filter Map in Java 8 Streams

Question

I was trying to filter Entries in HashMap using Streams API, but stuck in last method call Collectors.toMap. So, I don't have clue to implemement toMap method

    public void filterStudents(Map<Integer, Student> studentsMap){
            HashMap<Integer, Student> filteredStudentsMap = studentsMap.entrySet().stream().
            filter(s -> s.getValue().getAddress().equalsIgnoreCase("delhi")).
            collect(Collectors.toMap(k , v));
    }

public class Student {

        private int id;

        private String firstName;

        private String lastName;

        private String address;
    ...

    }

Any Suggestions?

Just so you're clear. Each parameter in Collectors.toMap takes a Function, so k and v don't exist. It would be toMap(s -> s.getKey(), s -> s.getValue()) which can be converted to Method References as in the answer by @Eran. Which I recommend even if they are a little longer — Novaterata
– Novaterata, Commented Jul 26, 2017 at 13:40
you might want to read this question and the one it is marked a duplicate of stackoverflow.com/questions/1992384/… — Novaterata
– Novaterata, Commented Jul 26, 2017 at 13:45

Eran · Accepted Answer · 2017-07-26 13:35:05Z

16

Just generate the output Map out of the key and value of the entries that pass your filter:

public void filterStudents(Map<Integer, Student> studentsMap){
    Map<Integer, Student> filteredStudentsMap = 
        studentsMap.entrySet()
                   .stream()
                   .filter(s -> s.getValue().getAddress().equalsIgnoreCase("delhi"))
                   .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
}

answered Jul 26, 2017 at 13:35

Eran

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5 Comments

Ankit Over a year ago

Do i need to cast result Map to HashMap<Integer,Student>?

Eran Over a year ago

@Ankit Are you asking whether the result would be a HashMap, or are you asking how to make sure the result is a HashMap? The 2 argument variant of Collectors.toMap returns a Map. There are other variants in which you can specify the type of Map you want created.

Novaterata Over a year ago

@Ankit You want to use the simplest Interface that that is necessary. It shouldn't matter if it's a HashMap (which it will be), but if you need a specific implementation like LinkedHashMap, then you need to use the 4 param toMap stackoverflow.com/questions/29090277/…

Ankit Over a year ago

@Eran: I was getting Compilation error Type mismatch: cannot convert from Map<Object,Object> to HashMap<Integer,Student>. Now, after casting things are working. Thanks

Jagannath Over a year ago

@Ankit Don't take the result by HashMap<Integer, Student>, take it by Map<Integer, Student>. You don't need any casts then.

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