Java : 범용 library 및 Annotation

• java.math package
  • BigInteger, BigDecimal::  Represents arbitrary-precision integers with no upper or lower bounds
    • Constructor
      • BigInteger(String val/integer): Creates a BigInteger instance from a string representation of an integer.
      • BigDecimal(String val/double val): Creates a BigDecimal instance from a double value.
    • Operations:
      • add(BigInteger val), subtract(BigInteger val), multiply(BigInteger val), divide(BigInteger val), mod(BigInteger val)
      • pow(int exponent): Raises this instance to the power of the specified exponent.
      • compareTo(BigInteger val): Compares this instance with another BigInteger, returning -1, 0, or 1.
      • instance data conversion : instance.toString() / instance.xxxvalue()
• java.math class
  • abs(val) : Returns the absolute value of an argument.
  • max,min (two value): Returns the greater, smaller of two values.
  • pow(base val, power val) : Returns the value of the first argument raised to the power of the second argument
  • ceil(val): Returns the smallest (closest to negative infinity) double value that is greater than or equal to the argument and is equal to a mathematical integer.
  • floor(val): Returns the largest (closest to positive infinity) double value that is less than or equal to the argument and is equal to a mathematical integer.
  • round(val): Returns the closest long or int, as indicated by the method's return type, to the argumen
• java.time package
  • java.time package contains classes for date and time manipulation
  • class0 : LocalDate
    • Format: yyyy-MM-dd (e.g., 2024-07-31).
    • now(): Gets the current date from the system clock.
    • of(int year, int month, int dayOfMonth): Creates a LocalDate instance for a specific date.
    • plusDays(long daysToAdd): Adds a specific number of days to the date.

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  • class1 : LocalDateTime : Represents a date-time without a time-zone in the ISO-8601 calendar system
    • Format: yyyy-MM-ddTHH:mm:ss (e.g., 2024-07-31T12:00:00).
    • method 1 : now(): Obtains the current date-time from the system clock in the default time-zone.
    • method2 : of(int year, int month, int dayOfMonth, int hour, int minute): Obtains an instance of LocalDateTime from a year, month, day, hour, and minute.
    • method3 : format(DateTimeFormatter formatter): 매개변수 formatter에서 정의된대로 다듬어서 string 반환
  • class2 : DateTimeFormatter
    
    • Formatter for printing and parsing date-time objects.
    • method1 : ofPattern(String pattern): Creates a formatter using the specified pattern.
    • method2 : format(TemporalAccessor temporal): Formats a date-time object using this formatter.
    • static field1 : DateTimeFormatter.ISO_LOCAL_DATE : yyyy-MM-ddTHH:mm:ss 형태의 formatter을 호출
    • static field2 : DateTimeFormatter.ISO.TIME : HH:mm:ss 형태의 formatter을 호출 , 시분초 
      • ISO Date and Time Standards 
        
        • DateTimeFormatter formatter = DateTime
        • Formatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        • String currentDate = now.format(formatter);
• System.currentTimeMillis() : is a method in Java that returns the current time in milliseconds since the Unix epoch. The Unix epoch is defined as the beginning of January 1, 1970, in UTC `(Coordinated Universal Time).
  • Time Zone Independence: The value returned by System.currentTimeMillis() is in UTC, making it independent of the system’s time zone.
  • unit of the time : The unit of time returned by System.currentTimeMillis() in Java is milliseconds. ( 1 second = 1,000 milliseconds, 1 minute = 60 seconds, 1 hour = 3,600 seconds, 1 day = 86,400 seconds)

 

 

 

 

• java.util.Random
  • the package generate pseudorandom numbers. The seed is derived from current time in millisecond.  You can either pass a seed explicitly to the constructor of a Random object 
  • method
    • nextBoolean(): Returns a random boolean value.
    • nextInt(): Returns a random integer.
    • nextInt(int bound): Returns a random integer between 0 (inclusive) and the specified bound (exclusive).
    • nextLong(): Returns a random long.
    • nextDouble(): Returns a random double between 0.0 (inclusive) and 1.0 (exclusive).
    • nextFloat(): Returns a random float between 0.0f (inclusive) and 1.0f (exclusive).
•  java.util.UUID
  • UUID :  Universal Unique Identifier, which is a standardized method of generating identifiers that are unique across different systems and networks 
    • A UUID is a 128-bit number, typically expressed as a string of 32 hexadecimal characters and broken down into five groups, separated by hyphens. This format is represented as 8-4-4-4-12, making a typical UUID look like this: 123e4567-e89b-12d3-a456-426614174000.
  • randomUUID(): Generates a random UUID (Version 4).
    • UUID uuid = UUID.randomUUID();
  • toString(): Returns a UUID as a string.

 

Java annotation

to provide metadata for methods where it is applied

 

 

@Target(ElementType.METHOD):

• Specifies that this annotation can only be applied to methods.

@Retention(RetentionPolicy.RUNTIME):

• Indicates that this annotation will be available at runtime. : This is necessary if you want to use reflection or an aspect-oriented programming (AOP) library (e.g., Spring AOP) to process this annotation.
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