Abstract classes and interfaces both define abstraction, but they are used for different design intentions.

Abstract Class

An abstract class cannot be instantiated directly. It provides a common base for related classes.

It can contain:

• Abstract methods.
• Concrete methods.
• Fields.
• Constructors.

abstract class ImportJob {
    void recordStart() { System.out.println("Import started"); }
    abstract void importData();
}

Use an abstract class when related classes share common state or behavior.

Interface

An interface defines a contract: what a class can do.

interface Payment {
    void pay();
}

class UpiPayment implements Payment {
    public void pay() {
        System.out.println("Paid using UPI");
    }
}

Use an interface when unrelated classes share a capability.

Key Difference

• Abstract class: what something is.
• Interface: what something can do.

Example:

• CsvImportJob is an ImportJob.
• CsvImportJob can be Retryable.
• WebhookDelivery can be Retryable.

Comparison

Interview Notes

• You cannot instantiate an abstract class.
• Abstract classes can have constructors.
• Interfaces cannot have constructors.
• Java classes can implement multiple interfaces.
• Prefer interfaces for flexible contracts; abstract classes for shared base behavior and state.