Java 8 Stream.distinct() list de duplication example

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Java 8 Stream.distinct() example

In this article, we will provide a Java 8 Stream distinct () example. Distinct () returns a Stream composed of different elements of the Stream. Distinct () is the method of the Stream interface. Distinct () uses hashCode () and equals () methods to get different elements. Therefore, our class must implement the hashCode () and equals () methods. If distinct () is processing an ordered flow, for repeated elements, the elements that appear first in the encounter order are retained, and it is stable to select different elements in this way. In the case of disordered flow, the selection of different elements is not necessarily stable and can be changed. Distinct() performs stateful intermediate operations. In the case of parallel flows with ordered flows, maintaining the stability of distinct () requires a high cost because it requires a lot of buffer overhead. If we don't need to maintain the consistency of encounter order, we should be able to use the unordered flow implemented by the BaseStream.unordered () method.

1. Stream.distinct()

The distinct() method is declared as follows:

Stream<T> distinct()

It is the method of the Stream interface. In this example, we have a list of string data types that contain duplicate elements

DistinctSimpleDemo.java

   
     
      
     
     
      
       package com.concretepage;
      
     

     
      
     
     
      
       import java.util.Arrays;
      
     

     
      
     
     
      
       import java.util.List;
      
     

     
      
     
     
      
       import java.util.stream.Collectors;
      
     

     
      
     
     
      
       public 
       class DistinctSimpleDemo {
      
     

     
      
     
     
          
       public static void main(String[] args) {
      
     

     
      
     
     
      
               List<String> list = Arrays.asList(
       "AA", 
       "BB", 
       "CC", 
       "BB", 
       "CC", 
       "AA", 
       "AA");
      
     

     
      
     
     
              
       long l = list.stream().distinct().count();
      
     

     
      
     
     
      
               System.out.println(
       "No. of distinct elements:"+l);
      
     

     
      
     
     
      
               String output = list.stream().distinct().collect(Collectors.joining(
       ","));
      
     

     
      
     
     
      
               System.out.println(output);
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
      
       }

Output

   
     
      
     
     
      
       No. of distinct elements:
       3
      
     

     
      
     
     
      
       AA,BB,CC

2. Stream.distinct() with List of Objects

In this example, we have a list of Book objects. To de duplicate the list, this class overrides hashCode () and equals ().

Book.java

   
     
      
     
     
      
       package com.concretepage;
      
     

     
      
     
     
      
       public 
       class Book {
      
     

     
      
     
     
          
       private String name;
      
     

     
      
     
     
          
       private 
       int price;
      
     

     
      
     
     
          
       public Book(String name, int price) {
      
     

     
      
     
     
      	
       this.name = name;
      
     

     
      
     
     
      	
       this.price = price;
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
          
       public String getName() {
      
     

     
      
     
     
      	
       return name;
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
          
       public int getPrice() {
      
     

     
      
     
     
      	
       return price;
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
          
       @Override
      
     

     
      
     
     
          
       public boolean equals(final Object obj) {
      
     

     
      
     
     
            
       if (obj == 
       null) {
      
     

     
      
     
     
               
       return 
       false;
      
     

     
      
     
     
      
             }
      
     

     
      
     
     
            
       final Book book = (Book) obj;
      
     

     
      
     
     
            
       if (
       this == book) {
      
     

     
      
     
     
               
       return 
       true;
      
     

     
      
     
     
      
             } 
       else {
      
     

     
      
     
     
               
       return (
       this.name.equals(book.name) && 
       this.price == book.price);
      
     

     
      
     
     
      
             }
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
          
       @Override
      
     

     
      
     
     
          
       public int hashCode() {
      
     

     
      
     
     
            
       int hashno = 
       7;
      
     

     
      
     
     
      
             hashno = 
       13 * hashno + (name == 
       null ? 
       0 : name.hashCode());
      
     

     
      
     
     
            
       return hashno;
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
      
       }

DistinctWithUserObjects.java

   
     
      
     
     
      
       package com.concretepage;
      
     

     
      
     
     
      
       import java.util.ArrayList;
      
     

     
      
     
     
      
       import java.util.List;
      
     

     
      
     
     
      
       public 
       class DistinctWithUserObjects {
      
     

     
      
     
     
          
       public static void main(String[] args) {
      
     

     
      
     
     
      
               List<Book> list = 
       new ArrayList<>();
      
     

     
      
     
     
      
               {
      
     

     
      
     
     
