How do I validate user input in Java

Question

I wrote a bmi calculator program and I want to validate user input so that the user would not enter a negative number for the height or weight input.

How do I do this? I am new to Java, so I have no idea.

import java.util.Scanner;
 
public class BMICalculator {
 
    public static void main(String[] args) throws Exception {
        calculateBMI();
    }
 
    private static void calculateBMI() throws Exception {
        System.out.print("Please enter your weight in kg: ");
        Scanner s = new Scanner(System.in);
        float weight = s.nextFloat();
        System.out.print("Please enter your height in cm: ");
        float height = s.nextFloat();
         
        float bmi = (100*100*weight)/(height*height);
         
        System.out.println("Your BMI is: "+bmi);
        printBMICategory(bmi);

        s.close();
       
    }
     

    private static void printBMICategory(float bmi) {
        if(bmi < 24) {
            System.out.println("You are underweight");
        }else if (bmi < 29) {
            System.out.println("You are healthy");
        }else if (bmi < 34) {
            System.out.println("You are overweight");
        }else {
            System.out.println("You are OBESE");
        }
    }
}

One easy way is to use a while loop to keep requesting an input until a valid input is given. while (height < 0) { height = s.nextFloat(); } — Locke
– Locke, Commented Feb 1, 2021 at 19:45

Ali Faris · Accepted Answer · 2021-02-01 20:21:28Z

2

you can keep asking for value until the user input a valid number

private float readZeroOrPositiveFloat(Scanner scanner , String promptMessage) 
{
     float value = -1;
     while(value < 0){
         System.out.println(promptMessage);
         value = scanner.nextFloat();
     }
     return value;
}


private static void calculateBMI() throws Exception {
    System.out.print("Please enter your weight in kg: ");
    Scanner s = new Scanner(System.in);
    float weight = readZeroOrPositiveFloat(s , "Please enter your weight in kg: ");
    float height = readZeroOrPositiveFloat(s , "Please enter your height in cm: ");
     
    float bmi = (100*100*weight)/(height*height);
     
    System.out.println("Your BMI is: "+bmi);
    printBMICategory(bmi);

    s.close();
   
}

answered Feb 1, 2021 at 20:21

Ali Faris

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Comments

XO56 · Accepted Answer · 2021-02-01 22:20:50Z

Just to handle negative inputs and keep asking for valid input,

boolean flag = true;
while(flag){
    System.out.print("Please enter your weight in kg: ");
    int weight = sc.nextInt();
    System.out.print("Please enter your height in cm: ");
    int height = sc.nextInt();
    if(weight >= 0 && height >= 0){
        float bmi = (100*100*weight)/(height*height);
        flag = false;
    }else{
        System.out.println("Invalid Input");
    }
}

To handle all the unexpected inputs and keep asking for valid input,

boolean flag = true;
    while(flag){
        Scanner sc = new Scanner(System.in);
        try{
            System.out.print("Please enter your weight in kg: ");
            int weight = sc.nextInt();
            System.out.print("Please enter your height in cm: ");
            int height = sc.nextInt();
            if(weight >= 0 && height >= 0){
                float bmi = (100*100*weight)/(height*height);
                flag = false;
            }else{
                System.out.println("Invalid Input");
            }
        }catch(Exception e){
            System.out.println("Invalid Input");
        }
    }

Jackie · Accepted Answer · 2021-02-01 20:06:16Z

0

When you get input from the scanner, you could use an if statement to make sure the value is not negative. For example:

if(input value <= 0){
    System.out.println("Invalid Input");
}else { *program continues* }

answered Feb 1, 2021 at 20:06

Jackie

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