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 Method Design
Designing methods with correct return types and access modifiers.

Step 1: Analyze the requirements and instance variables

The Car class has three private instance variables:

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

We need to evaluate which of the three options is a correctly designed method for this class.

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 access this getter-style calculation.
• Return Type: double. Since cost is a double and tax is a double, multiplying them results in a double. This matches the return type.
• 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 * (1 + tax);
}

• Return Type: String.
• Issue: The expression cost * (1 + tax) evaluates to a numeric double value, not a String. This will cause a compilation error because a double cannot be directly returned from a method declared to return a String.

Step 4: Evaluate Option 3

Let's look at the code for Option 3:

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

• Return Type: void.
• Issue: A void method cannot return any value. Attempting to use return with an expression in a void method causes a compilation error.

Answer:

Option 1