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 OKThis 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()); // OKNow the list contains objects that are not from the same class hierarchy.