Question:

Consider the following code snippet:

public class ConstantsDemo {
    public static void main(String[] args) {
        final double PI = 3.14159;
        double radius = 5.0;
        double circumference = 2 * PI * radius;

        int num1 = 10;
        final int num2 = 20;

        num1 = 15;
        // num2 = 25;   // Error: Cannot assign a value to final variable 'num2'

        System.out.println("Circumference of a circle with radius " + radius + " is: " + circumference);
        System.out.println("num1: " + num1);
        System.out.println("num2: " + num2);
    }
}

1. Explain the concept of variables and constants in programming.

2. In the given code snippet, what is the difference between the variables PI, radius, num1 and num2?

3. Why does the statement num2 = 25; result in a compilation error?

Answer:

1. Explanation of variables and constants:

In programming, variables are used to store and manipulate data. They act as placeholders that can hold different values during program execution. Variables have a type (e.g., int, double, String) and a name (e.g., radius, num1), which is used to refer to them within the program. The value of a variable can be changed during runtime.

On the other hand, constants are also used to store data, but their value remains fixed and cannot be changed once assigned. Constants are declared using the final keyword, meaning their value cannot be reassigned later in the code. Constants are often used for values that should remain constant throughout the program, such as mathematical constants (e.g., PI) or predefined values that should not be modified.

2. Difference between the variables PI, radius, num1, and num2:

• PI is a constant declared with the final keyword. It represents the mathematical constant pi (approximately 3.14159) and its value cannot be changed. It is of type double.
• radius is a variable of type double which represents the radius of a circle. It is not declared as final, so its value can be changed during program execution.
• num1 is a variable of type int. It is initially assigned the value 10, but its value is later changed to 15. It is not declared as final, so its value can also be changed.
• num2 is a constant of type int. It is declared using the final keyword and initially assigned the value 20. Its value cannot be changed during program execution.

3. Compilation error with num2 = 25;:

In the given code snippet, the statement num2 = 25; results in a compilation error because num2 is declared as a constant using the final keyword. Once a constant is assigned a value, it cannot be changed. Therefore, any attempt to modify the value of num2 will result in a compilation error.

The purpose of declaring a variable as a constant is to enforce immutability, ensuring that the value remains constant throughout the program. This aids in code maintainability and readability as it explicitly states that the value should not be modified.

By commenting out the line num2 = 25;, the code will compile successfully and output the following:

Circumference of a circle with radius 5.0 is: 31.4159
num1: 15
num2: 20