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What is `map` in a Java Stream

monknomo profile image Gunnar Gissel Originally published at gunnargissel.com Updated on ・1 min read

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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Discussion (2)

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mbtts profile image
mbtts

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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andrekelvin profile image
AndreKelvin

Thanks. I now fully understand this mapping thing