StreamGroupByExample.java

package gr.iserm.java.javaplayground.lambdas;

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collectors;

import static java.lang.System.out;

public class StreamGroupByExample {

    public static void main(String[] args) {

        String city = "Athens";

        //solution 1 - global state 'city' is associated with each item before iteration.
        out.println("----Solution 1-----");
        List<Person1> person1List = Arrays.asList(new Person1("Giannis", "Frangos"), new Person1("Nikos", "Lianeris"), new Person1("Eleftherios", "Tsallas"),
                new Person1("Panagis", "Lefteratos"), new Person1("Giannis", "Sermetziadis"), new Person1("Nikos", "Tester"));
        person1List.stream()
                .forEach(p -> p.setCity(city));
        person1List.stream()
                .collect(Collectors.groupingBy(Person1::createKey))
                .entrySet().stream()
                    .forEach(out::println);


        //solution 2 - using static key generation method. The keyGenerationFunction has no side-effects.
        out.println("----Solution 2-----");
        List<Person2> person2List = Arrays.asList(new Person2("Giannis", "Frangos"), new Person2("Nikos", "Lianeris"), new Person2("Eleftherios", "Tsallas"),
                new Person2("Panagis", "Lefteratos"), new Person2("Giannis", "Sermetziadis"), new Person2("Nikos", "Tester"));
        Function<Person2, Person2.Person2Key> keyGenerationFunction = Person2.createKeyStaticFunction(city);
        person2List.stream()
                .collect(Collectors.groupingBy(keyGenerationFunction))
                .entrySet().stream()
                .forEach(out::println);


        //solution 3 - using instance key generation method. The supplier returned by createKeyInstanceFunction is created
        // per item and has side-effects with person instance variables.
        out.println("----Solution 3-----");
        person2List = Arrays.asList(new Person2("Giannis", "Frangos"), new Person2("Nikos", "Lianeris"), new Person2("Eleftherios", "Tsallas"),
                new Person2("Panagis", "Lefteratos"), new Person2("Giannis", "Sermetziadis"), new Person2("Nikos", "Tester"));
        person2List.stream()
                .collect(Collectors.groupingBy(p -> p.createKeyInstanceFunction(city).get()))
                .entrySet().stream()
                .forEach(out::println);
    }


    private static class Person1 {

        private String name, lastname;
        private String city;

        public Person1(String name, String lastname) {
            this.name = name;
            this.lastname = lastname;
        }

        public String getName() {
            return this.name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getLastname() {
            return this.lastname;
        }

        public void setLastname(String lastname) {
            this.lastname = lastname;
        }

        public String getCity() {
            return this.city;
        }

        public void setCity(String city) {
            this.city = city;
        }

        public Person1Key createKey() {
            return new Person1Key(getCity()+"_"+getName());
        }

        @Override
        public String toString() {
            return "Person1{" +
                    "name='" + name + '\'' +
                    ", lastname='" + lastname + '\''+
                    '}';
        }

        public static class Person1Key {
            private String key;

            public Person1Key(String key) {
                this.key = key;
            }

            public String getKey() {
                return this.key;
            }

            public void setKey(String key) {
                this.key = key;
            }

            @Override
            public String toString() {
                return "Person1Key{" +
                        "key='" + key + '\'' +
                        '}';
            }

            @Override
            public boolean equals(Object o) {
                if (this == o) return true;
                if (o == null || this.getClass() != o.getClass()) return false;

                StreamGroupByExample.Person1.Person1Key that = (StreamGroupByExample.Person1.Person1Key) o;

                return this.key != null ? this.key.equals(that.key) : that.key == null;

            }

            @Override
            public int hashCode() {
                return this.key != null ? this.key.hashCode() : 0;
            }
        }

    }

    private static class Person2 {
        private String name, lastname;

        public Person2(String name, String lastname) {
            this.name = name;
            this.lastname = lastname;
        }

        public String getName() {
            return this.name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getLastname() {
            return this.lastname;
        }

        public void setLastname(String lastname) {
            this.lastname = lastname;
        }

        @Override
        public String toString() {
            return "Person2{" +
                    "name='" + name + '\'' +
                    ", lastname='" + lastname + '\'' +
                    '}';
        }

        public Supplier<Person2Key> createKeyInstanceFunction(String city) {
            return () -> new Person2Key(city+"_"+getName());
        }

        public static Function<Person2, Person2Key> createKeyStaticFunction(String city) {
            return (p) -> new Person2Key(city+"_"+p.getName());
        }

        public static class Person2Key {
            private String key;

            public Person2Key(String key) {
                this.key = key;
            }

            public String getKey() {
                return this.key;
            }

            public void setKey(String key) {
                this.key = key;
            }

            @Override
            public String toString() {
                return "Person2Key{" +
                        "key='" + key + '\'' +
                        '}';
            }

            @Override
            public boolean equals(Object o) {
                if (this == o) return true;
                if (o == null || this.getClass() != o.getClass()) return false;

                StreamGroupByExample.Person2.Person2Key that = (StreamGroupByExample.Person2.Person2Key) o;

                return this.key != null ? this.key.equals(that.key) : that.key == null;

            }

            @Override
            public int hashCode() {
                return this.key != null ? this.key.hashCode() : 0;
            }
        }
    }

}