How to Group and Partition Collectors in Java 8

See Java: Tips and Tricks for similar articles.

The Collectors class in Java 8 provides methods for grouping and partitioning data stored in collections. Grouping permits you to organize data that shares a common field value, e.g. department. Partitioning allows you to divide the data into two categories, or partitions. One partition satisfies the partitioning criteria whereas the other partition does not. For example, we might separate sales data into a partition that met or exceeded a sales goal and a partition that did not. Our case study below will apply grouping and partitioning technology to a collection of musical instruments.

  1. Open your text editor and create the Java program that will define the musical instrument class. Type in the following Java statements:
    
    public class MusicalInstrument {
    	private String name;
    	private String type;
    	private double price;
    	public MusicalInstrument(String name, String type, double price) {
    		setName(name);
    		setType(type);
    		setPrice(price);
    	}	
    	public String getName() {
    		return name;
    	}
    	public void setName (String name) {
    		this.name=name;
    	}
    	public String getType() {
    		return type;
    	}
    	public void setType (String type) {
    		this.type=type;
    	}
    	public double getPrice() {
    		return price;
    	}
    	public void setPrice (double price) {
    		this.price=price;
    	}
    	public String toString() {
    		return name + " is a " + type + " instrument that costs " + price + " dollars";
    	}	
    }		
    	
    This is a straightforward Java bean that defines name, type, and price properties for a musical instrument.
  2. Save your file as MusicalInstrument.java.
  3. Open a command prompt and navigate to the directory containing your new Java program. Then type in the command to compile the source and hit Enter.Compile Musical Instrument for Group and Partition
  4. Open your text editor and create the Java program that will use group and partition collectors for musical instrument data. Type in the following Java statements:
    
    import java.util.stream.*;
    import java.util.*;
    public class GroupAndPartitionCollectors {
    	private static List musicalInstruments=new ArrayList<>();
    	static {
    		musicalInstruments.add(new MusicalInstrument("Trumpet","brass",299.99));
    		musicalInstruments.add(new MusicalInstrument("Tuba","brass",1149.0));
    		musicalInstruments.add(new MusicalInstrument("Timpani","percussion",2339.0));
    		musicalInstruments.add(new MusicalInstrument("Snare drum","percussion",325.00));
    		musicalInstruments.add(new MusicalInstrument("Piano","keyboard",5179.99));
    		musicalInstruments.add(new MusicalInstrument("Trombone","brass",775.79));
    	}
    	public static void main (String args[]) {
    		System.out.println("Musical instruments in the collection:");
    		musicalInstruments
    		.stream()
    		.forEach(instrument->System.out.println(instrument) );
    		
    		System.out.println("Group by type:");
    		Map > typeMap= musicalInstruments
    		.stream()
    		.collect(Collectors.groupingBy(MusicalInstrument::getType));
    		typeMap.forEach( (key, value)-> {System.out.println(key); value.forEach(System.out::println);});
    		
    		System.out.println("Partition by Affordable (less than or equal to $500) or Expensive (greater than $500) :");
    		Map > priceMap= musicalInstruments
    		.stream()
    		.collect(Collectors.partitioningBy(musicalInstrument -> musicalInstrument.getPrice() > 500.0));
    		priceMap.forEach( (key, value)-> {System.out.println((key? "Expensive ":"Affordable")); value.forEach(System.out::println);});
    	}
    }
    	
    The stream will be created from the ArrayList that is defined on line 4. The program displays the entire collection of musical instruments before applying the groupingBy method. This method from the Collectors class is responsible for organizing the data into groups. In this case, the groups are defined by instruments belonging to the same type (e.g., "brass"). The collect method from the Stream class accepts the groupingBy method as its argument and returns a map containing the results. The program displays the results. Next, the partitioningBy method of the Collectors class is called to partition the data into "Affordable" instruments (price less than or equal to $500) and "Expensive" instruments (price greater than $500). I have highlighted the grouping method and the partitioning method in the image below:Java Source for Group Partition
  5. Save your file as GroupAndPartitionCollectors.java.
  6. Open a command prompt and navigate to the directory containing your new Java program. Then type in the command to compile the source and hit Enter.Compile Program with Group Partition
  7. You are ready to test your Java program. Type in the command to run the Java runtime launcher and hit Enter. The output displays the original list of musical instruments followed by the list of instruments collected by group and then the list of instruments partitioned by price.Run Program with Group Partition

Related Articles

  1. How to Check Object Type in Java
  2. How to Create a Jar File in Java
  3. How to Compile Packages in Java
  4. How to Throw an Exception in Java
  5. How to Create an Exception Class in Java
  6. How to Use the super Keyword to Call a Base Class Constructor in Java
  7. How to Use the Comparator.comparing Method in Java 8
  8. How to Use System.in in Java
  9. How to Call an Interface Method in Java
  10. How to Add a Time Zone in the Java 8 Date/Time API
  11. How to Rethrow an Exception in Java
  12. How to Use the instanceof Operator with a Generic Class in Java
  13. How to Instantiate an Object in Java
  14. How to Filter Distinct Elements from a Collection in Java 8
  15. How to Create a Derived Class in Java
  16. How to Skip Elements with the Skip Method in Java 8
  17. How to Create a Java Bean
  18. How to Implement an Interface in Java
  19. How to Compare Two Objects with the equals Method in Java
  20. How to Set PATH from JAVA_HOME
  21. How to Prevent Race Conditions in Java 8
  22. How to Write a Block of Code in Java
  23. How to Display the Contents of a Directory in Java
  24. How to Group and Partition Collectors in Java 8 (this article)
  25. How to Create a Reference to an Object in Java
  26. How to Reduce the Size of the Stream with the Limit Method in Java 8
  27. How to Write an Arithmetic Expression in Java
  28. How to Format Date and Time in the Java 8 Date/Time API
  29. How to Use Comparable and Comparator in Java
  30. How to Break a Loop in Java
  31. How to Use the this Keyword to Call Another Constructor in Java
  32. How to Write a Unit Test in Java
  33. How to Declare Variables in Java
  34. How to Override Base Class Methods with Derived Class Methods in Java
  35. How to Use Serialized Objects in Java
  36. How to Write Comments in Java
  37. How to Implement Functional Interfaces in Java 8
  38. How to Write Type Parameters with Multiple Bounds in Java
  39. How to Add Type and Repeating Annotations to Code in Java 8
  40. How to Use Basic Generics Syntax in Java
  41. How to Map Elements Using the Map Method in Java 8
  42. How to Work with Properties in Java
  43. How to Write while and do while Loops in Java
  44. How to Use the finally Block in Java
  45. How to Write for-each Loops in Java
  46. How to Create a Method in Java
  47. How to Continue a Loop in Java
  48. How to Handle Java Files with Streams
  49. How to Create an Interface Definition in Java
  50. How Default Base Class Constructors Are Used with Inheritance