This is how my map looks like.
Map<String, String> maps = List.stream().collect(Collectors.toMap(Cat::getName, Cat::getNumber, (c1,c2) -> c1));
If I already have "Lucy, 101" in maps then I am unable to add another value corresponding to the name Lucy i.e "Lucy, 102". Is there any way to change the merge function [ (c1,c2) -> c1 ] so that I can have two values corresponding to a single key (Lucy) in my maps?
By your requirement, in order to allow multiple values for the same key, you need to implement the result as Map<String, Collection<String>>.
In your case, you can merely use groupingBy + mapping collectors instead of toMap with the merge function:
Map<String, List<String>> maps = List.stream()
.collect(Collectors.groupingBy(
Cat::getName,
Collectors.mapping(Cat::getNumber, Collectors.toList())
);
However, you may want to consider a merge function as joining strings:
Map<String, String> maps = List.stream()
.collect(Collectors.toMap(
Cat::getName,
Cat::getNumber, (num1, num2) -> String.join("; ", num1, num2)
);
Then the map would contain "compound" values in the form of a string.
Since the type of your map is Map<String, String>, it can only return one string. However, you expect to get multiple strings from get. You need to change the type of the map to e.g. Map<String, List<String>> or Map<String, Collection<String>>.
Then, you can use groupingBy like this:
Map<String, List<String>> map = yourList.stream().collect(Collectors.groupingBy(
Cat::getName, // group by the name
Collectors.mapping( // map each group of cats to their numbers
Cat::getNumber, Collectors.toList() // collect each group to a list
)
));
If you are okay with multiple numbers in the same string (e.g. "101 102"), you can use Collectors.joining:
Map<String, String> map = yourList.stream().collect(Collectors.groupingBy(
Cat::getName,
Collectors.mapping(
Cat::getNumber, Collectors.joining(" ")
)
));
