Part 9 – Advance collectors in Java 8

December 15, 2018 Saurabh Oza Java 8 0

9

Advance Collectors in Java 8

What have we learned so far

1. Functional Interface And Default Methods In Java 8 –
PART 1 : FUNCTIONAL INTERFACE AND DEFAULT METHODS IN JAVA 8

2. Lambda Expression in Java 8
PART 2 – LAMBDA EXPRESSION IN JAVA 8

3. Method Reference in Java 8
PART 3 – METHOD REFERENCE IN JAVA 8

4. Optional in Java 8
PART 4 – OPTIONAL IN JAVA 8

5. filter(), findAny() in Java 8
PART 5 – FILTER(), FINDANY() IN JAVA 8

6. map() vs flatMap() in Java 8
PART 6 – MAP() VS FLATMAP() IN JAVA 8

7. Stream.peek() in Java 8
PART 7 – STREAM.PEEK() IN JAVA 8

8. Short circuiting operations of Streams in Java 8
PART 8 – SHORT CIRCUITING OPERATIONS OF STREAMS IN JAVA 8

9. Advance Collectors in Java 8
PART 9 – ADVANCE COLLECTORS IN JAVA 8

We have seen how collect method works with Collectors class, now its time to see some of the advanced scenarios to use advanced collectors:

1. joining() in Java 8

Collectors.joining will join all the results with delimiter specified in the parameter.

private static void joiningCollector(List<Student> students) {
 System.out.println("######## Executing joiningCollector() : ######## ");
 String allStudents = students.stream()
   .map(Student::getName)
   .collect(Collectors.joining(" | "));
 System.out.println("Collectors.joining() : "+allStudents);
 System.out.println("######## Ending the execution of joiningCollector() ######## "); 
}

Output
######## Executing joiningCollector() : ########
Collectors.joining() : Saurabh | Robert | John | Roman | Randy
######## Ending the execution of joiningCollector() ########

2. summaryStatistics() in Java 8

It is used to calculate the sum, min, max, avg and total count of the elements passed to this method. In below example we are finding the sum, min, max of the Students age.

private static void summaryStatisticsCollector(List<Student> students) {
 System.out.println("######## Executing summaryStatisticsCollector() : ######## ");
 DoubleSummaryStatistics statistics = students.stream()
   .mapToDouble(Student::getAge)
   .summaryStatistics();
 
 System.out.println("summaryStatistics() : "+statistics);
 System.out.println("Total Count : "+statistics.getCount());
 System.out.println("Total Sum : " + statistics.getSum());
 System.out.println("Minimum Age : " + statistics.getMin());
 System.out.println("Maximum Age : " + statistics.getMax());
 System.out.println("Average Age : " + statistics.getAverage());
 System.out.println("######## Ending the execution of summaryStatisticsCollector() ######## ");
}

Output
######## Executing summaryStatisticsCollector() : ########
summaryStatistics() : DoubleSummaryStatistics{count=5, sum=107.000000, min=17.000000, average=21.400000, max=26.000000}
Total Count : 5
Total Sum : 107.0
Minimum Age : 17.0
Maximum Age : 26.0
Average Age : 21.4
######## Ending the execution of summaryStatisticsCollector() ########

3. partitioningBy() in Java 8

We can partition a set of stream in two based on certain condition. So it will create two streams – one which satisfies the condition and another which does not satisfies the conditions.
Lets see below, suppose we want to divide the students who lives in Pune and who does not live in Pune.
partitioningBy will return a map containing two keys
true – which holds the stream which satisfy the condition.
false – which holds the stream which does not satisfy the condition. 

private static void partitioningByCollector(List<Student> students) {
 System.out.println("######## Executing partitioningByCollector() : ######## ");
 Map<Boolean,List<Student>> partition = students.stream()
   .collect(Collectors.partitioningBy(stud -> stud.getCity().equals("Pune")));
 
 System.out.println("Students living in Pune : "+partition.get(true));
 System.out.println("Students not living in Pune : "+partition.get(false));
 System.out.println("######## Ending the execution of partitioningByCollector() ######## ");
}

Output
######## Executing summaryStatisticsCollector() : ########
Students living in Pune : [Student [name=Saurabh, city=Pune, age=26], Student [name=Robert, city=Pune, age=25], Student [name=Roman, city=Pune, age=18]]
Students not living in Pune : [Student [name=John, city=Mumbai, age=21], Student [name=Randy, city=Mumbai, age=17]]
######## Ending the execution of summaryStatisticsCollector() ########

4. groupingBy() in Java 8

partitioningBy() is very simple form of grouping the stream into two, but in real life we might need more than that. groupingBy() is used to group the stream based on the condition passed to the groupingBy(). In below example we are passing the city as an input to the groupingBy() which will split the stream into total number of cities available in the stream and will return a map as Map<String,List<Student>>.

private static void groupingByCollector(List<Student> students) {
 System.out.println("######## Executing groupingByCollector() : ######## ");
 Map<String,List<Student>> groupBy = students.stream()
   .collect(Collectors.groupingBy(Student::getCity));
 
 System.out.println("groupingByCollector : "+groupBy);
 System.out.println("######## Ending the execution of groupingByCollector() ######## ");
}

Output
######## Executing groupingByCollector() : ########
groupingByCollector : {Pune=[Student [name=Saurabh, city=Pune, age=26], Student [name=Robert, city=Pune, age=25], Student [name=Roman, city=Pune, age=18]], Mumbai=[Student [name=John, city=Mumbai, age=21], Student [name=Randy, city=Mumbai, age=17]]}
######## Ending the execution of groupingByCollector() ########

5. mappingBy() in Java 8

groupingBy() splits the stream into set of groups but it collects the complete object on which groupingBy is done. Like in above example of groupingBy(), we got the Map<String,List<Student>> where we gets the key as grouping factor and value as List<Student> objects, but what if we want to collect the names of Students or age of Students ? In this case mappingBy() comes into picture, mappingBy() allows us to pick the particular property of the Object to store into map rather than storing the complete Object.

private static void mappingByCollector(List<Student> students) {
 System.out.println("######## Executing mappingByCollector() : ######## ");
 Map<String,Set<String>> mappingBy = students.stream()
   .collect(Collectors.groupingBy(Student::getCity, 
      Collectors.mapping(Student::getName, Collectors.toSet())));
 
 System.out.println("mappingByCollector : "+mappingBy);
 System.out.println("######## Ending the execution of mappingByCollector() ######## ");
}

Output
######## Executing mappingByCollector() : ########
mappingByCollector : {Pune=[Robert, Roman, Saurabh], Mumbai=[Randy, John]}
######## Ending the execution of mappingByCollector() ########

Source Code
Download source code of Java 8 features from below git repository :
java-8-features

Java 8 Features Tutorial

https://www.onlyfullstack.com/java-8-features-tutorial/

Lets go to our next tutorial where we will discuss below points :
 – What is Intermediate and Terminal Operations of Stream in Java 8?
 – Intermediate Operations in Java 8
 – Terminal Operations in Java 8
 – Difference between Intermediate and Terminal Operations in Java 8
 – Intermediate vs Terminal Operations in Java 8
PART 10 – INTERMEDIATE AND TERMINAL OPERATIONS OF STREAM IN JAVA 8