Question

a car class is to be created that will include private instance variables:

• num_doors (integer value)
• manufacturer (string)
• cost (double)

which of the following methods fit this best?

option 1:
public double get_cost(double tax)
{
return cost * (1 + tax);
}

option 2:
public string get_cost(double tax)
{
return cost * (tax);
}

option 3:
private void get_cost(double tax)
{
return cost * (1 + tax);
}

which option correctly fills in the missing code?

Explanation:

🆕 New Concept Discovered: Object-Oriented Encapsulation and Methods
Accessing and manipulating private instance variables safely.

Step 1: Analyze the Instance Variables

The Car class has three private instance variables:

• num_doors (integer)
• manufacturer (String)
• cost (double)

Because these variables are private, external classes cannot access them directly. We need public getter/accessor methods to retrieve or calculate values based on these variables.

Step 2: Evaluate Option 1

Let's look at the code for Option 1:

public double get_cost(double tax)
{
    return cost * (1 + tax);
}

• Access Modifier: public allows external classes to call this method to get the cost information.
• Return Type: double. The calculation cost * (1 + tax) multiplies a double (cost) by a double (1 + tax), which results in a double. This matches the return type perfectly.
• Logic: It correctly calculates and returns the total cost including tax.

Step 3: Evaluate Option 2

Let's look at the code for Option 2:

public String get_cost(double tax)
{
    return cost * (tax);
}

• Return Type: String.
• Logic: The expression cost * tax evaluates to a double. Returning a double when the method signature specifies a String return type will cause a compilation error.

Step 4: Evaluate Option 3

Let's look at the code for Option 3:

private void get_cost(double tax)
{
    return cost * (1 + tax);
}

• Access Modifier: private means other classes cannot use this method to retrieve the cost, defeating the purpose of a getter/accessor method.
• Return Type: void means the method cannot return any value. However, the body contains a return statement with a value, which will cause a compilation error.

Answer:

Option 1