Data Management with Java Streams: Comprehensive Examples for Beginners to Advanced Users

Ricardo Se.

Streams are one of the best ways to handle data in Java nowadays. Streams provide a versatile and efficient mechanism for data management. In this article, we explore practical examples of how to use them.

We’ll commence with straightforward examples and progressively delve into more advanced ones.

Consider the following class

public class Animal {

    private String name;
    private String type;
    private Double weight;
    private Double height;

    public Animal(String name, String type, double weight, double height) {
        this.name = name;
        this.type = type;
        this.weight = weight;
        this.height = height;
    }

    public String getName() {
        return name;
    }

    public String getType() {
        return type;
    }

    public Double getWeight() {
        return weight;
    }

    public Double getHeight() {
        return height;
    }

    @Override
    public String toString() {
        return String.format("(Name=%s,Type=%s,Weight=%s,Height=%s)",
                this.getName(),
                this.getType(),
                this.getWeight().toString(),
                this.getHeight().toString());

    }

    @Override
    public boolean equals(Object o) {
        if (!(o instanceof Animal)) {
            return false;
        }

        var animal = (Animal) o;

        return this.getName().equals(animal.getName()) &&
                this.getType().equals(animal.getType());

    }
}

We are going to use this data for the examples

public static Stream<Animal> animals() {
    List<Animal> animals = Arrays.asList(
            new Animal("Lion", "Mammal", 190.0, 3.5),
            new Animal("Elephant", "Mammal", 5000.0, 10.0),
            new Animal("Penguin", "Bird", 1.5, 0.5),
            new Animal("Snake", "Reptile", 10.0, 1.0),
            new Animal("Dolphin", "Mammal", 300.0, 2.0),
            new Animal("Eagle", "Bird", 5.0, 0.8),
            new Animal("Tiger", "Mammal", 250.0, 3.0),
            new Animal("Crocodile", "Reptile", 150.0, 2.5),
            new Animal("Giraffe", "Mammal", 1200.0, 5.5),
            new Animal("Hawk", "Bird", 2.0, 0.6),
            // Duplicated hawk
            new Animal("Hawk", "Bird", 2.5, 0.6)
    );

    return animals.stream();
}

We are going to use this auxiliary functions to print results

public static void printAnimals(Stream<Animal> animals) {
    animals.forEach(i -> print(i.getName() + " "));
}

public static void printStrings(Stream<String> params) {
    params.forEach(i -> print(i + " "));
}

public static void print(List<Animal> animals) {
    animals.forEach(i -> print(i.getName() + " "));
}

public static void print(Map<String, List<Animal>> animals) {
    animals.forEach((key, value) -> {
        print(key + "\n");
        print(value);
        print("\n");
    });
}

public static void print(String param) {
    System.out.print(param);
}

public static void println(String param) {
    System.out.println(param);
}

public static <T> void print(T object) {
    print(object.toString());

}

Basic examples

  1. Iterate over the stream printing the names
// 1. Iterate over the stream printing the names
animals().forEach(i -> println(i.getName()));

    //        Output:
    //        Lion
    //        Elephant
    //        Penguin
    //        Snake
    //        Dolphin
    //        Eagle
    //        Tiger
    //        Crocodile
    //        Giraffe
    //        Hawk
    //        Hawk

2. Filter the animals to get only the birds

// 2. Filter the birds to get only the birds
  var birds = animals().filter(i -> i.getType().equals("Bird"));
  printAnimals(birds);

  // Output:
  // Penguin Eagle Hawk Hawk

3. Find the first Hawk 

// 3. Find the first Hawk 

var hawk = animals()
        .filter(i -> i.getName().equals("Hawk"))
        .findFirst();

if (hawk.isPresent()) {
    print(hawk.get());
}

// Output:
// (Name=Hawk,Type=Bird,Weight=2.0,Height=0.6)

4. Get the animals sorted by height

// 4. Get the animals sorted by height


var smallestAnimalsFirst = animals()
        .sorted(Comparator.comparingDouble(i -> i.getHeight()));

smallestAnimalsFirst.forEach(i -> print(String.format("%s(%s) ", i.getName(), i.getHeight())));

        // Output:
        // Penguin(0.5) Hawk(0.6) Hawk(0.6) Eagle(0.8) Snake(1.0) Dolphin(2.0) Crocodile(2.5) Tiger(3.0) Lion(3.5) Giraffe(5.5) Elephant(10.0)

