Type safe Enum

✅ Why do we need Enum?

• Limit field to certain value

• User role can be only BASIC, GOLD, DIAMOND
• 👎🏻 how can we prevent entering basiiiic(오타) or diamond(소문자) or VIP(존재하지 않는 값)?
• 💊 ENUM

✅ Type safe enum pattern

• can only use enumerated fields

• if only enumerate BASIC, GOLD, DIAMOND, can only use these three values

• 👍🏻 type safe: only can use enumerated fields
• 👍🏻 data consistency

✅ Implement enum pattern

• 1️⃣ constant, static final enum field

• 2️⃣ private constructor, cannot be created from outside ENUM class

• ✔️ Example of enum pattern

public class ClassGrade {
    //same class, but different instance
    public static final ClassGrade BASIC = new ClassGrade(); //1️⃣ constant, static final
    public static final ClassGrade GOLD = new ClassGrade();
    public static final ClassGrade DIAMOND = new ClassGrade();

    //2️⃣ private constructor, cannot create from outside this class
    private ClassGrade() {
    }
}

• ClassGrade.BASIC, ClassGrade.GOLD, ClassGrade.DIMAOND is same class
• but different instance

✅ ENUM

• Java helps the use of enum pattern
• 👍🏻 type safe: only can use enumerated fields
• 👍🏻 data consistency

public enum Grade {
    BASIC, GOLD, DIAMOND
}

• same code as ClassGrade
• CANNOT be created as instance(private constructor)
• Grade.BASIC, Grade.GOLD, Grade.DIMAOND is same class
• but different instance

✅ Enum methods

• .values(): get enum values
• .name() : get name of value
• .ordinal(): get number of value
• .valueOf(): set enum value

Grade[] values = Grade.values(); //get all values of ENUM


for(Grade value : values){
    value.name(); //value 이름 얻기
    value.ordinal(); //몇 번째 value인지 return
}

String input = "GOLD";
Grade gold = Grade.valueOf(input); //set ENUM value

✅ Enum + Value

• want to set different discount value to user

• Basic user: 10% discount
• Gold user: 20% discount
• Diamond user: 30% discount

public enum Grade {
    BASIC(10), GOLD(20), DIAMOND(30);

    private final int discountPercent; //add value to ENUM

    Grade(int discountPercent) { //private -> need constructor
        this.discountPercent = discountPercent;
    }

    public int getDiscountPercent() { //getter
        return discountPercent;
    }
}

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