How to filter two maps and make third map by using Lambda expression java 8

Question

Map<String,String> persons = new HashMap<>();
persons.put("aaaa@testing","123456789");
persons.put("bbbb@testing","987654321");

Map<String,UsersDTO> users = new HashMap<>();
users.put("aaaa@testing", UsersDTO1);
users.put("bbbb@testing",UsersDTO2);

//Below one is the my required final map by using above two maps by using java 8 Lambdas
Map<String,UsersDTO> finalMap = new HashMap<>();
finalMap.put("123456789",UsersDTO1);
finalMap.put("987654321",UsersDTO2);

How to make finalMap by using the two maps above? This type of question might be there but I want to give special focus on this so that's why I am posting it. How to make by using the lambda expressions?

You want your final map to be <String,String> but then you are puting UserDtos to it? — Maciej Mościcki
– Maciej Mościcki, Commented Sep 12, 2019 at 19:29
Sorry it's typo mistake, finally i want <String,UserDto> generic type map — Srikanth Janapati
– Srikanth Janapati, Commented Sep 12, 2019 at 19:41

Naman · Accepted Answer · 2019-09-13 03:44:22Z

4

You could do that but note that you will get a Map<String,UserDto>:

Map<String,UsersDTO> finalMap =
        persons.entrySet().stream()
                .collect(Collectors.toMap(Map.Entry::getValue, e-> users.get(e.getKey())));

As suggested by Andreas, if the email doesn't have a match between the two maps, you could handle that case. For example by ignoring the entry :

Map<String, UsersDTO> finalMap =
        persons.entrySet().stream()
                .filter(e -> users.containsKey(e.getKey()))
                .collect(Collectors.toMap(Map.Entry::getValue, e -> users.get(e.getKey())));

edited Sep 13, 2019 at 3:44

Naman

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answered Sep 12, 2019 at 19:29

davidxxx

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2 Comments

Andreas Over a year ago

You're missing a filter() call to exclude persons entries without matching users entry.

davidxxx Over a year ago

Indeed.But not sure how the op would handle that (filtering, default value, exception) but added anyway an example with filtering.

Youcef LAIDANI · Accepted Answer · 2019-09-12 19:38:56Z

1

I think you mean something like so :

Map<String, UsersDTO> finalMap = users.entrySet().stream()
        .collect(Collectors.toMap(user -> persons.get(user.getKey()), Map.Entry::getValue));

Here is a simple ideone demo to the expected result.

As commented by Andreas, you need another filter to make sure that each entry has a matching :

Map<String, UsersDTO> finalMap = users.entrySet().stream()
        .filter(user -> persons.containsKey(user.getKey()))
        .collect(Collectors.toMap(user -> persons.get(user.getKey()), Map.Entry::getValue));

edited Sep 12, 2019 at 19:38

answered Sep 12, 2019 at 19:32

Youcef LAIDANI

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5 Comments

Andreas Over a year ago

You're missing a filter() call to exclude users entries without matching persons entry.

Youcef LAIDANI Over a year ago

@Andreas you are correct, without the filter it can cause some issues. fix it ;)

Srikanth Janapati Over a year ago

Can Please suggest where i can learn this complex lamdas, suggest me any resource for learing

Youcef LAIDANI Over a year ago

@SrikanthJanapati by practice and learning books for example amazon.fr/Java-Lambdas-Pragmatic-Functional-Programming-ebook/…

Srikanth Janapati Over a year ago

@YCF_L Thank you

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