Java 8 - Calling a multi argument method from Collection.stream.map()

Question

I've been using the java 8 Streams for a while. I came across a situation where I need to stream through a List and pass each element to a static method along with another argument. Is it possible in java 8?

........
String designation = "Engineer";
List<String> names = new ArrayList<>();
names.add("ABC");
names.add("DEF");
names.add("GHI");
names.stream().map(MyClass::createReport);
..........

class MyClass {
    public static void createReport(String name, String designation) {
       System.out.println(name+"\t"+designation);
    }
}

How can I pass the designation String via stream().map()?

Eran · Accepted Answer · 2018-11-29 14:18:25Z

11

Use a lambda expression:

names.stream().map(name -> MyClass.createReport(name,designation))...

answered Nov 29, 2018 at 14:18

Eran

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1 Comment

MC Emperor Over a year ago

Note that designation must be final or effectively final in order to use it inside a lambda expression.

Roman · Accepted Answer · 2018-11-29 15:44:27Z

2

You could write a curried version of the method createReport.

Curried `createReport`

We need to swap the order of the arguments because the designation for each name is the same. Additionly we just need to call the not curried method.

Function<String, Consumer<String>> createReportCurry = (designation) -> (name) -> {
    createReport(name, designation);
};

In Action

names.stream().forEach(createReportCurry.apply(designation))

edited Nov 29, 2018 at 15:44

answered Nov 29, 2018 at 15:35

Roman

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Comments

Nicholas K · Accepted Answer · 2018-11-29 14:27:39Z

0

You could also use the following as an alternative:

IntStream.range(0, names.size())
         .forEach(i -> MyClass.createReport(names.get(i), names.get(i)));

edited Nov 29, 2018 at 14:27

answered Nov 29, 2018 at 14:21

Nicholas K

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7 Comments

Felk Over a year ago

why the intstream?

Roland Over a year ago

designation was actually set/known from outside, so you may want to keep that and yes... why intstream? Erans answer is probably already simple enough...

Nicholas K Over a year ago

@Felk : How else would I get the index?

Roland Over a year ago

you don't need to... you can just use the stream on the list itself and use its contents (i.e. the name)... just check Erans answer...

Nicholas K Over a year ago

Well, its another alternative.

|

Herve MULUDIKI · Accepted Answer · 2018-11-30 09:46:06Z

0

The above answer are very good but let's try to explain why you couldn't run your code in the first place:

Your code:

names.stream().map(MyClass::createReport);names.stream().map(MyClass::createReport);

What the JVM undertands

names.stream().map(MyClass::createReport);names.stream().map(name -> MyClass.createReport(name));

Solutions use a basic lamba as @Eran or a Function as @Roman suggested.

answered Nov 30, 2018 at 9:46

Herve MULUDIKI

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