How to convert a list of Strings to a LinkedHashMap?

Question

I have a list:

private List <String> list;

I want to convert it to a LinkedHashMap (to preserve order), such that the first two values in the map are a LinkedHashMap entry and so on until the list is a LinkedHashMap:

private LinkedHashMap<String, String> linked;

This is what I have come up with. Admittedly I am new to the Collectors implementations so bear with me:

 linked = list.stream()
 .collect(Collectors.toMap(
 Function.identity(),
 String::valueOf, //Used to be String::length
 LinkedHashMap::new));

this is giving me an error on the LinkedHashMap constructor line:

Cannot resolve constructor of LinkedHashMap

This is an example of what the list may look like:

zero
test0
one
test1
two
test2

and what I want the Map to look like:

zero:test0
one:test1
two:test2

Thanks

• Youcef LAIDANI: String::length return Integer not String
• ernest_k: Did you mean to declare linked as Map<String, Integer>?
• dnault: Can you please give examples of what you want the LinkedHashMap to look like for different input lists?
• dk40149: I meant to declare Map<String, String> @ernest_k
• dk40149: please see OP @dnault

Answer 1

Why you complicate your code, a simple loop in your case solve the problem :

for (int i = 0; i < list.size(); i += 2) {
 linked.put(list.get(i), list.get(i + 1));
}

Quick, Ideone demo

Outputs

zero:test0
one:test1
two:test2

Answer 2

You missed merge function:

a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)

But to fill map with your expected values using stream api you can use forEach method and IntStream::iterate from java9

LinkedHashMap<String, String> linked = new LinkedHashMap<>();
IntStream.iterate(0, n -> n < list.size(), n -> n + 2)
 .forEach(i -> linked.put(list.get(i), list.get(i + 1)));

• dk40149: Hi thanks for the response. You're right, im missing the merge statement. How do i do that for <String, String>?
• dk40149: Hi thanks again. What is the error, Operator '+' cannot be applied to '<lambda parameter>', 'int it's not permitting lambda expressions in combination with mathematic operations ?
• dk40149: No, it's by itself. I get an error for the two mathematic operations n -> n < list.size() //Bad return type in lambda expression: boolean cannot be converted to in AND n -> n + 2 // Operator '+' cannot be applied to '<lambda parameter>'
• Ruslan: @dk40149 method iterate is available from java9

Answer 3

This my attempt with java 8.

 IntStream.range(0, list.size())
 .mapToObj(index -> {
 if (index % 2 == 0) {
 return new AbstractMap.SimpleImmutableEntry<>(list.get(index), list.get(index + 1));
 }
 return null;
 }
 )
 .filter(Objects::nonNull)
 .collect(Collectors.toMap(AbstractMap.SimpleImmutableEntry::getKey, AbstractMap.SimpleImmutableEntry::getValue));