Suppose i have a list of string
List<String> fourthList = Arrays.asList("abc","def","ghi");
I want to convert it into Map like {1=abc,2=def,3=ghi}.
Collectors in java is not allowing to me do that as it accept method only in keyMapper .
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This question has not been asked before or maybe i haven't found it. i was able to find convert list to map however in all the questions it mentioned user type variables .i was concerned about List of String type only .Rahul Pyasi– Rahul Pyasi2019-10-16 13:05:00 +00:00Commented Oct 16, 2019 at 13:05
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Map<Integer, String> collect = IntStream.rangeClosed(0, fourthList.size() - 1) .boxed() .collect(Collectors.toMap(i -> i + 1, fourthList::get)); System.out.println(collect);user2173372– user21733722019-11-05 11:21:20 +00:00Commented Nov 5, 2019 at 11:21
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1 Answer
As Per Hadi J's comment (recommended) you can use use :
IntStream.rangeClosed(0, list.size() - 1)
.mapToObj(i -> new AbstractMap.SimpleEntry<Integer, String>(i + 1, list.get(i)))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
Another way-around:
List<String> list = Arrays.asList("abc", "def", "ghi");
Converting List to Map<Integer, String> with key as integer and starting with 0...n:
AtomicInteger index = new AtomicInteger(0);
Map<Integer, String> map = list.stream()
.collect(Collectors.toMap(s -> index.incrementAndGet(), Function.identity()));
map.forEach((l, m) -> System.out.println(l + " " + m));
Output:
1 abc
2 def
3 ghi
I had done similar Map practices earlier this year, you can have a look: https://github.com/vishwaratna/ThatsHowWeDoItInJava8/blob/master/src/ListToMapWIthKeyAsInteger.java
6 Comments
DodgyCodeException
The problem is your use of a static variable
k makes this non-thread-safe, among many other drawbacks.
Rahul Pyasi
Thread safety is not a concern for me now , anyways Its good catch.
Rahul Pyasi
Thanks Vishwa it worked , however i have doubt on toMap signature as it accepts Function as argument however here it is taking variable here
Vishwa Ratna
@RahulPyasi , you are right
toMap signature accepts Function . Java lambda expressions are Java's first step into functional programming. A Java lambda expression is thus a function which can be created without belonging to any class.Rahul Pyasi
@VishwaRatna This is clear explanation . thanks for info!
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