如何将java.lang.Number []转换为java.lang.Byte []？

Question

I'm playing around with the Number wrappers of Java and I got the following error when I tried to cast a Number[] to Byte[]:

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Cannot cast from Number[] to byte[]
    Type mismatch: cannot convert from byte[] to Byte[]

The code it breaks on is: Byte[] coordinates;

ByteVector(Number... coordinates) {
    super(coordinates);
    this.coordinates = (Byte[])coordinates;
}

How can I write this so that I can input an array of any kind of Number and cast it to an array of Byte?

The constructor is called with new ByteVector(1,2); , so

Answer 1

You cannot cast arrays for a simple reason: It is not safe. Example

Number[] numbers = new Number[10];
numbers[0] = new Byte(10);
numbers[1] = new Byte(11);
Byte[] bytes = (Byte[]) numbers;
numbers[2] = new Double(0);

So if this were allowed, there would now be a Double in the Byte[] - can't happen! And this is actually true the other way too:

Byte[] numbers = new Byte[10];
numbers[0] = new Byte(10);
numbers[1] = new Byte(11);
Number[] bytesNumbers = (Number[]) numbers; // upcast - still not safe!
bytesNumbers[2] = new Double(0);

Answer 2

Number是可Serializable ，因此您可以简单地序列化数组。

Answer 3

You can use Java 8 Stream API to iterate over the array, convert each number to byte and collect to array:

this.coordinates = stream(numbers)
    .map(Number::byteValue)
    .toArray(Byte[]::new);

stream method is staticly imported java.util.Arrays.stream .

Note that you might lose precision here. For example if numbers contains 12345 it will be converted to 57 . Four doubles and floats, all decimal parts will be lost.