Which interface should be implemented for schedulable class?

A scheduled Apex class must implement the System.Schedulable interface. The interface has exactly one method:

public interface Schedulable {
    void execute(SchedulableContext ctx);
}

That’s the whole contract. Your class becomes schedulable the moment it provides that one method.

Minimal example

public class HelloScheduler implements Schedulable {
    public void execute(SchedulableContext ctx) {
        System.debug('Hello from the scheduler at ' + Datetime.now());
    }
}

Schedule it with:

System.schedule('Hello', '0 0 * * * ?', new HelloScheduler());

What SchedulableContext is

SchedulableContext is the context object passed to execute. It has one useful method:

public interface SchedulableContext {
    Id getTriggerId();
}

getTriggerId() returns the CronTrigger Id — the record that represents the schedule itself. Use it to inspect the schedule or modify it from within execute:

public void execute(SchedulableContext ctx) {
    CronTrigger ct = [
        SELECT CronExpression, TimesTriggered, NextFireTime
        FROM CronTrigger WHERE Id = :ctx.getTriggerId()
    ];
    System.debug('Times triggered: ' + ct.TimesTriggered);
}

Common combinations

A class can implement Schedulable together with other interfaces:

// Schedulable + Batchable — one class is both
public class NightlyCleanup implements
    Database.Batchable<sObject>,
    Schedulable
{
    public void execute(SchedulableContext ctx) {
        Database.executeBatch(this, 200);
    }
    public Database.QueryLocator start(Database.BatchableContext ctx) { /* ... */ }
    public void execute(Database.BatchableContext ctx, List<sObject> scope) { /* ... */ }
    public void finish(Database.BatchableContext ctx) {}
}

Apex distinguishes the two execute methods by parameter type — SchedulableContext vs Database.BatchableContext. No compiler conflict.

What the interface gives you

When you implement Schedulable:

Common mistakes

• Forgetting to make execute public — protected/private won’t work.
• Wrong return type — must be void.
• Wrong parameter type — must be SchedulableContext, not SchedulableContext[] or anything else.
• Trying to take constructor params for the schedule itself — the schedule is configured via System.schedule, not the constructor.

Common interview follow-ups

• Which package is the interface in? — System namespace. System.Schedulable and Schedulable are equivalent.
• Can a single class be scheduled multiple times? — Yes — each System.schedule call creates a separate CronTrigger.
• Can the scheduled class be virtual / abstract? — Yes, but the concrete subclass is what you actually schedule.

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