representation of the parent opportunity with new values for update
     * @param   childOppsJson   JSON string-representation of a list of child opportunities to create
     */
    @AuraEnabled
    public static void createChildrenUpdateParent(String parentOpp, String parentOppJson, String childOppsJson) {
        /* escape String for SOQL injection safety and validate that is for an Opportunity */
        Id parentOppId = Id.valueOf( String.escapeSingleQuotes(parentOpp) );
        if ( Schema.Opportunity.SObjectType != parentOppId.getSObjectType() ) {
            throw new AuraHandledException('This action is only available for Opportunity records.');
        }

        try {
            /* get all accessible fields for the running user to properly query for cloning */
            Set<String> accessibleFields = new Set<String>();
            Map<String, Schema.SObjectField> fields = Schema.Opportunity.SObjectType.getDescribe().fields.getMap();
            for ( String api : fields.keySet() ) {
                if ( fields.get(api).getDescribe().isAccessible() ) { accessibleFields.add(api); }
            }
            if ( (accessibleFields == null) || accessibleFields.isEmpty() ) {
                throw new ScheduleOppsException('There are no fields available to the current user.');
            }

            /* query for the Opportunity and related line items and check that the ID exists */
            List<Opportunity> parentList = Database.query( 'Select ' + String.join( new List<String>(accessibleFields), ', ' ) +
                                                                 // '(Select Id From OpportunityLineItems)' +
                                                            ' From Opportunity' +
                                                           ' Where Id = :parentOppId' );
                                                           
             List<OpportunityLineItem> Oit = [select Id from OpportunityLineItem where OpportunityId =  :parentOppId]; 
                                                   
                                                           System.debug( '-> parentList1 = ' + parentList );
            if ( (parentList == null) || parentList.isEmpty() ) {
                throw new ScheduleOppsException('This Opportunity cannot be found.');
            }

            /* convert the child Opportunity JSON string to a list of Opportunities */
            List<Opportunity> oppList = (List<Opportunity>)JSON.deserialize(childOppsJson, List<Opportunity>.class);
            if ( (oppList == null) || oppList.isEmpty() ) {
                throw new ScheduleOppsException('No child Opportunity records were passed for creation.');
            }

            /* loop through the child Opportunity records and create them as clones of the parent
             *   with updates to their fields as passed in via the JSON string */
            Opportunity parClone;
            for ( Integer i = 0; i < oppList.size(); ++i ) {
                parClone = parentList[0].clone(false, true, false, false);
                Map<String, Object> childVals = oppList[i].getPopulatedFieldsAsMap();
                for ( String field : childVals.keySet() ) {
                    parClone.put( field, childVals.get(field) );
                }
                parClone.Is_Child__c = true;
                oppList[i] = parClone;
            }

            /* convert the parent Opportunity JSON to an Opportunity to update to represent scheduling */
            Opportunity parent = (Opportunity)JSON.deserialize( parentOppJson, Opportunity.class );
           // parent.Include_in_Funnel__c = 'No';
            parent.StageName = 'Discovery';
            oppList.add(parent);

            /* upsert to create children and update parent, delete all parent-related line items */
            upsert oppList;
            
             if( Oit.size() > 0 )
            {
            delete Oit;
            }
            //delete parentList[0].OpportunityLineItems;
        } catch(Exception ex) {
            System.debug( 'EXCEPTION @ ' + ex.getLineNumber() + ' (' + ex.getLineNumber() + ': ' +
                    ex.getMessage() + '): ScheduleOppsCC.createChildrenUpdateParent()\n -> parentOpp = ' +
                    parentOpp + '\n -> parentOppJson = ' + parentOppJson +
                    '\n -> childOppsJson = ' + childOppsJson);
            throw new AuraHandledException('An error occurred, please reach out to your administrator for assistance.');
        }
    }


    private class ScheduleOppsException extends Exception {}

}

how to serialize the results of a SOQL query into JSON into a apex class.

SOQL query : SELECT Id, AccountId  form  Opportunity WHERE Id = :parentOppId

Hi All,
i am new to apex programming. in apex code below i have to pass one dml statement(opplist) to pass account id along with json string (parentOppJson) can anybody let me know how to do that.

/**
 * Controller used by the "scheduleOppsLwc" Lightning Web Component.
 
