For loop optimisation with Java8 - for-loop

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 ?

Answer 1

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>.

Answer 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.