Difference between map and filter on a java collection stream

问题

Suppose there is a list say

List<String> myList = new ArrayList<String>();
myList.add("okay");
myList.add("omg");
myList.add("kk");

I am doing this:

List<String> fianllist = myStream.map(item -> item.toUpperCase()).filter(item
->item.startsWith("O")).collect(Collectors.toList());

My question is what the difference between map and filter as both can take a lambda expression as a parameter. Can some one please explain?

回答1:

By using map, you transform the object values.

The map operation allows us to apply a function, that takes in a parameter of one type, and returns something else.

Filter is used for filtering the data, it always returns the boolean value. If it returns true, the item is added to list else it is filtered out (ignored) for eg :

List<Person> persons = …
Stream<Person> personsOver18 = persons.stream().filter(p -> p.getAge() > 18);

For more details on this topic you can visit this link

回答2:

Filter takes a predicate as an argument so basically you are validating your input/collection against a condition, whereas a map allows you to define or use a existing function on the stream eg you can apply String.toUpperCase(...) etc. and transform your inputlist accordingly.

来源：https://stackoverflow.com/questions/37154380/difference-between-map-and-filter-on-a-java-collection-stream