5. Map the stream to a new stream with only names and types

// 5. Map the stream to a new stream with only names and types

var justNamesAndTypes = animals().map(i -> new Object() {
    String name = i.getName();
    String type = i.getType();
});

justNamesAndTypes.forEach(i -> print(String.format("%s/%s ", i.name, i.type)));

// Output:
// Lion/Mammal Elephant/Mammal Penguin/Bird Snake/Reptile Dolphin/Mammal Eagle/Bird Tiger/Mammal Crocodile/Reptile Giraffe/Mammal Hawk/Bird Hawk/Bird

6. Get the smallest animal

// 6. Get the smallest animal

var smallestAnimal = animals()
        .min(Comparator.comparingDouble(i -> i.getHeight()));

if (smallestAnimal.isPresent()) {
    print(smallestAnimal.get());
}

// Output
// (Name=Penguin,Type=Bird,Weight=1.5,Height=0.5)

7. Get the biggest animal

// 7. Get the biggest animal
//
var biggestAnimal = animals()
        .max(Comparator.comparingDouble(i -> i.getHeight()));

if (biggestAnimal.isPresent()) {
    print(biggestAnimal.get());
}
// Output
// (Name=Elephant,Type=Mammal,Weight=5000.0,Height=10.0)

8. Group animals by type

//     8. Group animals by type

  var groupsByType = animals().
          collect(Collectors.groupingBy(i -> i.getType()));
  
  print(groupsByType);
        
//        Output:
//        Mammal
//        Lion Elephant Dolphin Tiger Giraffe
//        Reptile
//        Snake Crocodile
//        Bird
//        Penguin Eagle Hawk Hawk

9. Sum the heights of all animals

// 9. Sum the heights of all animals
var sumHeight = animals().mapToDouble(i -> i.getHeight()).sum();
print(sumHeight);

//Output:
// 30.0

10. Remove duplicated Hawks

// 10. Remove hawks duplicated

var removedDuplicated = animals()
        .filter(i -> i.getName().equals("Hawk"))
        .distinct();

print(removedDuplicated.findFirst().get());
// Output:
// (Name=Hawk,Type=Bird,Weight=2.0,Height=0.6)

Advances queries

11. Get name of smallest mammal or nothing

// 11. Get name of smallest mammal or nothing
        
var smallestMammal = animals()
        .filter(i -> i.getType().equals("Mammal"))
        .sorted(Comparator.comparingDouble(i -> i.getHeight()))
        .map(i -> i.getName())
        .findFirst();

if (smallestAnimal.isPresent()) {
    print(smallestMammal.get());
}

// Output:
// Dolphin

12. Get the smallest animal of each type

var smallestOfGroups = animals()
        .collect(Collectors.groupingBy(i -> i.getType()))
        .entrySet()
        .stream()
        .map(i -> new Object() {
            String group = i.getKey();
            String smallest = i.getValue()
                    .stream()
                    .min(Comparator.comparingDouble(j -> j.getHeight()))
                    .get()
                    .getName();
        });

smallestOfGroups
        .forEach(i -> println(String.format("%s: %s ", i.group, i.smallest)));
        
//       Output:
//        Mammal: Dolphin
//        Reptile: Snake
//        Bird: Penguin

13. Get the sum height of each group

// 13. Get the sum height of each group
var sumEachGroup = animals()
        .collect(Collectors.groupingBy(i -> i.getType()))
        .entrySet()
        .stream()
        .map(i -> new Object() {
            String group = i.getKey();
            Double sum = i.getValue()
                    .stream()
                    .mapToDouble(j -> j.getHeight())
                    .sum();
        });

sumEachGroup
        .forEach(i -> println(String.format("%s %s ", i.group, i.sum.toString())));
        
//        Output:
//        Mammal 24.0
//        Reptile 3.5
//        Bird 2.5

14. Get the animals that contains ‘in’ in the name sorted by height

//14. Get the animals that contains 'in' in the name sorted by height

var animalsWithIn = animals()
        .filter(i -> i.getName().toLowerCase().contains("in"))
        .sorted(Comparator.comparingDouble(i -> i.getHeight()));

printAnimals(animalsWithIn);
        
