leejunkim

Class vs Instance variables

2 min read

Instance variables

  • Belong to each individual object (instance) of a class.
    • They are created when a class is instantiated with the newkeyword
    • They last in the heap memory until the object’s life cycle
  • automatically initialized with default values, depending on their data type
    • reference type variables (참조자료형) have default null values
    • Primitive types have different default values based on their type
    • Basically they get overwritten when the constructor finishes if it assigns new values
  • Each object gets its own copy
public class Dog {
    String name;  // Instance variable
 
    public Dog(String name) {
        this.name = name;
    }
}
 
// Main.java
Dog dog1 = new Dog("Rex");
Dog dog2 = new Dog("Buddy");
 
System.out.println(dog1.name); // Rex
System.out.println(dog2.name); // Buddy

Class variables

  • Only created once when the class is loaded into memory
  • Belong to the class itself, not any individual object.
    • This is why it’s stored in the class area, and not the heap
  • No need to create an instance to access this, u can use the class itself!
    • you can technically create an instance but it’s not recommended
    • Shared by all instances
  • uses static keyword 
public class Dog {
    static int numberOfDogs = 0;  // Class variable
 
    public Dog() {
        numberOfDogs++;
    }
}
 
// Main.java
Dog dog1 = new Dog();
Dog dog2 = new Dog();
 
System.out.println(Dog.numberOfDogs); // 2
  • just use it with the class itself (like Dog.numberOfDogs)
  • With Garbage collection (GC)
    • Variables declared with static are not garbage collected for the entire runtime of the program because they belong to the class, not to individual instances
    • So, static variables live as long as the class is loaded, which often means for the entire duration the program runs
    • Because of this, you should carefully decide when to use static — unnecessary use can cause memory to be held longer than needed

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