Interface segregation

Definition

The interface segregation principle says no class should be forced to implement methods it does not use. Prefer several small interfaces over one big one.

Example code

A basic printer is not forced to scan

interface Printer { String print(String text); }
interface Scanner { String scan(); }

class BasicPrinter implements Printer {              // only what it can really do
    public String print(String text) { return "Printing: " + text; }
}

class OfficeMachine implements Printer, Scanner {    // a machine that can do both
    public String print(String text) { return "Printing: " + text; }
    public String scan() { return "Scanned page"; }
}

class Demo {
    public static void main(String[] args) {
        Printer simple = new BasicPrinter();
        OfficeMachine office = new OfficeMachine();
        System.out.println(simple.print("report"));
        System.out.println(office.print("invoice"));
        System.out.println(office.scan());
    }
}

// Output:
// Printing: report
// Printing: invoice
// Scanned page