[英]How to use Java Stream map for mapping between different types?
I have two arrays of equal size: 我有两个相同大小的数组:
int[] permutation
T[] source
I want to do smth like this 我想这样做
Arrays.stream(permutation).map(i -> source[i]).toArray();
But it won't work saying: Incompatible types: bad return type in lambda expression 但它不会起作用: 不兼容的类型:lambda表达式中的错误返回类型
Arrays.stream
with an int[]
will give you an IntStream
so map
will expect a IntUnaryOperator
(a function int -> int
). 带有
int[]
Arrays.stream
将为您提供一个IntStream
因此map
将需要一个IntUnaryOperator
(函数int -> int
)。
The function you provide is of the type int -> T
where T is an object (it would work if T
was an Integer
due to unboxing, but not for an unbounded generic type parameter, assuming it's a generic type). 你提供的函数是
int -> T
类型,其中T是一个对象(如果由于取消装箱而T
是一个Integer
,它将起作用,但对于无界泛型类型参数,假设它是泛型类型则不起作用)。
What you are looking for is to use mapToObj
instead, which expects an IntFunction
(a function int -> T
) and gives you back a Stream<T>
: 您正在寻找的是使用
mapToObj
,它需要一个IntFunction
(一个函数int -> T
)并返回一个Stream<T>
:
//you might want to use the overloaded toArray() method also.
Arrays.stream(permutation).mapToObj(i -> source[i]).toArray();
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