// Output:
// Penguin Dolphin

15. Get the names of the mammals with weight over 100

// 15. Get the names of the mammals with weight over 100
var namesMammalsOver100 = animals()
        .filter(i -> i.getType().equals("Mammal"))
        .filter(i -> i.getWeight() > 100.0)
        .map(i -> i.getName());

printStrings(namesMammalsOver100);

// Output:
// Lion Elephant Dolphin Tiger Giraffe

This is the whole code

package org.example;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {

    public static Stream<Animal> animals() {
        List<Animal> animals = Arrays.asList(
                new Animal("Lion", "Mammal", 190.0, 3.5),
                new Animal("Elephant", "Mammal", 5000.0, 10.0),
                new Animal("Penguin", "Bird", 1.5, 0.5),
                new Animal("Snake", "Reptile", 10.0, 1.0),
                new Animal("Dolphin", "Mammal", 300.0, 2.0),
                new Animal("Eagle", "Bird", 5.0, 0.8),
                new Animal("Tiger", "Mammal", 250.0, 3.0),
                new Animal("Crocodile", "Reptile", 150.0, 2.5),
                new Animal("Giraffe", "Mammal", 1200.0, 5.5),
                new Animal("Hawk", "Bird", 2.0, 0.6),
                // Duplicated hawk
                new Animal("Hawk", "Bird", 2.5, 0.6)
        );

        return animals.stream();
    }

    public static void main(String[] args) {

        println("1. Iterate over the stream printing the names");
        // 1. Iterate over the stream printing the names
        animals().forEach(i -> println(i.getName()));

    //        Output:
    //        Lion
    //        Elephant
    //        Penguin
    //        Snake
    //        Dolphin
    //        Eagle
    //        Tiger
    //        Crocodile
    //        Giraffe
    //        Hawk
    //        Hawk


        println("\n2. Filter the birds to get only the birds");
        // 2. Filter the birds to get only the birds
        var birds = animals().filter(i -> i.getType().equals("Bird"));
        printAnimals(birds);
        // Output:
        // Penguin Eagle Hawk Hawk


        println("\n3. Find the first Hawk");
        // 3. Find the first Hawk
        var hawk = animals()
                .filter(i -> i.getName().equals("Hawk"))
                .findFirst();

        if (hawk.isPresent()) {
            print(hawk.get());
        }
        // Output:
        // (Name=Hawk,Type=Bird,Weight=2.0,Height=0.6)


        println("\n4. Get the animals sorted by height");
        // 4. Get the animals sorted by height
        var smallestAnimalsFirst = animals()
                .sorted(Comparator.comparingDouble(i -> i.getHeight()));

        smallestAnimalsFirst.forEach(i -> print(String.format("%s(%s) ", i.getName(), i.getHeight())));
        // Output:
        // Penguin(0.5) Hawk(0.6) Hawk(0.6) Eagle(0.8) Snake(1.0) Dolphin(2.0) Crocodile(2.5) Tiger(3.0) Lion(3.5) Giraffe(5.5) Elephant(10.0)


        println("\n5. Map the stream to a new stream with only names and types");
        // 5. Map the stream to a new stream with only names and types
        var justNamesAndTypes = animals().map(i -> new Object() {
            String name = i.getName();
            String type = i.getType();
        });

        justNamesAndTypes.forEach(i -> print(String.format("%s/%s ", i.name, i.type)));
        // Output:
        // Lion/Mammal Elephant/Mammal Penguin/Bird Snake/Reptile Dolphin/Mammal Eagle/Bird Tiger/Mammal Crocodile/Reptile Giraffe/Mammal Hawk/Bird Hawk/Bird


        println("\n6. Get the smallest animal.");
        // 6. Get the smallest animal
        var smallestAnimal = animals()
                .min(Comparator.comparingDouble(i -> i.getHeight()));

        if (smallestAnimal.isPresent()) {
            print(smallestAnimal.get());
        }
        // Output
        // (Name=Penguin,Type=Bird,Weight=1.5,Height=0.5)


        println("\n7. Get the biggest animal");
        // 7. Get the biggest animal
        var biggestAnimal = animals()
                .max(Comparator.comparingDouble(i -> i.getHeight()));

        if (biggestAnimal.isPresent()) {
            print(biggestAnimal.get());
        }
        // Output
        // (Name=Elephant,Type=Mammal,Weight=5000.0,Height=10.0)