      
                  list.add(
       new Book(
       "Core Java", 
       200));
      
     

     
      
     
     
      
                  list.add(
       new Book(
       "Core Java", 
       200));
      
     

     
      
     
     
      
                  list.add(
       new Book(
       "Learning Freemarker", 
       150));        	
      
     

     
      
     
     
      
                  list.add(
       new Book(
       "Spring MVC", 
       300));
      
     

     
      
     
     
      
                  list.add(
       new Book(
       "Spring MVC", 
       300));
      
     

     
      
     
     
      
               }
      
     

     
      
     
     
              
       long l = list.stream().distinct().count();
      
     

     
      
     
     
      
               System.out.println(
       "No. of distinct books:"+l);
      
     

     
      
     
     
      
               list.stream().distinct().forEach(b -> System.out.println(b.getName()+ 
       "," + b.getPrice()));
      
     

     
      
     
     
      
           }
      
     

     
      
     
     
      
       }

Output

    
      
       
      
      
       
        No. of distinct books:
        3
       
      

      
       
      
      
       
        Core Java,
        200
       
      

      
       
      
      
       
        Learning Freemarker,
        150
       
      

      
       
      
      
       
        Spring MVC,
        300

3. Distinct by Property

distinct () does not provide a direct implementation of de duplication of object lists by attributes. It works based on hashCode () and equals (). If we want to de duplicate the object list according to the object properties, we can implement it by other methods. The following code snippet shows:

    
      
       
      
      
       
        static <T> 
        Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
       
      

      
       
      
      
       
                Map<Object,Boolean> seen = 
        new ConcurrentHashMap<>();
       
      

      
       
      
      
               
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == 
        null;
       
      

      
       
      
      
       
        }

The above method can be received as a parameter by the filter() of the Stream interface, as shown below:

list.stream().filter(distinctByKey(b -> b.getName()));

The distinguishbykey () method returns a hash map using ConcurrentHashMap To maintain the previously seen state Predicate Example, the following is a complete example of using object properties for de duplication.

DistinctByProperty.java

    
      
       
      
      
       
        package com.concretepage;
       
      

      
       
      
      
       
        import java.util.ArrayList;
       
      

      
       
      
      
       
        import java.util.List;
       
      

      
       
      
      
       
        import java.util.Map;
       
      

      
       
      
      
       
        import java.util.concurrent.ConcurrentHashMap;
       
      

      
       
      
      
       
        import java.util.function.Function;
       
      

      
       
      
      
       
        import java.util.function.Predicate;
       
      

      
       
      
      
       
        public 
        class DistinctByProperty {
       
      

      
       
      
      
           
        public static void main(String[] args) {
       
      

      
       
      
      
       
                List<Book> list = 
        new ArrayList<>();
       
      

      
       
      
      
       
                {
       
      

      
       
      
      
       
                	list.add(
        new Book(
        "Core Java", 
        200));
       
      

      
       
      
      
       
                	list.add(
        new Book(
        "Core Java", 
        300));
       
      

      
       
      
      
       
                	list.add(
        new Book(
        "Learning Freemarker", 
        150));
       
      

      
       
      
      
       
                	list.add(
        new Book(
        "Spring MVC", 
        200));
       
      

      
       
      
      
       
                	list.add(
        new Book(
        "Hibernate", 
        300));
       
      

      
       
      
      
       
                }
       
      

      
       
      
      
       
                list.stream().filter(distinctByKey(b -> b.getName()))
       
      

      
       
      
      
       
                      .forEach(b -> System.out.println(b.getName()+ 
        "," + b.getPrice()));   
       
      

      
       
      
      
       
            }
       
      

      
       
      
      
           
        private 
        static <T> 
        Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
       
      

      
       
      
      
       
                Map<Object,Boolean> seen = 
        new ConcurrentHashMap<>();
       
      

      
       
      
      
               
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == 
        null;
       
      

      
       
      
      
       
            }
       
      

      
       
      
      
       
        }

Output

    
      
       
      
      
       
        Core Java,
        200
       
      

      
       
      
      
       
        Learning Freemarker,
        150
       
      

      
       
      
      
       
        Spring MVC,
        200
       
      

      
       
      
      
       
        Hibernate,
        300

from : https://www.concretepage.com/java/jdk-8/java-8-distinct-example

Posted by phast1 on Fri, 08 Oct 2021 00:43:14 -0700

Programmer Group

Java 8 Stream.distinct() list de duplication example

2. Stream.distinct() with List of Objects

3. Distinct by Property

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