For loop optimisation with Java8

Question

How do achieve this with one line ?

I am currently trying to do this

Example :

{{"id" :"2", values: ["10","11", "12"]} , {"id" : "3", values : ["23"]}}

to

{{"id" :"2","value": "10"},{"id": "2","value":"11"},
 {"id" :"3","value":"23"} , {"id" : "2", "value":"12"}}

My java code is

Map<Integer, List<Integer>> attrMap = new HashMap<>();
//getAllData() & item.getValues() both returns List

getAllData().forEach(item - > {
    item.getValues().forEach(val - > {
        attrMap.computeIfAbsent(item.getId(), (k) - > 
        new ArrayList < > ()).add(val.getValue());
    });
});

How can i do it only 1 line ?

This question is kinda absurd. First of all the most important is REAL optimisation (time + data), the number of lines isjust for readability. When I see that some piece of code won'tchange anymore sometimes I remove line-breaks so i can scroll the code faster. But thats all. — Òscar Raya
– Òscar Raya, Commented Jul 14, 2017 at 8:56
I mean to say to avoid two foreach loop & do it with one foreach loop. Is it possible? — Sthita
– Sthita, Commented Jul 14, 2017 at 9:19

Holger · Accepted Answer · 2017-07-14 10:08:40Z

5

Since the IDs are unique, you can do it like

Map<Integer, List<Integer>> attrMap = getAllData().stream()
  .collect(Collectors.toMap(
    item -> item.getId(),
    item -> item.getValues().stream().map(i->i.getValue()).collect(Collectors.toList())));

But, of course, this will still have the performance characteristics of two nested loops. It would support parallel processing, though, but I doubt that your data will be large enough to draw a benefit from parallel processing.

Further, note that the resulting map still structurally matches your first pattern,

{{"id" :"2", values: ["10","11", "12"]} , {"id" : "3", values : ["23"]}}

you just converted item to an entry of the result Map and val to an element of a List<Integer>.

answered Jul 14, 2017 at 10:08

Holger

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Comments

Eugene · Accepted Answer · 2017-07-14 09:04:16Z

2

Assuming you have your input like this:

static class Data {
    private final int id;

    private final List<Integer> values;

    public int getId() {
        return id;
    }

    public List<Integer> getValues() {
        return values;
    }

    public Data(int id, List<Integer> values) {
        super();
        this.id = id;
        this.values = values;
    }
}

It could be done via:

 List<SimpleEntry<Integer, Integer>> result = Arrays.asList(
           new Data(2, Arrays.asList(10, 11, 12)), 
           new Data(3, Arrays.asList(23)))
              .stream()
              .flatMap(d -> d.getValues().stream().map(x -> new AbstractMap.SimpleEntry<>(d.getId(), x)))
            .collect(Collectors.toList());

    System.out.println(result); // [2=10, 2=11, 2=12, 3=23]

I am collecting those to Pair or AbstractMap.SimpleEntry.

answered Jul 14, 2017 at 9:04

Eugene

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2 Comments

Robin Topper Over a year ago

Don't think this is much shorter than the code from the OP.

Sthita Over a year ago

I mean to say to avoid two foreach loop & do it with one foreach loop. Is it possible?

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