All 32 Topics with Follow-up Questions

1. Java 8 Features

Answer: Java 8 introduced several landmark features:

• Lambda Expressions – Anonymous functions enabling functional-style programming: (a, b) -> a + b
• Stream API – Functional-style operations on collections: list.stream().filter().map().collect()
• Functional Interfaces – Interfaces with a single abstract method (e.g., Runnable, Comparator, Predicate)
• Default & Static Methods in Interfaces – Interfaces can now have method implementations
• Optional Class – Avoids NullPointerException by wrapping nullable values
• New Date/Time API (java.time) – LocalDate, LocalTime, LocalDateTime, ZonedDateTime
• Method References – Shorthand for lambdas: String::toUpperCase
• Collectors – Collectors.toList(), groupingBy(), joining() etc.

Follow-up Questions:

“What is the difference between map() and flatMap() in Stream?”

• map() transforms each element 1-to-1. flatMap() flattens a stream of streams into a single stream.

“What is a Functional Interface? Can it have default methods?”

• Yes. It must have exactly one abstract method. Default/static methods don’t count.

“What problem does Optional solve?”

• Reduces NullPointerExceptions by making nullability explicit. Use Optional.ofNullable(), .isPresent(), .orElse().