Today, I walk you through Java 8 Stream Distinct Examples using stream API. You can filter or collect all distinct elements from a collection by using distinct() of Stream API. In case, we have to process a collection of complex objects we need to write a custom function for the specific filtering purpose.
1. Java 8 distinct example
Now, we are finding all distinct strings without case sensitive from the String collection.
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public class StreamDistinctDemo { public static void main(String[] args) { Collection<String> list = Arrays.asList("java", "nodejs", "Java", "php", "C#", "c#", "JAVA"); // toLowerCase and distinct only List<String> distinctElements = list.stream().map(String::toLowerCase).distinct().collect(Collectors.toList()); // print the result System.out.println(distinctElements); } } |
Output
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[java, nodejs, php, c#] |
2. Java 8 stream distinct by key
Above distinct example is quite simple and easy for us. In fact, we need to deal with objects or complex types.
We’re considering a POJO as below
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public class People { public int age; public String gender; public String name; //getter and setter } |
Until now, Java doesn’t support any native API for filtering distinct by object property directly, even date object. We have to create a utility function which matches with our requirement. Our utility function will filter distinct by the name property of People object, like this.
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/** * Stateful filter. T is type of stream element, K is type of extracted key. */ public static <T> Predicate<T> distinctByKey(Function<? super T, Object> key) { Map<Object, Boolean> map = new ConcurrentHashMap<>(); return t -> map.putIfAbsent(key.apply(t), Boolean.TRUE) == null; } |
Here is a function that returns a predicate that maintains state about what it’s seen previously, and that returns whether the given element was seen for the first time.
Writting the demo program:
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public class StreamDistinctDemo2 { public static void main(String[] args) { People a = new People(28, "M", "Mr A"); People b = new People(25, "M", "Mr B"); People c = new People(21, "M", "Mr B"); People d = new People(21, "M", "Mr B"); // A collection of duplicated elements Collection<People> list = Arrays.asList(a, b, c,d); // Get distinct only List<People> distinctElements = list.stream().filter(distinctByKey(p -> p.getName())) .collect(Collectors.toList()); // print the distinct result System.out.println(distinctElements); // Get distinct with multiple keys List<People> distinctMultiKeys = list.stream().filter(distinctByKey(p -> p.getName() + p.getAge())) .collect(Collectors.toList()); // print the distinct result System.out.println(distinctMultiKeys); } public static <T> Predicate<T> distinctByKey(Function<? super T, Object> key) { Map<Object, Boolean> map = new ConcurrentHashMap<>(); return t -> map.putIfAbsent(key.apply(t), Boolean.TRUE) == null; } } |
Output
That’s all about the Java 8 Stream Distinct Examples.
References
Processing Data with Java SE 8 Streams
Download the complete source code, click link below
Java8StreamDistinctExamples.zip (165 downloads)
