Can you write a sample batch apex with Database.Stateful?

Here’s a realistic Batch class with Database.Stateful — it sums opportunity amounts per account across millions of records and writes the rollups in finish.

The class

public class OpptyRollupBatch implements
    Database.Batchable<sObject>,
    Database.Stateful
{
    // State surviving across chunks — possible because of Database.Stateful
    public Map<Id, Decimal> totalByAccount = new Map<Id, Decimal>();
    public Map<Id, Integer> countByAccount = new Map<Id, Integer>();
    public Integer recordsProcessed = 0;
    public Integer errors = 0;
    public List<String> errorList = new List<String>();

    public Database.QueryLocator start(Database.BatchableContext ctx) {
        return Database.getQueryLocator(
            'SELECT Id, AccountId, Amount, StageName ' +
            'FROM Opportunity ' +
            'WHERE IsClosed = false AND AccountId != null'
        );
    }

    public void execute(Database.BatchableContext ctx, List<Opportunity> scope) {
        for (Opportunity o : scope) {
            try {
                Decimal amt = (o.Amount == null) ? 0 : o.Amount;

                // Sum amounts per account, across chunks
                if (!totalByAccount.containsKey(o.AccountId)) {
                    totalByAccount.put(o.AccountId, 0);
                    countByAccount.put(o.AccountId, 0);
                }
                totalByAccount.put(o.AccountId, totalByAccount.get(o.AccountId) + amt);
                countByAccount.put(o.AccountId, countByAccount.get(o.AccountId) + 1);
                recordsProcessed++;
            } catch (Exception e) {
                errors++;
                errorList.add('Opp ' + o.Id + ': ' + e.getMessage());
            }
        }
    }

    public void finish(Database.BatchableContext ctx) {
        // Write the rollups in bulk — totalByAccount has *all* accounts processed
        List<Account> toUpdate = new List<Account>();
        for (Id accId : totalByAccount.keySet()) {
            toUpdate.add(new Account(
                Id = accId,
                Open_Opp_Total__c = totalByAccount.get(accId),
                Open_Opp_Count__c = countByAccount.get(accId)
            ));
        }

        // Chunk the update if we have >10K (DML row limit)
        Integer chunk = 1000;
        for (Integer i = 0; i < toUpdate.size(); i += chunk) {
            Integer end = Math.min(i + chunk, toUpdate.size());
            List<Account> slice = new List<Account>();
            for (Integer j = i; j < end; j++) slice.add(toUpdate[j]);
            Database.update(slice, false);
        }

        // Email summary
        AsyncApexJob job = [
            SELECT Status, CreatedBy.Email FROM AsyncApexJob WHERE Id = :ctx.getJobId()
        ];
        Messaging.SingleEmailMessage msg = new Messaging.SingleEmailMessage();
        msg.setToAddresses(new String[] { job.CreatedBy.Email });
        msg.setSubject('Opp Rollup Batch: ' + job.Status);
        msg.setPlainTextBody(
            'Opps processed: ' + recordsProcessed + '\n' +
            'Accounts updated: ' + totalByAccount.size() + '\n' +
            'Errors: ' + errors
        );
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { msg });
    }
}

How to invoke

Database.executeBatch(new OpptyRollupBatch(), 200);

What makes this pattern strong

A simpler stateful pattern

If they want a minimal example:

public class TaggerBatch implements Database.Batchable<sObject>, Database.Stateful {
    public Integer total = 0;

    public Database.QueryLocator start(Database.BatchableContext ctx) {
        return Database.getQueryLocator('SELECT Id FROM Lead WHERE IsConverted = false');
    }
    public void execute(Database.BatchableContext ctx, List<Lead> scope) {
        for (Lead l : scope) l.Description = 'Tagged ' + Date.today();
        update scope;
        total += scope.size();
    }
    public void finish(Database.BatchableContext ctx) {
        System.debug('Total leads tagged: ' + total); // shows the actual cumulative total
    }
}

Verified against: Apex Developer Guide — Database.Stateful Interface. Last reviewed 2026-05-17.