Java IF-ELSE IF-ELSE 跳过 if 和 else if 检查并自动打印出 else 语句

Question

Java 11.6 In this BMI calculator that will take in Person's weight, height and calculate the BMI. The BMI is calculated properly however in the classify BMI method the program skips the if and else if check and just prints out the else statement which is "obese" for every test. Is the loop implemented incorrectly? or the value of BMI is not initiating in the loop. When I print out the BMI, I do see that for every test the BMI does change so that is not a problem.

The PersonWeight.java class

import java.time.Year;


public class PersonWeight {


    private double height;
    private double weight;

    public PersonWeight() {

        height = 0;
        weight = 0;

    }

    public PersonWeight(double h, double w) {

        height = h;
        weight = w;

    }



    public void setHeight(double h) {
        this.height = h;
    }

    public double getHeight() {
        return height;
    }

    public void setWeight(double w) {
        this.weight = w;
    }

    public double getWeight() {
        return weight;
    }



    public double ComputeBMI() {

        double bmi = ((weight)/(height*height));
        return bmi;
    }



}

The test class that has the main method

import java.util.Scanner;
public class TestPersonWeight {



    public static void classifyBMI() {
        PersonWeight PersonWeight = new PersonWeight();
        double bmi = PersonWeight.ComputeBMI();


        if(bmi<18.5) {
            System.out.printf("Underweight");

        }else if (bmi >= 18.5 && bmi<25) {
            System.out.printf("Normal Weight");
        }else if (bmi >=25 && bmi<30) {
            System.out.printf("Overweight");
        }else  {
            System.out.printf("Obese");

        }
    }

        public static void main(String[] args){

        Scanner input = new Scanner(System.in);
        TestPersonWeight TestPersonWeight = new TestPersonWeight();
        PersonWeight PersonWeight = new PersonWeight()
        System.out.printf("Enter person's Height in Meters: ");
        double h = input.nextDouble();
        PersonWeight.setHeight(h);


        System.out.printf("Enter person's Weight in Kilograms: ");
        double w = input.nextDouble();
        PersonWeight.setWeight(w);

        PersonWeight.ComputeBMI();
        System.out.printf("%n Height: " + PersonWeight.getHeight());
        System.out.printf("%n Weight: " + PersonWeight.getWeight());
        System.out.printf("%n BMI: " + PersonWeight.ComputeBMI());
        classifyBMI();

    }
}

Answer 1

Your classifyBMI() method creates a new PersonWeight where the height and weight remain 0.0 , so you are dividing 0.0 by 0.0 , which results in NaN . Hence, none of the comparisons evaluates to true , and you end up with the else clause being executed.

You should change that method to be an instance method (ie not static ), and call it for the PersonWeight instance that you create in your main method.

Or, as an alternative, keep that method static , but pass to it the previously calculated bmi value.

ie in your main write:

double bmi = PersonWeight.ComputeBMI();
System.out.printf("%n Height: " + PersonWeight.getHeight());
System.out.printf("%n Weight: " + PersonWeight.getWeight());
System.out.printf("%n BMI: " + bmi);
classifyBMI(bmi);

and classifyBMI will become:

public static void classifyBMI (double bmi) {
    if(bmi < 18.5) {
        System.out.printf("Underweight");
    } else if (bmi >= 18.5 && bmi < 25) {
        System.out.printf("Normal Weight");
    } else if (bmi >= 25 && bmi < 30) {
        System.out.printf("Overweight");
    } else {
        System.out.printf("Obese");
    }
}

PS using the same identifier - PersonWeight - for both your class name and the variable name is a bad practice. Use personWeight for the variable.

Answer 2

The error is here:

PersonWeight personWeight = new PersonWeight();
double bmi = personWeight.computeBMI();

This always returns NaN (Not a number) because you compute on a fresh new object which has only zeroes in it. Division by zero is not allowed, hence the result is NaN. All if-comparisons fail because NaN cannot be compared to any number.

Solution 1: Move the ``classifyBMI() method inside the PersonWeight``` class and use the current class instance attributes (now a new class with zeroes) for the calculation.

Solution 2: Pass the filled PersonWeight``` instance to the classifyBMI()``` method as an argument and use that for the calculation.

Solution 3: Pass the bmi value to the ``classifyBMI()``` method as an argument and use that for the calculation.

The following code demonstrates all three solutions:

import java.util.Scanner;

public class Main
{

    public static class PersonWeight
    {
        private double height;
        private double weight;

        public PersonWeight()
        {
            height = 0;
            weight = 0;
        }

        public void setHeight(double h)
        {
            this.height = h;
        }

        public double getHeight()
        {
            return height;
        }

        public void setWeight(double w)
        {
            this.weight = w;
        }

        public double getWeight()
        {
            return weight;
        }

        public double computeBMI()
        {
            double bmi = ((weight) / (height * height));
            return bmi;
        }

        private void classifyBMI1()
        {
            double bmi = ((weight) / (height * height));

            if (bmi < 18.5)
            {
                System.out.println("Underweight");
            }
            else if (bmi >= 18.5 && bmi < 25)
            {
                System.out.println("Normal Weight");
            }
            else if (bmi >= 25 && bmi < 30)
            {
                System.out.println("Overweight");
            }
            else
            {
                System.out.println("Obese");
            }
        }
    }

    private static void classifyBMI2(PersonWeight personWeight)
    {
        double bmi = personWeight.computeBMI();
        if (bmi < 18.5)
        {
            System.out.println("Underweight");
        }
        else if (bmi >= 18.5 && bmi < 25)
        {
            System.out.println("Normal Weight");
        }
        else if (bmi >= 25 && bmi < 30)
        {
            System.out.println("Overweight");
        }
        else
        {
            System.out.println("Obese");
        }
    }

    private static void classifyBMI3(double bmi)
    {
        if (bmi < 18.5)
        {
            System.out.println("Underweight");
        }
        else if (bmi >= 18.5 && bmi < 25)
        {
            System.out.println("Normal Weight");
        }
        else if (bmi >= 25 && bmi < 30)
        {
            System.out.println("Overweight");
        }
        else
        {
            System.out.println("Obese");
        }
    }

    public static void main(String[] args) throws Exception
    {
        Scanner input = new Scanner(System.in);
        PersonWeight personWeight = new PersonWeight();

        System.out.printf("Enter person's Height in Meters: ");
        double h = input.nextDouble();
        personWeight.setHeight(h);

        System.out.printf("Enter person's Weight in Kilograms: ");
        double w = input.nextDouble();
        personWeight.setWeight(w);

        System.out.printf("%nHeight: %f%n", personWeight.getHeight());
        System.out.printf("Weight: %f%n", personWeight.getWeight());
        System.out.printf("BMI: %f2%n", personWeight.computeBMI());

        personWeight.classifyBMI1();
        classifyBMI2(personWeight);
        classifyBMI3(personWeight.computeBMI());
    }
}

I also fixed other things:

Variable names and method names should start with a small letter. Upper case beginnings are reserved for class name. To print simple strings use println() instead of printf() for better performance. When you use printf() to output variables, then use placeholders like %f instead of string concatenation. Again this provides better performance.