I have classes Employee and Address as follows:

public static class Employee {
    private List<Address> address;
    
    // getters, etc.
}

public static class Address {
    private String city;
    
    // getters, etc.
}

I am learning streams, I am trying to iterate over the objects from list of list and create a set of unique values. I was able to get it working by using nested for loop. How can I convert below code to streams?

public static Set<String> getCityUniquName(List<Employee> emp){

    Set<String> cityUniquName = new HashSet<>();
    for(Employee e: emp){
        List<Address> addList = e.getAddress();
        for(Address add: addList){
            cityUniquName.add(add.getCity());
        }
    }
   return cityUniquName;
}

CodePudding user response：

Use flatMap to flatten the Address list and then use map to get the city from each Address object and then collect into Set

Set<String> cityUniquName = emp.stream()
                .flatMap(Employee::getAddress)
                .map(Address::getCity)
                .collect(Collectors.toSet());

CodePudding user response：

Since each Employee is associated with a collection of Address instances, you need to apply of the stream operations that allow to flatten the data, namely either flatMap(), or mapMulty().

Note that flatMap() expects a function producing a Stream,not a Collection like shown in another answer.

Stream.flatMap

To turn a Stream of Employee into a Stream of Address in the mapper function of flatMap() we need to extract the collection of addresses and create a stream over it.

The only thing left is to get the city name and collect the result into a set.

public static Set<String> getCityUniqueName(List<Employee> emp) {
    
    return emp.stream()
        .flatMap(e -> e.getAddress().stream())
        .map(Address::getCity)
        .collect(Collectors.toSet());
}

Stream.mapMulti

mapMulti() expects a BiConsumer, which in turn takes two arguments: stream element (having an initial type) and a Consumer of the resulting type. Every object offered to the consumer would appear in the resulting stream.

public static Set<String> getCityUniqueName1(List<Employee> emp) {
    
    return emp.stream()
        .<Address>mapMulti((e, c) -> e.getAddress().forEach(c))
        .map(Address::getCity)
        .collect(Collectors.toSet());
}