Immutable

✅ Summary

• Immutable: cannot change reference value
• 1️⃣ final field, 2️⃣ no setter

✅ Primitive 🆚 Reference

• 🆚 Primitive

int a = 10;
int b = a;

b = 20;

• ❓ 이제 a의 값은? 여전히 10
• ❓ 이제 b의 값은? 20

• b만 바꿨으니 a의 값은 바뀌지 않음

• int a and int b are primitive type

• primitive values DO NOT share memory

• 🆚 Reference

Address addressA  = new Address("seoul");
Address addressB  = addressA;

addressB.setValue("busan");

• ❓ 이제 addressA의 값은? seoul
• ❓ 이제 addressB의 값은? seoul

• b만 바꿨지만 a의 값도 바뀜

• addressA and addressB are reference type
• they both reference(point) to the SAME memory

✅ Side effect of reference

• I only want to change addressB

• however, end up changing all reference values

• 👎🏻 change addressB, but change addressA as well!

Address addressA  = new Address("seoul");
Address addressB  = addressA;

addressB.setValue("busan"); //👎🏻 this will change both A and B

• 💊 immutable

✅ Immutable

• reference value의 복사를 막을수는 없다

• 🔐 복사한 reference value는 값을 바꿀 수 없게 락을 걸어버리자

• 💡 how to make immutable class
• final field
• no setter

//👎🏻 before, not immutable
public class Address {
    private String value; //👎🏻 not final

    public void setValue(String value) { //👎🏻 setter
        this.value = value;
    }
}

//👍🏻 after, immutable
public class ImmutableAddress {
    private final String value; //1️⃣ final
    //2️⃣ no setter
}

• ✔️ result

public static void main(String[] args) {
    Address addressA  = new Address("seoul");
    Address addressB  = addressA;
    addressB.setValue("busan"); //👎🏻 this will make both A, B change

    ImmutableAddress immutableAddresssA = newImmutableAddress("seoul");
    ImmutableAddress immutableAddresssB =immutableAddresssA;

    // 👍🏻 address value is final, cannot change
    // 👍🏻 no setter, cannot change
    immutableAddresssB = new ImmutableAddress("busan");
    }
    //if want to change AddresssB, have to create a new instance

✅ How to change immutable

• need to create a new instance, change value, and return the new instance

• old instance will remain unchanged

• ✔️ Immutable obj class

public class ImmutableObj {
    private final int value; //1️⃣ final

    public ImmutableObj(int value) {
        this.value = value;
    }

    //2️⃣ no setter

    public ImmutableObj add(int addValue){ //return new instance
        int result = value + addValue;
        return new ImmutableObj(result);
        //do not change this instance value
        //create a new instance to return the new value
    }
}

• ✔️ Main class using method add()

public class ImmutableMain {
    public static void main(String[] args) {
        ImmutableObj immutableObj = new ImmutableObj(10);
        //create new instance
        ImmutableObj newImmutableObj = immutableObj.add(20); //method add()

        System.out.println(immutableObj.getValue()); //10
        System.out.println(newImmutableObj.getValue()); //30
    }
}

• old instance immutableObj did not change
• created new instance newImmutableObj and returned new value

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