Primitive type, Reference type

✅ Primitive type

• int, long, double, boolean
• save value itself
• int A = 10;

✅ Reference type

• class is reference type
• String
• save memory value

Student student = new Student();
student.age = 10;

• reference type can be initialized as null
• null means address has not been set yet

📌 대원칙: 자바는 항상 변수의 값을 복사해서 대입한다

자바는 항상 변수의 값을 복사해서 대입한다
Java is always pass-by-value, even for wrapper classes or objects

✅ copy value

• ✔️ primitive type
• a, b have different memory address
• 기본형은 변수의 실제 값을 복사해서 대입
• copies the actual value
• 나도 종이를 가지고 있고, 상대방도 종이를 가지고 있는데 내 종이에 있는 값을 그대로 상대방 종이에 복사해 줌
• 내 종이 내용을 바꾼다고 해서 상대방 종이 내용은 바뀌지 않음

    public static void main(String[] args) {
        int a = 10;
        int b = a;

        System.out.println("a: " + a);
        System.out.println("b: " + b);

        a = 20;
        System.out.println("a is changed to 20");
        System.out.println("a: " + a); // a = 20
        System.out.println("b: " + b); // b = 10

        b = 30;
        System.out.println("b is changed to 30");
        System.out.println("a: " + a); // a = 20
        System.out.println("b: " + b); // b = 30
    }

✅ copy reference

• ✔️ reference type
• 참조형은 변수에 들어있는 참조값을 복사해서 대입
• value being copied is a reference to the object, not the object itself.
• 나와 상대방은 구글 클라우드에서 같은 문서를 공유함
• 내 문서를 바꾸면 상대방 문서 내용도 바뀜

    public static void main(String[] args) {
        Data dataA = new Data();
        dataA.value = 10;
        Data dataB = dataA; //dataB copy address of dataA

        System.out.println("dataA 참조값=" + dataA); //Data@7344699f
        System.out.println("dataB 참조값=" + dataB); //Data@7344699f
        System.out.println("dataA.value = " + dataA.value); //10
        System.out.println("dataB.value = " + dataB.value); //10

        //dataA 변경
        dataA.value = 20;
        System.out.println("a is changed to 20");
        System.out.println("dataA.value = " + dataA.value); //20
        System.out.println("dataB.value = " + dataB.value); //20

        //dataB 변경
        dataB.value = 30;
        System.out.println("b is changed to 30");
        System.out.println("dataA.value = " + dataA.value); //30
        System.out.println("dataB.value = " + dataB.value); //30
    }

💡 심화 pritive type: copy value

int a = 10;
int b = changeNum(a);

System.out.println(a); //10
System.out.println(b); //20

public static int changeNum(int i){
    return i+10;
}

• a는 바뀌지 않음

• int는 pritive type, 따라서 copy value

• 바뀐 a의 값을 프린트하기 위해서는 다른 변수 b에 저장해서 프린트해야 함

• b는 바뀐 a의 값을 대입했으니 바뀜

• method parameter도 pritive type일때는 copy value
• 내가 아무리 changeNum안에서 i를 지지고 볶아봤자 a는 변하지 않아요

💡 심화 reference type:

• instance의 attribute은 primitive type인데, 이걸 바꾸면 어떻게 될까?

        Student studentA = new Student(10);
        Student studentB = studentA; //point to ssme reference
        Student studentC = new Student(studentA); //copy constructor

        studentA.setAge(20);
        System.out.println(studentA.getAge()); //20
        System.out.println(studentB.getAge()); //20
        System.out.println(studentC.getAge()); //10

        studentB.setAge(30);
        System.out.println(studentA.getAge()); //30
        System.out.println(studentB.getAge()); //30
        System.out.println(studentC.getAge()); //10

• StudentA: 나이 10살로 정함
• StudentB: studentA를 대입, copy by value of Reference
  • same reference as studentA
  • studentA and B are same objects

• StudentC: copy constuctor, studentA를 사용해 새로운 student instance 생성
  • new Student object, with copied age value
  • 새로운 memory address, creates a new object by copying data from another object of the same class
  • DOES NOT share reference(deep copy) ❌
  • 따라서 age 변하지 않음

        // method에 class instance(copy reference)를 parameter로 넘기면
        // instance의 attribute는 바뀔까?
        changeAge(studentC);
        System.out.println(studentC.getAge()); //11

        changeAgeReturn(studentC);
        System.out.println(studentC.getAge()); //12
        int age = changeAgeReturn(studentC);
        System.out.println(age); //13
        System.out.println(studentA.getAge()); //30


    public static void changeAge(Student student){
        student.setAge(student.age + 1);
    }

    public static int changeAgeReturn(Student student){
        student.setAge(student.age + 1);
        return student.age;
    }

• instance는 copy by reference value이므로 age가 바뀜

✅ Null

• reference type can be initialized as null
• null means no address has been allocated

        Data data1 = null;
        System.out.println(data1); //no address, null

        Data data2 = new Data();
        System.out.println(data2); //has address
        System.out.println(data2.value); //0

        data2 = null; //free address space
        System.out.println(data2); //now no address, null

• GC will free unused address space
• after data2 = null, the address space that data2 was using will be freed by garbage collector
• ⭐️ GC is big advantage of Java, keeps freeing unused address space, making memory use efficient

✅ NullPointerException

• cannot point null
• because null means no address space

    public static void main(String[] args) {
        Data data1 = null;
        data1.value = 100; //cannot point null! NullPointerException
    }