        println("\n8. Group animals by type");
        // 8. Group animals by type
        var groupsByType = animals().
                collect(Collectors.groupingBy(i -> i.getType()));

        print(groupsByType);
//        Output:
//        Mammal
//        Lion Elephant Dolphin Tiger Giraffe
//        Reptile
//        Snake Crocodile
//        Bird
//        Penguin Eagle Hawk Hawk


        println("\n9. Sum the heights of all animals");
        // 9. Sum the heights of all animals
        var sumHeight = animals().mapToDouble(i -> i.getHeight()).sum();
        print(sumHeight);

        //Output:
        // 30.0


        println("\n10. Remove hawks duplicated");
        // 10. Remove hawks duplicated
        var removedDuplicated = animals()
                .filter(i -> i.getName().equals("Hawk"))
                .distinct();

        print(removedDuplicated.findFirst().get());
        // Output:
        // (Name=Hawk,Type=Bird,Weight=2.0,Height=0.6)


        // 11. Get name of smallest mammal or nothing
        println("\n 11. Get name of smallest mammal or null");
        var smallestMammal = animals()
                .filter(i -> i.getType().equals("Mammal"))
                .sorted(Comparator.comparingDouble(i -> i.getHeight()))
                .map(i -> i.getName())
                .findFirst();

        if (smallestAnimal.isPresent()) {
            print(smallestMammal.get());
        }
        // Output:
        // Dolphin


        // 12. Get the smallest animal of each type
        println("\n 12. Get the smallest animal of each type ");

        var smallestOfGroups = animals()
                .collect(Collectors.groupingBy(i -> i.getType()))
                .entrySet()
                .stream()
                .map(i -> new Object() {
                    String group = i.getKey();
                    String smallest = i.getValue()
                            .stream()
                            .min(Comparator.comparingDouble(j -> j.getHeight()))
                            .get()
                            .getName();
                });

        smallestOfGroups
                .forEach(i -> println(String.format("%s: %s ", i.group, i.smallest)));

//       Output:
//        Mammal: Dolphin
//        Reptile: Snake
//        Bird: Penguin


        println(" 13. Get the sum height of each group");
        // 13. Get the sum height of each group
        var sumEachGroup = animals()
                .collect(Collectors.groupingBy(i -> i.getType()))
                .entrySet()
                .stream()
                .map(i -> new Object() {
                    String group = i.getKey();
                    Double sum = i.getValue()
                            .stream()
                            .mapToDouble(j -> j.getHeight())
                            .sum();
                });

        sumEachGroup
                .forEach(i -> println(String.format("%s %s ", i.group, i.sum.toString())));

//        Output:
//        Mammal 24.0
//        Reptile 3.5
//        Bird 2.5


        //14. Get the animals that contains 'in' in the name sorted by height
        println("\n 14. Get the animals that contains 'in' in the name sorted by height");

        var animalsWithIn = animals()
                .filter(i -> i.getName().toLowerCase().contains("in"))
                .sorted(Comparator.comparingDouble(i -> i.getHeight()));

        printAnimals(animalsWithIn);

        // Output:
        // Penguin Dolphin


        // 15. Get the names of the mammals with weight over 100
        println("\n 15. Get the names of the mammals with weight over 100");
        var namesMammalsOver100 = animals()
                .filter(i -> i.getType().equals("Mammal"))
                .filter(i -> i.getWeight() > 100.0)
                .map(i -> i.getName());

        printStrings(namesMammalsOver100);

        // Output:
        // Lion Elephant Dolphin Tiger Giraffe
    }

    public static void printAnimals(Stream<Animal> animals) {
        animals.forEach(i -> print(i.getName() + " "));
    }

    public static void printStrings(Stream<String> params) {
        params.forEach(i -> print(i + " "));
    }

    public static void print(List<Animal> animals) {
        animals.forEach(i -> print(i.getName() + " "));
    }

    public static void print(Map<String, List<Animal>> animals) {
        animals.forEach((key, value) -> {
            print(key + "\n");
            print(value);
            print("\n");
        });
    }

    public static void print(String param) {
        System.out.print(param);
    }

    public static void println(String param) {
        System.out.println(param);
    }

    public static <T> void print(T object) {
        print(object.toString());

    }
}

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