被忽略的Math.round

Question

    double energy, mass;
    double speedOfLight = 299792458.0;
    // Get mass
    System.out.print("Mass? ");
    mass = keyboard.nextDouble();
    // Calculate energy
    energy = mass*Math.pow(speedOfLight, 2);
    // Round to 1 decimal place
    energy = Math.round(energy * 10.0) / 10.0;
    System.out.printf("The energy is " +energy+ " Joules");
    // Close scanner
    keyboard.close();

它正在返回，“能量是8.987551787368176E16焦耳”，而它应该返回，“能量是89875517873681760.0焦耳”

Answer 1

Math.round按预期工作，返回最接近的整数(但输出为scientific notation格式)。问题出在号码的打印上。您可以使用printf的format specifiers以您想要的格式打印数字：

System.out.printf("The energy is %.1f Joules", energy);