Is Interface segregation principle only a substitue for Single responsibility principle?

Question

Is interface segregation principle only a substitue for single responsibility principle ?

I think that if my class fulfill SRP there is no need to extract more than one

Answer 1

No.

A class can implement multiple interfaces but it should implement the methods only applicable to it.

Assume that you have 10+ different capabilities like Climb, Think, Learn, Apply. Class Dog can have 2 capabilities and class Cat can have 2 capabilities and class Man can have 6 capabilities. It makes sense to implement only applicable capabilities in respective classes.

Have a look at this code.

public class ISRDemo{
    public static void main(String args[]){

        Dog dog = new Dog("Jack",16);
        System.out.println(dog);

        Learn dl = dog;
        dl.learn();
        ProtectOwner p = dog;
        p.protectOwner();

        Cat cat = new Cat("Joe",20);
        System.out.println(cat);
        Climb c = cat;
        c.climb();
        Remember r = cat;
        cat.doRemember();       

        Man man = new Man("Ravindra",40);   
        System.out.println(man);
        Think t = man;
        t.think();
        Learn l = man;
        l.learn();
        Apply a = man;
        a.apply();
        PlaySports pm = man;
        pm.playSports();
        Remember rm = man;
        rm.doRemember();

    }
}

class Dog implements Learn,ProtectOwner{
    private String name;
    private int age;
    public Dog(String name,int age){
        this.name = name;
        this.age = age;
    }

    public void learn(){
        System.out.println(this.getClass().getSimpleName()+ " can learn");
    }
    public void protectOwner(){
        System.out.println(this.getClass().getSimpleName()+ " can protect owner");
    }
    public String toString(){
        return "Dog :"+name+":Age:"+age;
    }
}
class Cat implements Climb,Remember {
    private String name;
    private int age;
    public Cat(String name,int age){
        this.name = name;
        this.age = age;
    }
    public void climb(){
        System.out.println(this.getClass().getSimpleName()+ " can climb");
    }
    public void doRemember(){
        System.out.println(this.getClass().getSimpleName()+ " can remember");
    }
    public String toString(){
        return "Cat :"+name+":Age:"+age;
    }
}
interface ProtectOwner {
    public void protectOwner();
}
interface Remember{
    public void doRemember();
}
interface Climb{
    public void climb();
}
interface Think {
    public void think();
}
interface Learn {
    public void learn();
}
interface Apply{
    public void apply();
}
interface PlaySports{
    public void playSports();
}

class Man implements Think,Learn,Apply,PlaySports,Remember{
    String name;
    int age;

    public Man(String name,int age){
        this.name = name;
        this.age = age;
    }
    public void think(){
        System.out.println(this.getClass().getSimpleName() + " can think");
    }
    public void learn(){
        System.out.println(this.getClass().getSimpleName() + " can learn");
    }
    public void apply(){
        System.out.println(this.getClass().getSimpleName() + " can apply");
    }
    public void playSports(){
        System.out.println(this.getClass().getSimpleName() + " can play sports");
    }
    public void doRemember(){
        System.out.println(this.getClass().getSimpleName() + " can remember");
    }
    public String toString(){
        return "Man :"+name+":Age:"+age;
    }
}

output:

java ISRDemo
Dog :Jack:Age:16
Dog can learn
Dog can protect owner
Cat :Joe:Age:20
Cat can climb
Cat can remember
Man :Ravindra:Age:40
Man can think
Man can learn
Man can apply
Man can play sports
Man can remember

In above example, interface segregation recommends to define 10 capabilities in 10 interfaces in stead of declaring all of them in fat interface. But it does not mean that you need different classes to meet single responsibility criteria.

Have a look at implementation of Dog, Cat and Man classes in same example.