Java Stream API: how to convert to a Map of Sets

Question

Lets say I have following an class named Employee and a List of these Employee-classes:

class Employee {

private String praefix;
private String middleFix;
private String postfix;
private String name;

public Employee(String praefix, String middleFix, String postfix, String name) {
    this.praefix = praefix;
    this.middleFix = middleFix;
    this.postfix = postfix;
    this.name = name;
}

public String getPraefix() {
    return praefix;
}

public void setPraefix(String praefix) {
    this.praefix = praefix;
}

public String getMiddleFix() {
    return middleFix;
}

public void setMiddleFix(String middleFix) {
    this.middleFix = middleFix;
}

public String getPostfix() {
    return postfix;
}

public void setPostfix(String postfix) {
    this.postfix = postfix;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}



List<Employee> employees = new ArrayList<>();
employees.add(new Employee("A", "B", "C", "Michael Phelps"));
employees.add(new Employee("A", "B", "C", "Cristiano Ronaldo"));
employees.add(new Employee("D", "E", "F", "Usain Bolton"));
employees.add(new Employee("D", "E", "F", "Diego Armando Maradona"));
employees.add(new Employee("D", "E", "F", "Lionel Messi"));

Is it possible to convert it to following Map with Java Stream-API?

{A.B.C=[Cristiano Ronaldo, Michael Phelps], D.E.F=[Aydin Korkmaz, Diego Armando Maradona, Usain Bolton]}

Eugene · Accepted Answer · 2018-05-15 20:01:06Z

6

  Map<String, List<String>> result = employees.stream()
            .collect(Collectors.groupingBy(
                    x -> String.join(".",
                            x.getPraefix(),
                            x.getMiddleFix(),
                            x.getPostfix()),
                    Collectors.mapping(Employee::getName, Collectors.toList())

edited May 15, 2018 at 20:01

answered May 15, 2018 at 19:43

Eugene

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6 Comments

Ivan Over a year ago

Stream.of(x.getPraefix(), x.getMiddleFix(), x.getPostfix()) .collect(Collectors.joining(".") could be replaced with simple concatenation using +

Eugene Over a year ago

@Ivan ooh my! thx, this is what happens when I try to do two things at a time.

Vasan Over a year ago

That leaves out the "." delimiter though. I think the first one was better.

Eugene Over a year ago

@Vasan neither :) how about a 3-rd option (that I actually would use myself)

Vasan Over a year ago

Ah yes, I'd forgot about StringJoiner. +1

|

Ousmane D. · Accepted Answer · 2018-05-15 20:07:47Z

1

You could also use the toMap collector:

Map<String, List<String>> resultSet = employees.stream()
             .collect(toMap(e ->
               String.join(".", e.getPraefix(), e.getMiddleFix(), e.getPostfix()),
               v -> new ArrayList<>(Collections.singletonList(v.getName())),
               (left, right) -> {left.addAll(right); return left;}));

edited May 15, 2018 at 20:07

answered May 15, 2018 at 19:57

Ousmane D.

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2 Comments

Eugene Over a year ago

this is exactly what I wanted to have today in default jdk, ArrayList.of(...), or HashSet.of(...), would make this a little nicer

Ousmane D. Over a year ago

@Eugene couldn't agree more, it's definitely long-winded with the current approach.

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