Law of Demeter - Least Knowledge Principle

Design-Principles · 25 Sep 2019

A class should know as less as possible about other classes implementation. There is a law for it!

A class should only know about first implementation of its “immediate fried” classes. This is important to avoid coupling and make your system more flexible, reusable, maintainable and with low coupling.

Example:

Let’s check the allowed usages:

class A
{
    private C c;

    public void doB(B b)
    {
        b.do(); // Allowed!
    }

    public void doC()
    {
        this.c.do(); // Allowed!
    }

    public void doD()
    {
        D d = new D();

        d.do(); // Allowed!
    }
}

And these are not allowed:

class A
{
    private C c;

    public void doB(B b)
    {
        b.do().doAgain(); // NOT Allowed!
    }

    public void doC()
    {
        this.c.do().doAgain(); // NOT Allowed!
    }

    public void doD()
    {
        D d = new D();

        d.do().doAgain(); // NOT Allowed!
    }

    public void setC(C2 c)
    {
        this.c = c;

        this.c.do(); // NOT Allowed, cause C2 is a base class of C and A only knows C!
    }
}