Must Java Developer Interview Questions And Answers

Explain how the Liskov Substitution Principle requires substituting a superclass with a subclass without breaking behavior, illustrated by birds, sparrows, and ostriches and the flying capability.

Explain how try-with-resources automates closing, eliminating finally blocks, supports multiple resources in proper order, and relies on compiler-generated closable resource classes to call close after the block.

Demonstrate how throw without an argument inside a catch block rethrows the caught exception, preserving its stack trace, and compare it with throwing a new exception for logging.

Compare predicate, function, and supplier in Java eight by highlighting their purposes. Predicate tests boolean conditions, function transforms input to output, and supplier provides values without input.

Explore the spliterator interface, its role in traversing and partitioning data sources for parallel or sequential processing, and how try advance, try split, and for-each-remaining enable stream-like operations.

Explore how this behaves differently in a lambda expression versus an anonymous class: the lambda uses the enclosing instance, while the anonymous class creates a new scope and instance.

A class implementing two interfaces with the same default method triggers a compiler ambiguity error; override the conflicting method to provide a concrete implementation and resolve the conflict.

Volatile is a field-level modifier that guarantees visibility across threads but does not guarantee thread safety. Synchronized provides atomicity, visibility, and mutual exclusion with locking, making it heavier than volatile.

When two Java interfaces share a default method and a class implements both, override the method to resolve conflict, and call the interface default via super, addressing the diamond problem.

Learn how Java implements lambdas at runtime using the invokedynamic instruction, the lambda meta factory, and a dynamically created hidden class that implements the functional interface.

Parallel streams split work into subtasks and run on multiple CPU cores, offering speedups for large CPU-bound data sets but incurring overhead from task splitting and thread management.

Explore how sealed classes, abstract classes, and interfaces differ in controlling inheritance, with sealed classes enabling exhaustive modeling, abstract classes providing shared code, and interfaces defining capabilities.

Learn how pattern matching with instance of in Java 16 integrates type checking and casting, eliminating separate casts, reducing boilerplate, and avoiding class cast exceptions to improve readability and safety.

Understand how sealed classes control inheritance: use extends for standard subclassing, and permits to restrict which classes may extend a sealed class, as shown with shape and circle.

Discover the Java 11 http client API, a modern asynchronous and synchronous replacement for http url connection, with fluent, chainable design, built in body handlers, and web socket support.

Explore how the lines method in Java, introduced in Java eight, reads lines from files or strings via a lazy, memory-efficient stream, with use cases like filtering and mapping lines.

Understand Java 10's var for local variable type inference, deducing types from initializers, useful for streams and lambdas; use only for locals—not fields or parameters, when types are obvious.

Enable auto configuration automatically creates beans based on classpath and properties, reducing boilerplate. It uses the Spring Boot application annotation, import selector, Spring factories, and conditional annotations to tailor beans.