How to convert List<Integer> to int[] in Java?
This is similar to this question: How to convert int[] to Integer[] in Java?
I'm new to Java. How can i convert a List<Integer> to int[] in Java? I'm confused because List.toArray() actually returns an Object[] , which can be cast to nether Integer[] or int[] .
Right now I'm using a loop to do so:
int[] toIntArray(List<Integer> list){
int[] ret = new int[list.size()];
for(int i = 0;i < ret.length;i++)
ret[i] = list.get(i);
return ret;
}
I'm sure there's a better way to do this.
Unfortunately, I don't believe there really is a better way of doing this due to the nature of Java's handling of primitive types, boxing, arrays and generics. In particular:
List<T>.toArray won't work because there's no conversion from Integer to int int as a type argument for generics, so it would have to be an int -specific method (or one which used reflection to do nasty trickery). I believe there are libraries which have autogenerated versions of this kind of method for all the primitive types (ie there's a template which is copied for each type). It's ugly, but that's the way it is I'm afraid :(
Even though the Arrays class came out before generics arrived in Java, it would still have to include all the horrible overloads if it were introduced today (assuming you want to use primitive arrays).
No one mentioned yet streams added in Java 8 so here it goes:
int[] array = list.stream().mapToInt(i->i).toArray();
Thought process:
Stream#toArray returns Object[] , so it is not what we want. Also Stream#toArray(IntFunction<A[]> generator) doesn't do what we want because generic type A can't represent primitive int int , because its toArray method will most probably also return int[] array (returning something else like Object[] or even boxed Integer[] would be unnatural here). And fortunately Java 8 has such stream: IntStream so now only thing we need to figure out is how to convert our Stream<Integer> (which will be returned from list.stream() ) to that shiny IntStream . Here mapToInt method comes to rescue. All we need to do is provide some mapping from Integer to int . We could use something like Integer#getValue which returns int :
mapToInt( (Integer i) -> i.intValue())
(or if someone prefers mapToInt(Integer::intValue) )
but similar code can be generated using unboxing, since compiler knows that result of this lambda must be int (lambda in mapToInt is implementation of ToIntFunction interface which expects body for int applyAsInt(T value) method which is expected to return int ).
So we can simply write
mapToInt((Integer i)->i)
or simpler (since Integer i type can be inferred by compiler because List<Integer>#stream() returns Stream<Integer> )
mapToInt(i -> i)
In addition to Commons Lang, you can do this with Guava's method Ints.toArray(Collection<Integer> collection) :
List<Integer> list = ...
int[] ints = Ints.toArray(list);
This saves you having to do the intermediate array conversion that the Commons Lang equivalent requires yourself.
链接地址: http://www.djcxy.com/p/46088.html上一篇: 用零填充字符串