Java 8: Filters

If you are unfamiliar with Java 8 Streams then I suggest you first check out our Introduction to Streams before reading this article.

Java 8 Filters are an Intermediate Operation that can be performed on a Stream. Traditionally if you had a collection of data such as a List and you wanted to filter out some data, you would need to think either think about the repercussions of mutating the data or simply make a copy of the collection. With Java 8 Filters you are working with Streams so you never end up mutating the original data.

Let’s say you have a list of names and you want to Filter out any names that start with the letter A.

Given the following ArrayList of Strings

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List<String> strings = new ArrayList<String>();
strings.add("Alice");
strings.add("Arianna");
strings.add("Jennifer");
strings.add("Melissa");

You can use the filter operation

Filter

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strings.stream()
    .filter(e -> !e.startsWith("A"))
    .forEach(System.out::println);

Which produces the following output to the console:

Console Output

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Jennifer
Melissa

Let’s say you want to also filter any names that start with M. You could either chain another filter Operation or you add the check to your existing filter. Let’s take a look at both.

Add to the existing Filter

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strings.stream()
    .filter(e -> !e.startsWith("A") && !e.startsWith("M"))
    .forEach(System.out::println);

or

Chain New Filter

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strings.stream()
    .filter(e -> !e.startsWith("A"))
    .filter(e -> !e.startsWith("M"))
    .forEach(System.out::println);

Both Operations produce the same result:

Console Output

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Jennifer