1. Subtype Substitution Principle

2. Type
Every variable in Java has a type. Specifically, every reference variable has two types: static type and dynamic type.
A type in Java can be one of the following:
Primitive type: int, float, double, boolean, and char.
Any classes:
Built-in classes: Object, String, and Integer etc.
User-defined classes: Student, Person, and University etc.
Interfaces

3. Subtype Type B is a subtype of Type A if
A is an interface and B implements A, OR,
A are B both are interfaces and B extends A, OR,
A is a class and B extends A
As special case, any type A is a subtype of A A A OR B B

4. How Does Compiler Do?
Recall compiler tries to detect as many runtime errors as possible.
Static Type checking is one of the ways for compiler to avoid runtime errors.
Why the static type of the RHS of assignment should be a subtype of the static type of the LHS of assignment.

5. Subtype Type Substitution
If type B is a subtype of type A and everywhere the code expects A, then a B can be used instead.
“The code expects A” follows the “first write first dominate (FWFD)” principle. For instance: …
public Person m(Student s) { ……
return new Student(); }
Person p = new Student();
producer
x.m(new Collegestudent ());
client/consumer