使用Java流将对象映射到多个对象

Question

I have a question regarding Java streams. Let's say I have a stream of Object and I'd like to map each of these objects to multiple objects. For example something like

IntStream.range(0, 10).map(x -> (x, x*x, -x)) //...

Here I want to map each value to the same value, its square and the same value with opposite sign. I couldn't find any stream operation to do that. I was wondering if it would be better to map each object x to a custom object that has those fields, or maybe collect each value into intermediate Map (or any data structure).

I think that in terms of memory it could be better to create a custom object, but maybe I'm wrong.

In terms of design correctness and code clarity, which solution would be better? Or maybe there are more elegant solutions that I'm not aware of?

Answer 1

You can use flatMap to generate an IntStream containing 3 elements for each of the elements of the original IntStream :

System.out.println(Arrays.toString(IntStream.range(0, 10)
                                            .flatMap(x -> IntStream.of(x, x*x, -x))
                                            .toArray()));

Output:

[0, 0, 0, 1, 1, -1, 2, 4, -2, 3, 9, -3, 4, 16, -4, 5, 25, -5, 6, 36, -6, 7, 49, -7, 8, 64, -8, 9, 81, -9]

Answer 2

Besides using a custom class such as:

class Triple{
private Integer value;
public Triple(Integer value){
 this.value = value;
}

public Integer getValue(){return this.value;}
public Integer getSquare(){return this.value*this.value;}
public Integer getOpposite(){return this.value*-1;}
public String toString() {return getValue()+", "+this.getSquare()+", "+this.getOpposite();}
}

and run

IntStream.range(0, 10)
         .mapToObj(x -> new Triple(x))
         .forEach(System.out::println);

you could use apache commons InmmutableTriple to do so. for example:

 IntStream.range(0, 10)
.mapToObj(x -> ImmutableTriple.of(x,x*x,x*-1))
.forEach(System.out::println);

maven repo: https://mvnrepository.com/artifact/org.apache.commons/commons-lang3/3.6

documentation: http://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/tuple/ImmutableTriple.html