Final, Constant

✅ Final

• final: can not change after initialization
• final value can not be altered after initializatoin

• 처음 값 할당 후에는 변수의 값을 변경하지 못하게 막아버린다

• 👎🏻 without final

int data0;
data0 = 10;
data0 = 20; //change value possible

• 👍🏻 with final
• 처음 한번만 값 할당 가능
• final이 붙으면 바꿀 수 없음

final int data1;
data1 = 10; //can set value once
//data1 = 20; //🔴compile error

✅ static final

• static: 인스턴스끼리 공유
• final: 한번 생성 후에는 바꿀 수 없다

public class Student{
    static final int STUDENT_AGE = 15; //STATIC FINAL write in CAPITAL LETTERS
    final int grade;

    public Student(int grade){
        this.grade = grade; //initiate student with grade, grade will never change
    }
}

public class StudentMain{
    public static void main(String[] args){
        Student student1 = new Student(100);
        Student student2 = new Student(90);
        Student student3 = new Student(80);
    }
}

• ✔️ 이렇게 생성하면 all student 1, 2, 3 will have age = 15
  • static STUDENT_AGE will be shared among all student instance
  • static STUDENT_AGE is saved only once in method memory
  • since final STUDENT_AGE, age can not be altered
• ✔️ each student will have grade 100, 90, 80
  • since final int grade, grade can not be altered

✅ Constant

• 하나만 존재하고 : static
• 변하지 않는 값: final
• constant is used to make a fixed value to use the value itself
• constant is normally written in CAPITAL_LETTERS

public class Constant {

    public static final int HOURS_IN_DAY = 24;
    public static final int MINUTES_IN_HOUR = 60;
    public static final int SECONDS_IN_MINUTE = 60;

    public static final int MAX_USERS = 1000;
}

public class ConstantMain {
    public static void main(String[] args){
        Constant.HOURS_IN_DAY;
        Constant.MAX_USERS;
    }
}

• 👍🏻 유지보수가 쉽다.
• MAX_USERS를 바꾸고 싶으면 Constant클래스에 가서 한번만 바꾸면 됨

✅ Final reference value can be altered

public class Data {
    public int value;
}

• public int value is not final

public class FinalRefMain {
    public static void main(String[] args) {
        final Data data = new Data(); //data instance is final
        //data = new Data(); //🔴final value can not be altered after initialization

        data.value = 10;
        data.value = 20; //🟢even when instance final, reference value can be altered
        //value is not final
    }
}

• final Data data = new Data() is final
• however, value field in Data class is not final
• so value field can be altered as many times

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