使用DecimalFormat进行格式化会引发异常-“无法将给定对象格式化为数字". [英] Formatting using DecimalFormat throws exception - "Cannot format given Object as a Number"
问题描述
这可能看起来像是一个重复的问题，但是我尝试了以下所有链接，但找不到正确的答案.
This might look like a repeated question but I tried in all the below links and can't get a proper answer.
但是我没有发现问题所在.这是我的代码
But I'm not getting what's wrong. Here is my code
DecimalFormat twoDForm = new DecimalFormat("#.##");
double externalmark = 1.86;
double internalmark = 4.0;
System.out.println(String.valueOf((externalmark*3+internalmark*1)/4));
String val = String.valueOf((externalmark*3+internalmark*1)/4);
String wgpa1=twoDForm.format(val); // gives exception
String wgpa2=twoDForm.format((externalmark*3+internalmark*1)/4)); // works fine
System.out.println(wgpa1);
format方法采用对象类型参数，所以这就是为什么我传递了一个给出异常的String对象
The format method takes Object type argument, so that's why I passed a String object which gives exception
线程"main"中的异常java.lang.IllegalArgumentException:无法 将给定对象的格式设置为数字.
但是当我给double值作为参数时，程序运行良好.但是，如果该方法是用Object类型参数定义的，为什么我在传递String时却得到了异常而在传递double时却没有得到了异常?
But when I give double value as argument the program works fine. But if the method is defined with Object type argument why I'm getting an exception while passing a String and not getting exception while passing double?
推荐答案
DecimalFormat的format()方法已重载.
在实际情况下，您正在调用:
In the working case, you are invoking :
public final String format(double number)
在失败的情况下，您正在调用:
And in the failing case, you are invoking :
public final String format (Object obj)
第一个方法采用一个非常具体的参数.它需要一个double.
The first method takes a very specific argument. It expects a double.
第二种情况不是这样，第二种情况接受的类型非常广泛:Object，因此在运行时对传递的类型进行检查.
This is not the case of the second one, which the type accepted is very broad : Object and where so the check on the type passed is performed at runtime.
通过提供一个不是double而是String的参数，调用的方法是第二个方法.
By providing a argument that is not a double but a String, the method invoked is the second one.
在内部，此方法依赖于format(Object number, StringBuffer toAppendTo, FieldPosition pos)方法，该方法期望使用number自变量，该自变量是Number类(Short，Long，... Double)的实例. :
Under the hood, this method relies on the format(Object number, StringBuffer toAppendTo, FieldPosition pos) method that expects to a number argument that is an instance of the Number class (Short, Long, ... Double):
@Override
public final StringBuffer format(Object number,
StringBuffer toAppendTo,
FieldPosition pos) {
if (number instanceof Long ||
number instanceof Integer ||
number instanceof Short ||
number instanceof Byte ||
number instanceof AtomicInteger ||
number instanceof AtomicLong ||
(number instanceof BigInteger && ((BigInteger)number).bitLength () < 64)) {
return format(((Number)number).longValue(), toAppendTo, pos);
} else if (number instanceof BigDecimal) {
return format((BigDecimal)number, toAppendTo, pos);
} else if (number instanceof BigInteger) {
return format((BigInteger)number, toAppendTo, pos);
} else if (number instanceof Number) {
return format(((Number)number).doubleValue(), toAppendTo, pos);
} else {
throw new IllegalArgumentException("Cannot format given Object as a Number");
}
}
但是，当您将其传递给String实例时，情况并非如此.
But it is not the case as you passed to it a String instance.
要解决此问题，可以像成功案例中那样传递double原语，或者将String转换为Number的实例，例如Double和Double.valueOf(yourString).
我建议采用第一种方法(传递double)，因为它在已经使用double原语的代码中更加自然.
第二个需要从String到Double的附加转换操作.
To solve the problem, either pass a double primitive as in the success case or convert your String into an instance of Number such as Double with Double.valueOf(yourString).
I advise the first way (passing a double) as it is more natural in your code that already uses double primitives.
The second one requires a additional conversion operation from String to Double.
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