点后第16个元素的意外舍入

Question

可能重复： Moving decimal places over in a double

我正在创建一个基本的实现装饰模式，使用冰淇淋和补充作为主题。除了突然出现的一些奇怪的输出行为之外，一切正常。

在看似随机的地方，成本将在圆点后面的第16个元素上舍入，这很奇怪，因为我使用的数字是1.00 (Vanilla)、1.10 (Chocolate)、0.30 (Sprinkles)、0.40 (WhippedCream)和0.35 (Nuts)。

为了重新创建它，您需要以下类和接口：

public interface Icecream {
    public double getCost();
    public String getDescription();
}

public class Vanilla implements Icecream {
    @Override
    public double getCost() {
        return 1.00;
    }

    @Override
    public String getDescription() {
        return "Vanilla ice";
    }
}

public class Chocolate implements Icecream {
    @Override
    public double getCost() {
        return 1.10;
    }

    @Override
    public String getDescription() {
        return "Chocolate ice";
    }
}

public abstract class Decorator implements Icecream{
    protected Icecream decoratedIcecream;

    public Decorator(Icecream decoratedIcecream) {
        this.decoratedIcecream = decoratedIcecream;
    }

    public abstract double getCost();
    public abstract String getDescription();
}

public class Sprinkles extends Decorator {
    public Sprinkles(Icecream decoratedIcecream) {
        super(decoratedIcecream);
    }

    @Override
    public double getCost() {
        return decoratedIcecream.getCost() + 0.30;
    }

    @Override
    public String getDescription() {
        return decoratedIcecream.getDescription() + ", Sprinkles";
    }
}

public class WhippedCream extends Decorator {
    public WhippedCream(Icecream decoratedIcecream) {
        super(decoratedIcecream);
    }

    @Override
    public double getCost() {
        return decoratedIcecream.getCost() + 0.40;
    }

    @Override
    public String getDescription() {
        return decoratedIcecream.getDescription() + ", Whipped cream";
    }
}

public class Nuts extends Decorator {
    public Nuts(Icecream decoratedIcecream) {
        super(decoratedIcecream);
    }

    @Override
    public double getCost() {
        return decoratedIcecream.getCost() + 0.35;
    }

    @Override
    public String getDescription() {
        return decoratedIcecream.getDescription() + ", Nuts";
    }
}


public class StartUp {
    public static void main(String[] args) {
        Icecream icecream = new Vanilla();
        showDetails(icecream);

        icecream = new Sprinkles(icecream);
        showDetails(icecream);

        icecream = new WhippedCream(icecream);
        showDetails(icecream);

        Icecream icecream2 = new Nuts(new Chocolate());
        showDetails(icecream2);

        icecream2 = new Sprinkles(icecream2);
        showDetails(icecream2);

        Icecream icecream3 = new Vanilla();
        showDetails(icecream3);

        icecream3 = new Nuts(icecream3);
        showDetails(icecream3);

        icecream3 = new Sprinkles(icecream3);
        showDetails(icecream3);
    }

    private static void showDetails(Icecream icecream){
        System.out.println("Cost: " + icecream.getCost() + "\t" + "Description: " + icecream.getDescription());
    }
}

这将产生以下结果：

Cost: 1.0   Description: Vanilla ice
Cost: 1.3   Description: Vanilla ice, Sprinkles
Cost: 1.7000000000000002    Description: Vanilla ice, Sprinkles, Whipped cream
Cost: 1.4500000000000002    Description: Chocolate ice, Nuts
Cost: 1.7500000000000002    Description: Chocolate ice, Nuts, Sprinkles
Cost: 1.0   Description: Vanilla ice
Cost: 1.35  Description: Vanilla ice, Nuts
Cost: 1.6500000000000001    Description: Vanilla ice, Nuts, Sprinkles

我不太愿意添加这么多代码，但考虑到错误似乎是随机出现的(一次是我添加了奶油，一次是我创建巧克力+坚果，两次是我添加了零星的代码)，我无法确定代码的哪些部分是相关的，哪些是不相关的。

为什么它有时在任何地方都被声明为双倍，并且总是在圆点后面有2位数的数字？

Answer 1

这就是浮点数(double和float)的现实.他们使用一个数学公式(我不详细介绍)来存储他们的数据，所以有些值不是你所期望的(这就是为什么银行和其他货币系统使用BigDecimal而不是double或其他类似的东西)。

因为，在您的例子中，您处理的是相当小的度量单位，所以我可能会把它简化为小数点。

下面是要输出的一小部分代码，四舍五入到小数点2位：

String dblOutput = new DecimalFormat("#.##").format(icecream.getCost());
System.out.println("Cost: " + dblOutput + "\t" + "Description: " + icecream.getDescription());

Answer 2

这只是浮点四舍五入的结果。请记住，在基数2中，一个很好的结束小数点，比如0.1，可能是一个无限重复的小数点。当小数点被四舍五入时，我们得到这些奇怪的舍入错误。

解决办法是永远不要直接打印浮点值。在DecimalFormat中使用Java.text或get货币实例来格式化(和舍入)输出