The initial Skype interview went well. It was a basic "getting to know you" interview about my CV and experience, and they asked me a few technical questions on the basics of OOP.

Following that, I went in for a tech test and two more meetings.

The tech test was a UML class diagram exercise. The two people conducting it were very friendly and helpful, guiding me through the process. They did ask some challenging questions, but their intention was to help me complete the diagram.

After the tech test, I had another interview with the manager who conducted the Skype interview. This was a very friendly and relaxed session. We discussed myself, my career goals, and some more basic interview questions. We also talked about a project and the technologies I had used.

The subsequent interview was very short, with a senior manager. It was quick, and they simply went through my CV, asking some basic questions about myself. Again, this was a very friendly and relaxed interaction.

Overall, this was one of the better interview processes I have experienced. Everyone was very friendly and relaxed.

Explain the core principles of OOP.