private static void practiceIf(){
    Scanner scanner = new Scanner(System.in);
    System.out.println("Please input your weight(Kg):");
    String weightValue = scanner.nextLine();
    System.out.println("Please input your height(m):");
    String heightValue = scanner.nextLine();

    double weight = 0.0;
    double height = 0.0;

    try {
        weight = Double.parseDouble(weightValue);
    } catch (NumberFormatException e) {
        e.printStackTrace();
        System.out.printf("Invalid weight ‘%s’",weightValue);
        return;
    }
    try {
        height = Double.parseDouble(heightValue);
    } catch (NumberFormatException e) {
        e.printStackTrace();
        System.out.printf("Invalid height ‘%s’",heightValue);
        return;
    }

    double bmi = weight/Math.pow(height, 2);
    System.out.printf("Your bmi is %.2f", bmi);
    String levelMessage = "OK";
    if (bmi < 18.5){
        levelMessage = "TOO LIGHT";
    }else if (bmi >= 18.5 && bmi <= 25){
        levelMessage = "NORMAL";
    }else if (bmi > 25 && bmi <= 28){
        levelMessage = "TOO HEAVY";
    }else if (bmi > 28 && bmi <= 32){
        levelMessage = "FAT";
    }else if (bmi > 32){
        levelMessage = "TOO FAT";
    }
    System.out.printf(" and it's %s \n", levelMessage);
}

输出：

Please input your weight(Kg):
71
Please input your height(m):
1.88
Your bmi is 20.09 and it's NORMAL 

Process finished with exit code 0