 * @since   2021-10-05
 * @version 2021-10-05
 */
public class ScheduleOppsCC {

    /**
     * Creates new child installment Opportunities related to the Opportunity
     * used for the "Schedule Opportunities" quick action and updates the parent
     * as needed to CPQ-related details.
     * @param   parentOpp       ID of Opportunity from quick action context
     * @param   parentOppJson   JSON string-representation of the parent opportunity with new values for update
     * @param   childOppsJson   JSON string-representation of a list of child opportunities to create
     */
    @AuraEnabled
    public static void createChildrenUpdateParent(String parentOpp, String parentOppJson, String childOppsJson) {
    
     system.debug('--> Schema.Opportunity.SObjectType '  + Schema.Opportunity.SObjectType);
     
        
     Id parentOppId = Id.valueOf( String.escapeSingleQuotes(parentOpp) );
     system.debug('--> parentOppId '  + parentOppId);
     
        
       if ( Schema.Opportunity.SObjectType == parentOppId.getSObjectType() )
       
      
        {
           try {
           
            List<Opportunity> parentOp = (List<Opportunity>)JSON.deserialize(parentOppJson, List<Opportunity>.class);
         System.debug( '-> parentOp = ' + parentOp );
         
           //List<Opportunity> oppList = [ SELECT Id, AccountId , RecordTypeId , StageName, Name,CloseDate  FROM Opportunity  WHERE Id = :parentOppId ];
           
                                          
                                           
                                           
        if ( (parentOp != null) && !parentOp .isEmpty() ) {
            List<Opportunity> childOpps = (List<Opportunity>)JSON.deserialize(childOppsJson, List<Opportunity>.class);
            system.debug('--> childOpps after deserialize ' + childOpps);
            List<Opportunity> insertingChildren = new List<Opportunity>();
            for ( Integer i = 0; i < childOpps.size(); ++i ) {
           
            
              Opportunity opp = new Opportunity(AccountId = parentOp [0].AccountId, RecordTypeId = parentOp [0].RecordTypeId  );
              opp.Is_Child__c = true;
              opp.Parent_OpportunityID__c = parentOppId;
              opp.StageName = parentOp[0].StageName;
              opp.Name = parentOp[i].Name;
              opp.CloseDate = childOpps[i].CloseDate;
              opp.Primary_Contact__c = childOpps[i].Primary_Contact__c;
              opp.First_Installment_Date__c = parentOp [0].First_Installment_Date__c;
              opp.Installment_Period__c =  parentOp [0].Installment_Period__c;
              opp.Last_Installment_Date__c = parentOp [0].Last_Installment_Date__c;  
              opp.Number_of_Installments__c = parentOp [0].Number_of_Installments__c; 

              opp.MINT_Lines_Magenta_IoT__c = childOpps[i].MINT_Lines_Magenta_IoT__c;
              opp.Probability = parentOp [0].Probability;
              opp.Scheduled_as_Child__c = childOpps[i].Scheduled_as_Child__c;
              opp.Scheduled_with_Children__c = childOpps[i].Scheduled_with_Children__c;
              opp.Schedule_Type__c = parentOp [0].Schedule_Type__c;
              
              opp.Voice_Lines__c = childOpps[i].Voice_Lines__c;
              opp.Yellow_IoT_Lines__c = childOpps[i].Yellow_IoT_Lines__c;                     

                // update opp with records from childOpps[i] //
               
                insertingChildren.add(opp);
                 
            } 
            System.debug( '-> insertingChildren1 = ' + insertingChildren);
            try{
                   insert insertingChildren;
                    System.debug( '-> insertingChildren2 = ' + insertingChildren);
           } catch(DmlException e) 
           { System.debug('The following exception has occurred: ' + e.getMessage()); }
        } 
              
             
                List<OpportunityLineItem> oliList = [ SELECT Id
                           FROM OpportunityLineItem
                          WHERE OpportunityId = :parentOppId ];
                System.debug( '-> oliList = ' + oliList );
                delete oliList;
                /* insert (List<Opportunity>)JSON.deserialize(childOppsJson, List<Opportunity>.class);
                delete [ SELECT Id
                           FROM OpportunityLineItem
                          WHERE OpportunityId = :parentOppId ]; */

                
                   for( Opportunity op : parentOp  ){
                    
                   op.StageName = 'Discovery';
                 }
                
                
                System.debug( '-> parent = ' + parentOp );
                update parentOp  ;
          } catch (Exception ex) {
                System.debug( 'EXCEPTION @ ' + ex.getLineNumber() + ' (' + ex.getLineNumber() + ': ' +
                        ex.getMessage() + '): ScheduleOppsCC.createChildrenUpdateParent()\n -> parentOpp = ' +
                      parentOpp + '\n -> parentOppJson = ' + parentOppJson +
                       '\n -> childOppsJson = ' + childOppsJson);
              throw new AuraHandledException('An error occurred, please reach out to your administrator for assistance.');
            }
       }
       throw new AuraHandledException('This action is only available for Opportunity records.');
   }

}