The Liskov Substitution Principle

Substitutability is a principle in object-oriented programming introduced by Barbara Liskov in a 1987 conference keynote stating that, if class B is a subclass of class A, then wherever A is expected, B can be used instead:

class A {...}
class B extends A {...}

public void method(A obj) {...}

A a = new B(); // Assignment OK
method(new B()); // Passing as parameter OK

This also applies when the type is an interface, where there doesn’t need to any hierarchical relationship between the objects:

interface Foo {
    void bar();
}

class A implements Foo {
    void bar() {...}
}

class B implements Foo {
    void bar() {...}
}

List<Foo> foos = new ArrayList<>();
foos.add(new A()); // OK
foos.add(new B()); // OK

Now the list contains objects that are not from the same class hierarchy.