Filtering elements from List using Java Stream API

Using Stream API filter and collect methods

Please note that in below examples, in order to get a Stream Object from List, you can also use List#parallelStream() method, but use it only if the List implementation supports thread-safety and parallelism (for example CopyOnWriteArrayList).

Filtering means selecting only those elements from a list that satisfy a condition — keeping every person older than 30, every order with a value above $100, every string starting with a specific prefix. The result is a new, smaller list; the original list is unchanged. Before Java 8, this required a loop, an if check, and a separate result list. The Stream API introduced in Java 8 replaces that boilerplate with a single filter(predicate) call that reads like the plain-English description of what you want.

The pattern is: call stream() on the list, chain filter() with your condition expressed as a lambda, then collect the matching elements into a new list with collect(Collectors.toList()).

In order to follow below example, create a class called Person and declare two variables, name as String and age as integer.

public class Person { private String name; private int age; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } @Override public String toString() { return "[name : " +name + " , age : "+age +"]"; } }

Let's write a program to filter Person(s) with age greater than 30, but less than 40.

public static void filterListElementsUsingStream() { List<Person> persons = List.of( new Person("Person A", 29), new Person("Person B", 32), new Person("Person C", 45), new Person("Person D", 39) ); System.out.println("BEFORE filtering : " + persons); List<Person> fruitsFiltered = persons.stream(). filter(person -> person.getAge() >30 && person.getAge()<40) .collect(Collectors.toList()); System.out.println("AFTER filter : " + fruitsFiltered); }

Explanation of above code:

First we called stream() method on List<Person> that returns a Stream<Person> objects.
Then on the Stream object, we called filter() method with a predicate person.getAge() >30 && person.getAge() >40. This essentially pass every Person object from the list and check whether predicate condition is passed.
Finally, collecting the result into a List.

When you run the above program, you will get the below output.

BEFORE filtering : [[name : Person A , age : 29], [name : Person B , age : 32], [name : Person C , age : 45], [name : Person D , age : 39]]
AFTER filter : [[name : Person B , age : 32], [name : Person D , age : 39]]

Above example complete source code can be found here on Github

Explanation of above code:

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