java 8制作两个倍数的流

Question

I'm practicing streams in java 8 and im trying to make a Stream<Integer> containing the multiples of 2. There are several tasks in one main class so I won't link the whole block but what i got so far is this:

Integer twoToTheZeroth = 1;
UnaryOperator<Integer> doubler = (Integer x) -> 2 * x;
Stream<Integer> result = ?;

My question here probably isn't related strongly to the streams, more like the syntax, that how should I use the doubler to get the result?

Thanks in advance!

Answer 1

You can use Stream.iterate .

Stream<Integer> result = Stream.iterate(twoToTheZeroth, doubler);

or using the lambda directly

Stream.iterate(1, x -> 2*x);

The first argument is the "seed" (ie first element of the stream), the operator gets applied consecutively with every element access.

EDIT:

As Vinay points out, this will result in the stream being filled with 0s eventually (this is due to int overflow). To prevent that, maybe use BigInteger :

Stream.iterate(BigInteger.ONE, 
               x -> x.multiply(BigInteger.valueOf(2)))
      .forEach(System.out::println);

Answer 2

Arrays.asList(1,2,3,4,5).stream().map(x -> x * x).forEach(x -> System.out.println(x));

所以你可以在map来电者中使用doubler