Java memory, Static, psvm

✅ Method, Stack, Heap

✔️ Method

• class

✔️ Stack

• methods
• program is run

✔️ Heap

• Instance created
• new
• class instance, array

💡 Method

• shared data to run program
• class: field, method, constructor…
• static: shared among instances
• literal

💡 Stack

• methods run in main class

• local variables, parameter, method return values…

• LIFO
• main() start ➡️ method1 start ➡️ method2 start

• method2 end ➡️ method1 end ➡️ main() end

• method in class 🆚 method in main class
• method in class: method area, shared data for creating instances
• method in main class: stack area

💡 Heap

• Instance, array
• new instance
• GC is run in heap area

• if value is not referenced in heap area, GC would free address space

• student class 🆚 student 1 instnace

• new student 1 instance is made from student class

  • student class: method area
  • student 1 instance made from student class: heap area
  • student 2 instance made from student class: heap area
    • student 1, 2 instance will be each allocated address space in heap area
    • however, student class is used commonly for student 1, student 2

public static void main(String[] args) {
        method1();
    }

    static void method1() {
        Data data1 = new Data(10);
        method2(data1);
    }

    static void method2(Data data) {
    }

• main() start
• ➡️ method1 start
• ➡️ new data1 in heap allocated address space
• ➡️ method2 start
• ➡️ method2 end
• ➡️ method1 end
• ➡️ new data1 in heap is not referenced anymore
• ➡️ GC clean
• ➡️ main() end

✅ Static

• student class로 student 1, 2, 3...등 만들건데, 인스턴스간에 공유하는 변수를 지정할 수는 없을까?

• 1️⃣ static is to share among instances
• 2️⃣ static variables are saved in method memory, only once
  • normally, instance is created in heap
  • however, bc static is shared among instances, it is saved in method memory
  • only saved once(shared among instance)

• 3️⃣ static variables can be used without creating instance

• 👎🏻 Before(without static)

public class Data1 {
    public String name;
    public int count; //👎🏻 not static

    public Data1(String name){
        this.name = name;
        count++; //when data instance is created, ++
    }
}

public class DataCountMain1 {
    public static void main(String[] args) {
        Data1 data1 = new Data1("A");
        System.out.println("data: " + data1.count);

        Data1 data2 = new Data1("B");
        System.out.println("data: " + data2.count);

        Data1 data3 = new Data1("C");
        System.out.println("data: " + data3.count);
    }
}

• ✔️ result

data: 1
data: 1
data: 1

• when instance is created, count is 0

• thus, making count++ will be 1

• 👍🏻 After(with static)

public class Data3 {
    public String name;
    public static int count; //👍🏻 implement static

    public Data3(String name){
        this.name = name;
        count++;
    }
}

public class DataCountMain3 {
    public static void main(String[] args) {
        Data3 data1 = new Data3("A");
        System.out.println("data: " + Data3.count); //static

        Data3 data2 = new Data3("B");
        System.out.println("data: " + Data3.count); //static

        Data3 data3 = new Data3("C");
        System.out.println("data: " + Data3.count); //static
    }
}

• ✔️ result

data: 1
data: 2
data: 3

• static variables can be used without creating instance
• Data3.count ⭕️

• data1.count ❌

• static int count is shared among instance data 1, 2, 3
• thus every time instance is created, 기존 count++
• because static int count is shared among instance, saved in method part of memory
  • only once saved in method memory
• 🆚 String name(not static) will be saved in heap part of memory
  • saved 3 times in heap memory
  • (created instance 3 times)

Local variable 🆚 Instance variable 🆚 Class variable

public class Data3 {
public String name; //class field, instance variable
public static int count; //class field, class variable, static
}

• Local variable: saved on memory stack, short life
• Instance variable: saved on memory heap, cleaned by GC
• Class variable: saved on memory method, longest life as variable

✅ Static Method

• 1️⃣ static method can be used without creating instance

• 👎🏻 Before(without static)

public class DecoUtil1 {
    public String deco(String str){
        return  str;
    }
}

public class DecoMain1 {
    public static void main(String[] args) {
        String s = "hello";

        DecoUtil1 utils = new DecoUtil1(); // 👎🏻 create instance
        utils.deco(s); // 👎🏻 then use method
    }
}

• 👍🏻 After(with static)

public class DecoUtil2 {
    public static String deco(String str){ //static
        return  str;
    }
}

public class DecoMain2 {
    public static void main(String[] args) {
        String s = "hello";

        DecoUtil2.deco(s); //call static method without creating instance
    }
}

• 2️⃣ static method can only use static field and static method
• 🆚 instance method can use both static, instance

public class DecoData {
    private int instanceValue; //instance variable
    private static int staticValue; //static

    public static void staticCall(){ //static
        staticValue++; //static can use static
        //instanceValue++; //🔴compile error, static can only use static
        //instanceMethod(); //🔴static can only use static
    }

    public void instanceMethod(){
        staticValue++; //🟢instance can use both static and not static
        instanceValue++;
        staticCall();
    }
}

• 🤨 Why static method can only use static?
• static is created in method
• static can be used without creating instance
• thus, if static calls an instance field or method, the instance might not have been created yet!

public class DecoDataMain {
    public static void main(String[] args) {
        DecoData.staticCall(); //static method is called from class, do not need instance to be created

        DecoData data = new DecoData();
        data.instanceMethod(); //instance method is called from instance, after creating instance
    }
}

✅ psvm Main()

• main method is also static

public static void main(String[] args) {}

• 1️⃣ can use main() method without creating main instance
• main() is used to start the programs
• 2️⃣ methods called in main() should also be static

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