Question 1

What is the Java Stream API and what is it used for?

Accepted Answer

Processing sequences of elements functionally (filter, map, reduce) in a declarative style without modifying the source. Streams let you write: list.stream().filter(x -> x > 0).map(x -> x * 2).collect(toList()). They support lazy evaluation and can be parallelized with .parallelStream().

Question 2

What is Optional<T> in Java and why is it used?

Accepted Answer

A container that may or may not hold a non-null value — used to avoid NullPointerException and express intent explicitly. Optional<T> forces callers to handle the absent case. Instead of returning null, return Optional.empty(). Use .orElse(), .ifPresent(), or .map() to work with it safely.

Question 3

What is the difference between HashMap and ConcurrentHashMap in Java?

Accepted Answer

ConcurrentHashMap is thread-safe using segment-level locking; HashMap is not thread-safe and must not be used across threads without external synchronization. HashMap in a multi-threaded environment can cause infinite loops or data corruption. ConcurrentHashMap allows safe concurrent reads/writes. For read-heavy workloads, also consider Collections.synchronizedMap().

Question 4

What are lambda expressions in Java 8+?

Accepted Answer

Anonymous functions that implement functional interfaces, enabling a functional programming style. (x, y) -> x + y is a lambda. It implements a functional interface (one abstract method). Used with Stream API, Comparators, Runnable, etc. Enables concise, readable code.

Question 5

What is the difference between Comparable and Comparator in Java?

Accepted Answer

Comparable defines the natural ordering inside the class itself; Comparator defines custom external ordering. Comparable: class implements compareTo() — one natural order. Comparator: separate class/lambda with compare() — multiple custom orderings. Use Comparator when you can't modify the class or need multiple sort orders.

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