Java 8 Lambda To Handle for > if > else

Question

I need to run two different tasks based on a whole mess of filters

I'm just now grasping the concept of Lambdas, so I guess my question is, how do you handle multiple "else" conditions containing complicated logic... within a Lambda?

I know I can use filter and map to select certain pieces of data. How does "nesting" work with filters?

Could I do something like:

//Iterate over list of sites
sites.stream()
//Check if current site is active
.map(isActive ? 
//{Do something with site because site is active};
//Set a variable for later user maybe?
:
//{Do something else involving the current site because it's not?};
//Set a different variable maybe?
);

//use variable from first map?

Can someone provide me with some proper syntax and maybe a basic explanation of what I'm doing to my data when I run through these abstract processes that are doing me a bamboozle.

Secondly, if I wanted to run these two map processes in parallel, would I just do this?

sites.stream().parallel()?

As usual, thanks for helping with my ignorance!

I think you better stick to traditional programming for these types of tasks. — Yassin Hajaj
– Yassin Hajaj, Commented Mar 31, 2016 at 18:11
Just so I'm completely clear...if I have two simple program paths, do task 1 if true, do task 2 if false, lambdas aren't really the way to go? — TheFunk
– TheFunk, Commented Mar 31, 2016 at 18:42
Maybe you should put more logic into your site objects and let them decide what to do, see Tell-Don't-Ask principle. This way you can simply call sites.forEach(Site::doSomething) from outside and put the logic in the Site class. — siegi
– siegi, Commented Apr 2, 2016 at 7:27

Johnny Willer · Accepted Answer · 2016-03-31 18:46:56Z

4

Some example of lambda usage:

You can group your sites by active and not active, after that you can perform some logic to both. Put the big logic into a method and then call in the map() function.

You can use filter() as well to get only a part of the stream.

public class Lambda {

    static class Site {

        public boolean isActive;
        public boolean otherCondition;
        public boolean isActive() {
            return isActive;
        }

        public boolean isOtherCondition() {
            return otherCondition;
        }

        public void setProperty(String property) {

        }
    }
    public static Site someOperation(Site site) {

        //some code

        return site;
    }
    public static void main(String[] args) {

        ArrayList<Site> sites = new ArrayList<>();

        Map<Boolean, List<Site>> groupedSites = sites.stream().collect(Collectors.groupingBy(Site::isActive));          
        groupedSites.get(Boolean.TRUE).stream().filter(Site::isOtherCondition).map(Lambda::someOperation).forEach(p -> p.setProperty("something"));

    }
}

answered Mar 31, 2016 at 18:46

Johnny Willer

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3 Comments

TheFunk Over a year ago

Thank you! This is very helpful! I can now leverage multiple processors to work on multiple lists at one time and the code is much less complex. Perhaps a little more confusing, but less complex overall!

Holger Over a year ago

You can replace Collectors.groupingBy(Site::isActive) by Collectors.partitioningBy(Site::isActive). Then you will get a map optimized for mapping exactly true and false to a value and it will have an empty list in case there’s no match, so the follow-up groupedSites.get(Boolean.TRUE).stream() won’t throw a NullPointerException in that case. Also, mind that there is an overload that accept a downstream collector to perform operation right when collecting instead of storing everything into a list and perform a stream operation afterwards.

Johnny Willer Over a year ago

@Holger oh, thank you, I didn't know that method, useful comment.

Muhammad Hewedy · Accepted Answer · 2016-03-31 18:33:29Z

3

I would put the complex logic into a separate method and reference to it in the map(…).

sites.stream()
    .map(Foo::bar)
    …;

This would use a method in a class Foo that could look like this:

public static String bar(Site site)
{
    return site.toString();
}

Of course you can use arbitrary logic in the method but it should be free of side effects.

And yes, if the method used for mapping is thread safe, you can use parallel() to run the mappings in parallel.

edited Mar 31, 2016 at 18:33

Muhammad Hewedy

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answered Mar 31, 2016 at 18:25

siegi

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Comments

Louis Wasserman · Accepted Answer · 2016-03-31 18:27:06Z

1

You just use a plain, simple if:

.map(site -> {
   if (isActive) {
     ...whatever...
   } else {
     ...whatever else...
   }
 })
 ...

answered Mar 31, 2016 at 18:27

Louis Wasserman

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1 Comment

Yassin Hajaj Over a year ago

@TheFunk There isn't. Lambdas and streams aren't always the best solution.

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