If both static and const fix a value, what is the difference between them?

While both static and const (or final in Java) are used to define values that cannot change, they serve different purposes and have distinct characteristics. Let’s break them down:

static
Definition: The static keyword is used to declare class-level variables or methods. It means that the variable or method belongs to the class itself rather than to any particular instance (object) of the class.

Characteristics:
Single Copy: There is only one copy of a static variable shared among all instances of the class. If the value is changed in one instance, it affects all instances.
Accessed Without an Instance: Static methods and variables can be accessed directly using the class name without creating an object of the class.

Use Cases:
Constants: Static variables can be used to define constants for a class that all instances share.
Utility Methods: Static methods are often used for utility or helper methods that don’t require instance data.
Example:

class MathUtils {
    static final double PI = 3.14159;  // Constant value
    
    static int add(int a, int b) {      // Static method
        return a + b;
    }
}

// Accessing static members
double circumference = 2 * MathUtils.PI * radius;  // Accessing static variable
int sum = MathUtils.add(5, 10);  // Accessing static method

const / final
Definition: The const keyword is used in languages like C++ to declare constant values that cannot be modified after they are initialized. In Java, the equivalent is the final keyword.

Characteristics:
Immutable Values: Once a const or final variable is assigned a value, it cannot be changed. This applies to the variable itself, not to the object it references (if it’s a reference type).
Instance Level: const/final can be instance-level (associated with an object) or class-level (when declared as static final).

Use Cases:
Constants: They are used to define values that should remain constant throughout the application (like mathematical constants).
Method Parameters: You can also use const to protect method parameters from being modified.

class Circle {
    final double radius;  // Instance variable

    Circle(double r) {
        radius = r;  // Must be initialized in constructor
    }

    double area() {
        return Math.PI * radius * radius;  // Use of final variable
    }
}

Circle circle = new Circle(5);
// circle.radius = 10;  // Error: Cannot modify final variable

Summary of Differences

Use static when you want to share a variable or method among all instances of a class.
Use const/final when you want to ensure that a value cannot be modified after it’s assigned.

Instances of a class mean?