Interface Segregation Principle (ISP)

Clients should not be forced to depend upon interfaces they do not use.

Split large interfaces into smaller, focused ones.

Problem

A BasicPlayer is forced to implement record and stream it doesn't support:

// ❌ Fat interface
interface MediaPlayer {
  play(file: string): void;
  record(source: string): void;
  stream(url: string): void;
}

class BasicPlayer implements MediaPlayer {
  play(file: string) { /* ok */ }
  record() { throw new Error('Not supported'); } // forced!
  stream() { throw new Error('Not supported'); } // forced!
}

Solution

Split into role-specific interfaces:

interface Playable   { play(file: string): void; }
interface Recordable { record(source: string): void; }
interface Streamable { stream(url: string): void; }

class BasicPlayer implements Playable {
  play(file: string) { /* ... */ }
}

class ProRecorder implements Playable, Recordable {
  play(file: string)   { /* ... */ }
  record(src: string)  { /* ... */ }
}

Key Insight

Each class implements only what it physically supports. No more throwing NotSupportedException from required methods.

Real-world Example

Printer management:

• Printable, Scannable, Faxable, Stapleable
• An inkjet implements Printable + Scannable; a laser-only printer just Printable