Interfaces & complete decoupling

The pure contract

An interface is a contract with (classically) no implementation at all — it says what methods a type provides, never how. Where an abstract class can be "partly built," an interface is "nothing but the API." This gives complete decoupling: code written against an interface neither knows nor cares which concrete class actually shows up at run time.

Classic rules (the ones an exam tests):

• Interface methods are implicitly public and abstract — you write the signature, no body.
• A class uses implements (not extends) and must provide a body for every method.
• A field declared in an interface is implicitly public static final — a constant, not instance state.
• A class may implement many interfaces (implements A, B, C) — Java's controlled form of "multiple inheritance," because interfaces carry no state to collide.

Worked example — the project's Fixable

The Greenhouse system needs anything that can recover from a fault to expose two operations — attempt a fix and record what happened. That is a contract, so it is an interface:

public interface Fixable {
  void fix();
  void log();
}

Two unrelated classes can both satisfy it without sharing a base class:

class FixWindow implements Fixable {
  public void fix() { /* close & reseal the window */ }
  public void log() { /* append to fix.log */ }
}

class FixPower implements Fixable {
  public void fix() { /* switch to backup power */ }
  public void log() { /* append to fix.log */ }
}

Now recovery code can hold Fixable f and call f.fix(); f.log(); against either class — it is decoupled from the concrete type. A class can also be a Fixable and something else at once:

class Restart extends Event implements Fixable, Runnable {
  public void action() { /* reschedule events */ }
  public void fix()    { /* ... */ }
  public void log()    { /* ... */ }
  public void run()    { /* ... */ }
}

It extends exactly one class but implements as many interfaces as it needs — each adds a capability without adding state.

A note on default methods (Java 8+)

Modern Java lets an interface supply a default method with a body so implementers can inherit it. The AU exam is anchored to Thinking in Java 4e (pre-Java-8), so treat the classic "no bodies" rule as the exam answer, and default methods as real-world context.