Interfaces
By the end of this lesson, you'll be able to:
- Define an interface as a pure contract with no implementation at all
- Implement an interface in a class using implements
- Explain the difference between an interface and an abstract class
Prerequisites: "Abstract Classes"
A contract with no body at all
public interface Payable {
Decimal calculatePayment();
}
public class Contractor implements Payable {
public Decimal hourlyRate;
public Integer hoursWorked;
public Decimal calculatePayment() {
return hourlyRate * hoursWorked;
}
}An interface can't have any method bodies, fields, or constructors — it's purely a promise: "any class that implements Payable guarantees it has a calculatePayment() method." How each class fulfils that promise is entirely up to it.
One class, several unrelated interfaces
public interface Payable {
Decimal calculatePayment();
}
public interface Schedulable_Custom {
String getSchedule();
}
public class Contractor implements Payable, Schedulable_Custom {
public Decimal calculatePayment() { return 5000; }
public String getSchedule() { return 'Mon-Fri, 9-5'; }
}Unlike extends (a class can only extend one parent), a class can implements multiple interfaces at once — interfaces describe unrelated capabilities a class can mix and match, not a single "is-a" lineage.
Interface vs abstract class
An abstract class can mix real, shared implementation with abstract methods still to be filled in, and a class can only extend one of them. An interface has zero implementation at all, and a class can implement many of them together. Reach for an interface when you're describing an unrelated capability ("this can be paid," "this can be scheduled") rather than a shared lineage.
Exercise
Write an interface Printable with a method String toPrintFormat(). Implement it in a class Invoice that returns "Invoice #" plus an invoiceNumber field.
Show hint
public interface Printable { String toPrintFormat(); }
Interfaces Quiz
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Summary
An interface is a pure contract — a list of method signatures with zero implementation — that any class can promise to fulfil using implements, regardless of what it inherits from.