What is `map` in a Java Stream
#java#functional#java8
Gunnar Gissel

Gunnar Gissel @monknomo

About: Saving fish by writing code! Applications developer in fisheries, specializing in webapps and moving 'enterprise-y' legacy systems to modern agile systems - Email or tweet me if you want to talk!

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What is `map` in a Java Stream

Publish Date: Jun 13 '18
28 2

Originally published at www.gunnargissel.com

Mapping is how to convert one type of object to another with a stream. Say you have a set of Fruit and you want to show people what is in your set. It would be helpful to have a list of fruit names to do so.

fruitList.stream().map(fruit::getName).collect(Collectors.toList);
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That's pretty simple; you can imagine how to do that in real life with a basket of fruit.

Pick up a piece of fruit, write its name down. Pick up another piece of fruit, write its name down, etc.

Mapping also lets you you can't easily simulate in real life. Say you have a Fruit set and you want Oranges, instead of Apples (I think this is closer to transmutation than swapping, but it's a metaphor, ymmv).

You can do that, with Java:

fruitList.stream().map(fruit -> {
    if( fruit instanceof Apple){
        return new Orange();
    }
    return  fruit;
}).collect(Collectors.toSet);
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Comments 2 total

  • AndreKelvin
    AndreKelvinJun 14, 2018

    Thanks. I now fully understand this mapping thing

  • mbtts
    mbttsJun 15, 2018

    Good article and thank you for sharing.

    For those not aware it is also possible to use a method reference for the first example:

    fruitList.stream()
         .map(Fruit::getName)
         .collect(toList());

    This is just a more succinct syntax for invoking the #getName method on each and every instance of Fruit in the stream. As such even though it may appear to be invoked on the class it is not a static method.

    Also a small API thing - it is Collectors#toList not #asList (and #toSet).

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