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.
- Open your text editor and create the Java program that will define the musical instrument class. Type in the following Java statements:
This is a straightforward Java bean that defines name, type, and price properties for a musical instrument.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"; } }
- Save your file as
MusicalInstrument.java
. - 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.
- 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:
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 theimport 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);}); } } groupingBy
method. This method from theCollectors
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"). Thecollect
method from theStream
class accepts thegroupingBy
method as its argument and returns a map containing the results. The program displays the results. Next, thepartitioningBy
method of theCollectors
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: - Save your file as
GroupAndPartitionCollectors.java
. - 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.
- 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.
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