Gunnar Gissel

Posted on Jun 13, 2018 • Edited on Jun 22, 2018 • Originally published at gunnargissel.com

What is `map` in a Java Stream

#java #functional #java8

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);

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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Top comments (2)

mbtts • Jun 15 '18

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).