Java Backend Developer Interview Questions (Level-1)

Question 1: What are the main features introduced in Java 8?

Answer:
Java 8 introduced several powerful features that transformed Java into a more modern programming language:

• Lambda Expressions: Enable functional programming with concise syntax for anonymous functions.
  
• Stream API: Allows bulk data processing operations on collections in a functional style.
  
• Functional Interfaces: Interfaces with only one abstract method (e.g., Runnable, Predicate, Function).
  
• Optional Class: A container object to avoid NullPointerException.
  
• Default Methods in Interfaces: Lets interfaces have method implementations.
  
• New Date and Time API: Replaces java.util.Date with immutable, thread-safe classes like LocalDate, LocalTime.
  
• Method References: A shorthand for lambda expressions using ::.
  

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Question 2: What are functional interfaces?

Answer:
A functional interface is an interface that has exactly one abstract method. These interfaces are the basis for lambda expressions and method references in Java 8.

Key Points:

• Annotated with @FunctionalInterface (optional but recommended).
  
• Support functional programming in Java.
  
• Examples:
  
  • Runnable → void run()
    
  • Predicate<T> → boolean test(T t)
    
  • Function<T, R> → R apply(T t)
    

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Question 3: Explain Stream and its key methods.

Answer:
A Stream in Java represents a sequence of elements that support sequential or parallel aggregate operations.

Types of Operations:

• Intermediate Operations (return a new Stream):
  
  • filter(): Filters elements based on a condition.
    
  • map(): Transforms each element.
    
  • sorted(): Sorts the stream.
    
• Terminal Operations (produce a result):
  
  • collect(): Converts stream into a collection.
    
  • forEach(): Iterates over elements.
    
  • reduce(): Aggregates elements.
    

Streams promote functional-style operations and are lazy, meaning computation happens only during terminal operations.

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Question 4: What is the difference between map() and flatMap()?

Answer:

Feature	map()	flatMap()
Definition	Transforms each element into another form	Transforms and flattens nested collections
Output	Stream of transformed elements	Single flat Stream from nested collections
Use Case	List<String> → List<Integer>	List<List<String>> → List<String>

Example:

java

map(): List<Optional<String>> → List<Optional<String>>  

flatMap(): List<Optional<String>> → List<String>

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Question 5: What is Optional in Java 8?

Answer:
Optional is a container object which may or may not contain a non-null value. It helps in writing null-safe code.

Important Methods:

• Optional.of(value): Wraps a non-null value.
  
• Optional.empty(): Creates an empty Optional.
  
• Optional.ifPresent(action): Executes action if value is present.
  
• Optional.orElse(defaultValue): Returns value if present; otherwise, returns default.
  

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Question 6: How do default methods work in interfaces?

Answer:
Java 8 allows interfaces to have default implementations using the default keyword.

Example:

java

interface MyInterface {

    default void show() {

        System.out.println(“Default Method”);

    }

}

This helps in extending interfaces without breaking existing implementations.

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Question 7: What is the purpose of Collectors?

Answer:
The Collectors utility class in Java 8 helps accumulate elements of a Stream into collections or summary results.

Popular Collectors:

• toList(), toSet(): Collects elements into list or set.
  
• joining(): Joins strings with delimiter.
  
• groupingBy(): Groups elements by a classifier.
  
• partitioningBy(): Splits elements into two groups based on predicate.
  

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Question 8: How does the Date and Time API differ from java.util.Date?

Answer:
Java 8 introduced a modern, immutable, and thread-safe Date and Time API under java.time package:

Feature	Old API (Date)	New API (LocalDate, etc.)
Mutability	Mutable	Immutable
Thread Safety	Not thread-safe	Thread-safe
Formatting	SimpleDateFormat	DateTimeFormatter
Time Zone Support	Limited	ZonedDateTime provides full support

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Question 9: What are method references in Java 8?

Answer:
Method references are shorthand syntax for calling methods using ::. They improve code readability.

Types:

• Static method: ClassName::staticMethod
  
• Instance method of object: instance::instanceMethod
  
• Constructor: ClassName::new
  

Example:

java

List<String> list = Arrays.asList(“a”, “b”);

list.forEach(System.out::println);

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Question 10: What is parallelStream() in Java 8?

Answer:
parallelStream() enables parallel processing of elements using multiple threads. It’s suitable for processing large datasets where order is not critical.

Example:

java

List<Integer> numbers = Arrays.asList(1, 2, 3);

numbers.parallelStream().map(n -> n * 2).forEach(System.out::println);

Use with caution when:

• Order matters.
  
• The dataset is small.
  
• The operation is not CPU-intensive.
  

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Question 11: What is the difference between Stream.findFirst() and Stream.findAny()?

Answer:

Method	Behavior	Use Case
findFirst()	Returns the first element (ordered)	For ordered or sequential streams
findAny()	Returns any element (non-deterministic)	For parallel streams where order doesn’t matter

Example:

java

Optional<String> first = list.stream().findFirst();

Optional<String> any = list.parallelStream().findAny();

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