Java 8 Features Comparison Summary
Fast recall
Quick revision
Core answer
Collection.stream(). These features support declarative and composable code, but they do not make data immutable or require lambda syntax.Whether you understand how Java 8 language syntax, functional interfaces, and library APIs fit together rather than listing isolated features.
Saying Streams or Optional require lambdas. Their methods accept functional-interface objects; a lambda or method reference is simply the concise way to provide one.
Explain how functional interfaces connect the features, how default methods aided API evolution, and why none of this guarantees immutability or better performance.
Interview-ready guide
Interview answer
Java 8's features connect through functional interfaces.
A lambda is an expression that supplies the implementation of a functional interface, so APIs can accept behavior without an anonymous-class wrapper. Streams use this in operations such as filter, map, and reduce, while operations such as count and distinct need no lambda. Optional and CompletableFuture also have methods that accept functional interfaces for transformations and callbacks.
A method reference is a shorter form when a lambda only calls an existing method. Default methods let interfaces add behavior compatibly; that is how Collection could gain stream() without forcing every existing implementation to add a new method body. Together these features make small declarative pipelines easier to write, while normal object-oriented code remains part of the design.
Java 8 Features Comparison Summary
Direct Answer
Java 8 connected several language and library features around functional interfaces. Lambdas and method references are concise ways to supply behavior; Streams use that behavior in pipelines, and Optional uses it for present-or-empty transformations. Default methods helped existing interfaces gain new operations such as Collection.stream(). These features support declarative and composable code, but they do not make data immutable or require lambda syntax.
Code Example
Here several Java 8 features cooperate in one small, compilable example:
import java.util.Comparator;
import java.util.List;
import java.util.Optional;
public class OrderProcessor {
enum Status { PENDING, COMPLETED }
static final class Order {
private final long id;
private final long customerId;
private final double total;
private final Status status;
Order(long id, long customerId, double total, Status status) {
this.id = id;
this.customerId = customerId;
this.total = total;
this.status = status;
}
long getId() { return id; }
long getCustomerId() { return customerId; }
double getTotal() { return total; }
Status getStatus() { return status; }
}
interface OrderRepository {
List<Order> findByCustomerId(long customerId);
Optional<Order> findById(long orderId);
}
private final OrderRepository repository;
OrderProcessor(OrderRepository repository) {
this.repository = repository;
}
Optional<Order> findHighestCompletedOrder(long customerId) {
return repository.findByCustomerId(customerId).stream()
.filter(order -> order.getStatus() == Status.COMPLETED)
.max(Comparator.comparingDouble(Order::getTotal))
.map(this::addLoyaltyBonus);
}
void processOrder(long orderId) {
repository.findById(orderId)
.filter(order -> order.getTotal() > 100)
.ifPresent(this::sendConfirmationEmail);
}
private Order addLoyaltyBonus(Order order) {
return new Order(order.getId(), order.getCustomerId(),
order.getTotal() * 1.1, order.getStatus());
}
private void sendConfirmationEmail(Order order) {
System.out.println("Confirmed order " + order.getId());
}
}
The stream uses a lambda for filtering and a method reference for comparison. Its max result is an Optional, whose map runs only when an order exists. The second method uses an Optional filter and a bound method reference.
Common Mistakes & Pitfalls
- Saying Streams depend on lambda syntax: Stream operations depend on functional-interface contracts. Lambdas, method references, and anonymous classes are different ways to supply them.
- Assuming functional style guarantees immutability: stream lambdas can mutate referenced objects or external state. Prefer non-interfering operations, but do not claim the API enforces immutable data.
- Promising default methods never break code: they improve source and binary compatibility, but unrelated inherited defaults can conflict and a new implementation can change behavior.
Check your understanding
Answer these without looking back at the article.
- What is a functional interface?
- How do streams use lazy evaluation?
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