Interview_literal/StringBuilder/Exception/Generic/lambda, stream/Functional programming/Functional Interface/Annotation

✅ String literal과 new String(““)의 차이

• String literal: saved in constant pool

• new String(""): saved in memory heap

• constant pool에 저장되면 동일한 문자열에 대해서는 하나의 참조를 재사용
• constant pool에서는 같은 값이 존재한다면, 새로 정의한 변수여도 같은 주소값을 가진다

✅ What is literal?

• fixed, constant value
• int PI = 3.14;
• String greeting = "Hello, world!";

String 🆚 StringBuilder 🆚 StringBuffer

• String: immutable
• StringBuilder: mutable(same memory), single thread
• StringBuffer: mutable(same memory), multithread safe

Exception 🆚 Error

• Exception: event that disrupts flow, usually recoverable
  • checked exception: exception in compile time, handled with try-catch or throws, IO exception, SQL exception
  • unchecked exception: exception in runtime, NPE, IAE
• Error: system level failure, usually not recoverable
  • Out of Memory, JVM crash

✅ Exception 클래스의 예시

• IO exception: checked exception
• SQL exception: checked exception
• Null Pointer Exception: unchecked runtime exception
• Illegal Argument Exception: unchecked runtime exception, divide by 0

Checked Exception 🆚 Unchecked Exception

• Checked Exception: exception in compile time, "You must deal with me, or your code won’t compile.", thus, try-catch or throws
• Unchecked Exception: exception in runtime, ` “You can deal with me, but I won’t force you — until I crash your app.”`

throw 🆚 throws

• throw: throw exception

public void checkAge(int age) {
    if (age < 18) {
        throw new IllegalArgumentException("Age must be 18 or older.");
    }
}

• throws: declare exception in method declaration

public void myMethod() throws IOException, SQLException {
    // method code
}

✅ try-catch-finally 구문에서 finally의 역할

• try: code that might throw exception
• catch: what to do when exception occurs

• finally: code that must be run regardless of exception

• finally: after IO, return DB connectoin pool regardless of exception

Throwable 🆚 Exception의 차이

• Throwable: super class of both Exception and Error
• Exception: child class of Throwable, a problem developer can fix

✅ 제네릭(Generic)이란 무엇이고, 왜 사용할까요?

• create class, interface, methods that operate on typed parameter
• postpone the decision of instance type until we use the type
• decide type from outside class
• 👍🏻 type safe at compile time
• 👍🏻 write one class/method that works with many types

List<String> stringList = new ArrayList<>();
List<Integer> integerList = new ArrayList<>();

👍🏻 only need to create one List, ArrayList class
- can create ArrayList with both String and Integer

✅ What is Lambda?

• an anonymous method(method without name) written in one line
• omit return, method name
• (parameters) -> expression

User user = userRepository.getByUserId(userId)
    .orElseThrow(() -> new UserNotFoundException());

✅ What is functional programming(함수형)?

• focus on function, data flow
• focus on immutable

• in Java, lambda, Stream API (map(), filter())

• 🆚 OOP
• focus more on Object and how they interact
• focus on mutable
• inheritence, polymorphism, abstract, encapsulation

✅ What is functional interface?

• interface that has exactly one abstract method
• functional interface is designed to be used with lambda expressions
• 자바가 자주 사용할 것 같은 lambda 함수 형태를 함수형 인터페이스로 만들어 제공해준 것

@FunctionalInterface //add annotation, limit to one abstract method
public interface Consumer<T> {
    void accept(T t); //only one abstract method
}

Consumer<String> printer = s -> System.out.println("Hello " + s); //use functional interface
printer.accept("Java"); // Output: Hello Java

• also can use functional interface with List, Map

public static void main(String[] args) {
    List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

     //replace all: method provided by functional interface
    // 각 요소에 10을 곱함
    list.replaceAll( (x) -> x * 10 );

     //forEach: method provided by functional interface
    list.forEach( (x) -> System.out.println(x) );
}

✅ What is Stream?

• API to efficiently process data in Collections, like Array or List
• operate on data, and produce results in pipeline

List<String> names = userRepository.findByName();

List<String> result = names
    .stream() //create stream
    .filter(name -> name.startsWith("A")) //intermediate operations
    .map(String::toUpperCase) //intermediate operations
    .collect(Collectors.toList()); //terminal operations

✅ 람다와 스트림은 왜 생겨났을까요?

• to support functional programming in Java
• example of functional programming in Java is lambda and Stream API

✅ 어노테이션이란?

• metadata to add to class, method, variable, parameters…
• @Override
• @Autowired: inject dependency into class automatically
• @Controller: tell Spring MVC controller to handle web requests
• @RestController: @Controller + @ResponseBody, so return JSON or XML
• @GetMapping, @RequestMapping, @PostMapping: map HTTP methods
• @Service: this class is business logic service class
• @Repository: this class is DAO(Data Access Object) and interacts with interface
• @Component
• @Value: inject value from application.yaml
• @RequestParam: extract query parameter from request URL
• @PathVariable: extract variable from URL

✅ 어노테이션 사용 이유

• compiler uses annotation for checks, like missing @Override
• metadata for tools, @Autowired for dependency injection
• clean code, other developers can know @Repository, @Controller

✅ 리플렉션이란

• inspect and manipulate classes/members at runtime
• 구체적인 클래스 타입을 몰라도 그 클래스의 method, variable에 접근할 수 있게 해준다
• can access private field, methods
• can create objects dynamically
• ⚠️ can break encapsulation

✅ System.out.println 클래스는 성능이 좋지 않다고 하는데 이유?

• lock can occur in blocking I/O and multithreading
• println is synchronized lock

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