The 30-minute interview focused on Java Collections, Java 8 features, and OOP concepts. The interviewer asked in-depth questions about various collection types, their performance, and use cases. They also explored Java 8 enhancements like the Streams API, lambda expressions, and functional interfaces. Additionally, they tested my understanding of OOP principles like inheritance, polymorphism, encapsulation, and abstraction. I was given coding challenges involving collections and asked to solve them using Java 8 features, particularly streams and lambdas, to demonstrate clean and modern coding practices.