Class, System, Math 클래스

✅ Class 클래스

• Class 클래스를 통해서 get metadata about a class 할 수 있다

• ✔️ get class type information: class name, super class, interface, access modifier…
• ✔️ reflection: get field, method, constructor info in class
• ✔️ dynamic loading: Class.forName(), newInstance()
• ✔️ annotation

✅ How to get metadata of class

• get metadata of String class
• reflection of a class

Class clazz = String.class; //get metadata of String class

clazz.getDeclaredFields(); //get fields
clazz.getDeclaredMethods(); //get methods
clazz.getInterfaces(); //get interface
clazz.getSuperclass() //get parent class

✅ System 클래스

• get metadata about system

• get time
• get env variable
• get system info
• copy array
• exit program

//get time
long currentTime = System.currentTimeMillis();
long nanoTime = System.nanoTime();

//get env variables
System.getenv();

//get system properties, 자바의 환경설정
//encoding info, java version
System.getProperties();
System.getProperty("java.version"); //get java version

//copy array
char[] originalArray = {'h', 'e', 'o'};
char[] copyArray = new char[3];
System.arraycopy(originalArray,0, copyArray, 0, originalArray.length);

//exit program
System.exit(0);

✅ Math 클래스

• useful methods for math

Math.max(10, 20);
Math.min(10, 20);
Math.abs(-10);

Math.ceil(2.1); //올림
Math.floor(2.1); //내림
Math.round(2.1); //반올림

Math.sqrt(4); //제곱근
Math.random(); //0.0 ~ 1.0 사이의 랜덤 숫자

• ✔️ random
• if there is seed, random result will always be same

Random random = new Random();

random.nextInt(); //random Integer
random.nextDouble(); //random Double
random.nextBoolean(); //random Boolean(true or false)

random.nextInt(10); //random Integer between 0~9

Random randomSeed = new Random(1); //if seed is same, random result is same
randomSeed.nextInt(); //always return same int

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