Static vs. Instance Members

By the end of this lesson, you will be able to distinguish static from instance variables and methods, explain when to use each, and avoid common static/instance confusion errors.

The Two Kinds of Members

Every field (variable) and method in a Java class is either static or instance (non-static).

Instance members: belong to a specific object; each object has its own copy.

Static members: belong to the class itself; shared by all objects of that class.

Instance Variables

public class Dog {
    private String name;  // instance variable — each Dog has its own name
    private int age;
    
    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

Dog d1 = new Dog("Rex", 3);
Dog d2 = new Dog("Bella", 5);
// d1.name = "Rex"; d2.name = "Bella" — separate copies

Each object created from the class has its own set of instance variables.

Static Variables

public class Dog {
    private String name;
    private static int count = 0;  // tracks total dogs created
    
    public Dog(String name) {
        this.name = name;
        count++;  // increments the SHARED counter
    }
    
    public static int getCount() {
        return count;
    }
}

Dog d1 = new Dog("Rex");   // count = 1
Dog d2 = new Dog("Bella"); // count = 2
System.out.println(Dog.getCount()); // 2

A static variable (class variable) is shared by all objects of the class. There's only one copy, regardless of how many objects exist.

When to use static variables: for data that belongs to the class concept, not to any individual object — counters, constants, configuration.

Constants are almost always static: public static final double PI = 3.14159;

Instance Methods

public String getName() {
    return name;  // accesses THIS object's name field
}
d1.getName()  // "Rex"
d2.getName()  // "Bella"

Instance methods operate on a specific object and can access its instance variables.

Static Methods

public static int max(int a, int b) {
    return (a > b) ? a : b;
}
// Called on the class name, not an object:
int m = ClassName.max(5, 10);
// Or just max(5, 10) within the same class.

Static methods do not belong to any specific object. They cannot access instance variables or call instance methods (without an object reference).

Calling static methods: use the class name: , .