This three-chapter handout covers the basic concepts of object-oriented programming (OOP) in Java, in line with the official curriculum [7]. The general aim of this handout is to introduce 2nd year students in the Mathematical Computing (MI) stream to computational thinking in problem solving, to understand the basic foundations of object-oriented programming and to master the techniques of designing advanced programs in the Java language. It does not, of course, require any prior knowledge of the subject, but nevertheless assumes as a given the basics of procedural programming, such as types, variables, loops, functions, etc., learned in the first year of MI.At the end of the course, students will be able to: 1) Write a functional program in Java.2) Find a solution to a simple problem using an object-oriented approach. 3) Use the notion of inheritance as a more advanced development technique to enrich, specialize and prioritize code.