[JAVA] Difference between instance variable and class variable

1.First of all

The difference between instance variables and class variables was ambiguous, so I summarized them for the future. I hope it helps.

2. What is an instance variable?

It is a variable that belongs to each instance. Specifically, the instance variable area (instance) is secured by the ** new keyword **, and the instance variable is initialized by the initial value assignment and the constructor (process executed when the instance is created). Note that each created instance is recognized as a separate instance, so the value of the instance variable will differ for each instance. Therefore, it is used when you want to handle different information for each object (instance of class).

3. What is a class variable?

A variable declared with the ** static keyword **, which is common information shared by all instances. Changing the value of a class variable from one object affects all other objects. Therefore, it is used when you want to handle information that is common to the entire object.

Postscript (2020/1/1) From @ shiracamus's comment

If there is an inheritance relationship between classes, class variables can have the same name for each inheriting class, and all inherited classes are traced and the first value reached is used. Therefore, even if a class variable has the same name, its value may differ depending on the referencing class. (Add a concrete example here to 5.)

4. Each feature

5. Specific example

Instance variables

Main.java

public class Main {
  public static void main(String[] args) {
    Sample sampleA = new Sample();
    Sample sampleB = new Sample();
    
    sampleA.a = 10;
    sampleB.a = 20;
    System.out.println(sampleA.a);
    System.out.println(sampleB.a);
  }
}

public class Sample {

  public int a;
}

Output result

10
20

I set different values for the instance variables of each object, so As a result, different values are output.

Class variables

Main.java

public class Main {
  public static void main(String[] args) {
    Sample sampleA = new Sample();
    Sample sampleB = new Sample();

    sampleA.setNum(30);
    sampleB.setNum(40);
    System.out.println(sampleA.getNum());
    System.out.println(sampleB.getNum());
  }
}

public class Sample {
  
  public static int b;
  
  public void setNum(int value) {
    b = value;
  }

  public int getNum() {
    return b;
  }
}

Output result

40
40

Class variables share a common value across all instances, and Because changes to one object affect other objects As a result, the last set 40 is output as the value of the class variable.

bonus

Main.java

public class Main {
  public static void main(String[] args) {
    var s1 = new Sample();
    var s2 = new Sample();
    System.out.println("Base: " + Base.a + ", " + Base.b);
    System.out.println("Sample: " + Sample.a + ", " + Sample.b);
    System.out.println("s1: " + s1.a + ", " + s1.b);
    System.out.println("s2: " + s2.a + ", " + s2.b);
    Base.a = 10;
    Sample.a = 20;
    s1.a = 30;
    s2.a = 40;
    System.out.println("Base: " + Base.a + ", " + Base.b);
    System.out.println("Sample: " + Sample.a + ", " + Sample.b);
    System.out.println("s1: " + s1.a + ", " + s1.b);
    System.out.println("s2: " + s2.a + ", " + s2.b);
    Base.b = 100;
    Sample.b = 999;
    System.out.println("Base: " + Base.a + ", " + Base.b);
    System.out.println("Sample: " + Sample.a + ", " + Sample.b);
    System.out.println("s1: " + s1.a + ", " + s1.b);
    System.out.println("s2: " + s2.a + ", " + s2.b);
  }
}

class Base {
    static int a = 0;
    static int b = 1;
}

class Sample extends Base {
    static int b = 2;
}

Output result

Base: 0, 1
Sample: 0, 2
s1: 0, 2
s2: 0, 2
Base: 40, 1
Sample: 40, 2
s1: 40, 2
s2: 40, 2
Base: 40, 100
Sample: 40, 999
s1: 40, 999
s2: 40, 999

Reference article

SOFTWARE ENGINEERING [tutorialspoint] (https://www.tutorialspoint.com/What-is-the-difference-between-class-variables-and-instance-variables-in-Java)