map from string to multiple different strings and add to list with Java stream

Question

I'm new to Java 8 and Streams . I got a PolicyDefinition object, that got to two method : getAlias,getName which both returns a string . Is there an elegant way to create a list with all aliases and names of policy definitions using Stream (created from collection of PolicyDefinition) in one statement ?

with two statements its not a problem :

List<String> policyNames = 
              policyDefinitions.stream()
                               .map(definition -> definition.getName())
                               .collect(Collectors.toList());

List<String> policyAlias = 
              policyDefinitions.stream()
                               .map(definition -> definition.getAlias())
                               .collect(Collectors.toList());

But Is it possible in one ?

Thanks a lot for the help

Boris the Spider · Accepted Answer · 2018-04-02 13:00:37Z

4

flatMap it!

List<String> policyNames = policyDefinitions.stream()
    .flatMap(definition -> Stream.of(definition.getName(), definition.getAlias()))
    .collect(Collectors.toList());

As mentioned in the comments - for tidyness, create a method in Definition

public Stream<String> allNames() {
    return Stream.of(getName(), getAlias())
}

Then

List<String> policyNames = policyDefinitions.stream()
    .flatMap(Definition::allNames)
    .collect(Collectors.toList());

OP comments "I forgot to mention that getAlias might be null, what do you do than[sic]"

In that case, use Optional:

public Stream<String> allNames() {
    return Stream.concat(Stream.of(getName()), Optional.ofNullable(getAlias()).stream())
}

edited Apr 2, 2018 at 13:00

answered Apr 2, 2018 at 11:14

Boris the Spider

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4 Comments

nadavgam Over a year ago

I forgot to mention that getAlias might return null, what do you do than ?

Ousmane D. Over a year ago

Note, that optional only had a stream method as of JDK9, an alternative for JDK8 could be to create your own method that does such thing or filter the nulls out like

List<String> policyNames = policyDefinitions.stream()     .flatMap(Definition::allNames).filter(Objects::nonNull)     .collect(Collectors.toList());

Boris the Spider Over a year ago

@Aominè Good point Optional.map(Stream::of).orElseGet(Stream::empty) was a favourite of mine.

Holger Over a year ago

Or simply Stream.of(getName(), getAlias()).filter(Objects::nonNull)

user9588409 · Accepted Answer · 2018-04-03 00:28:59Z

0

Also you can create a Map with Alias as a Key and Name as a Value using groupingBuy operator

answered Apr 3, 2018 at 0:28

user9588409

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