Java 8 Features Interview Questions

Java 8 introduced Lambda Expressions, Stream API, Functional Interfaces, Method References, Optional, Default Methods, and the new Date and Time API.

A Lambda Expression is a concise way to represent an anonymous function that can be passed as an argument or stored in a variable.

Lambda Expressions reduce boilerplate code, improve readability, support functional programming, and simplify collection processing.

A Functional Interface is an interface that contains exactly one abstract method and can be implemented using lambda expressions.

Examples include Runnable, Callable, Comparator, Predicate, Function, Consumer, and Supplier.

Stream API provides a functional approach to processing collections with operations such as filtering, mapping, sorting, and reducing.

Collections store data in memory, while Streams process data without modifying the source collection.

map() transforms each element of a stream into another form and returns a new stream.

filter() selects elements that satisfy a given condition and returns a new stream.

reduce() combines stream elements into a single result such as sum, maximum value, or concatenated string.

Optional is a container object that may or may not contain a value, helping avoid NullPointerException.

Method References provide a shorthand syntax for referring to methods using the :: operator.

A Default Method is a method with implementation inside an interface, introduced to support backward compatibility.

A Parallel Stream processes elements concurrently using multiple threads from the ForkJoinPool.

The ForkJoin Framework enables parallel task execution by recursively splitting large tasks into smaller subtasks.

Name concrete APIs (collections, concurrency, streams, annotations) tied to Java 8 Features. Interviewers want syntax-level clarity plus when you would reach for each feature in production.

Prefer specific checked/unchecked exceptions, wrap lower-level failures with context, and avoid empty catch blocks. For Java 8 Features, decide what is recoverable vs fatal to the request thread.

JUnit 5 with Arrange-Act-Assert, Mockito for collaborators, and parameterized tests for edge cases. Keep Java 8 Features logic pure where possible so tests stay fast.

Incorrect equals/hashCode break HashMap/HashSet behavior. If Java 8 Features stores objects in collections, explain the contract and mutability pitfalls.

Retain only needed references, prefer streaming for large datasets, and avoid accidental caches. Relate leaks or GC pressure to a concrete Java 8 Features scenario.