Here's a detailed explanation and corrected answer you can post on StackOverflow:

You're experiencing the issue because of a common mistake in your constructor and how you're calling the BMI calculation method. Let's walk through it step-by-step.

🔍 1. Problem in Constructor

In your Person constructor:

public Person(String userFirstName, double userHeightInches, double userWeightPounds) {
    this.firstName = firstName;
    this.heightInches = heightInches;
    this.weightPounds = weightPounds;
}

You're using the wrong variable names inside the constructor. Instead of using userFirstName, userHeightInches, and userWeightPounds, you're using firstName, heightInches, and weightPounds which are the class fields. As a result, you're assigning null and 0.0 values to your object fields.

✅ Fix it like this:

public Person(String userFirstName, double userHeightInches, double userWeightPounds) {
    this.firstName = userFirstName;
    this.heightInches = userHeightInches;
    this.weightPounds = userWeightPounds;
}

🔍 2. Incorrect BMI Calculation Call

In your displayBMI() method, you're passing 0 for both height and weight:

double userWeightPounds = 0;
double userHeightInches = 0;

double BMI = anyPerson.calculateBMI(userWeightPounds, userHeightInches);

So you're calling the calculateBMI() method with zeros, even though the anyPerson object already has the correct height and weight.

✅ You have two options to fix this:

Option 1: Change calculateBMI() to use object fields:

Update the Person class method to:

public double calculateBMI() {
    return (this.weightPounds / (this.heightInches * this.heightInches)) * 703;
}

And then call it in displayBMI() like this:

double BMI = anyPerson.calculateBMI();

Option 2: Keep method parameters but pass actual values:

double userWeightPounds = anyPerson.getWeightPounds();
double userHeightInches = anyPerson.getHeightInches();

double BMI = anyPerson.calculateBMI(userHeightInches, userWeightPounds);

But Option 1 is cleaner and more object-oriented.

✅ Full Minimal Fixes Summary

• Fix constructor variable names.

• Use the actual object values to calculate BMI.

• Optionally, update calculateBMI() to not require parameters.

🧪 Bonus Fix: Add BMI to the toString()

If you want to include BMI in the output string, modify your toString() like this:

@Override
public String toString() {
    double bmi = calculateBMI();
    return this.firstName + " weighs " + this.weightPounds + " lbs and is " + this.heightInches +
           " inches tall. Your BMI is " + String.format("%.2f", bmi);
}

✅ Final Output Example:

Input:

John
70
150

Output:

John weighs 150.0 lbs and is 70.0 inches tall. Your BMI is 21.52
Healthy

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