New & Simpler way of creating Collections in Java 9

In this article, we are going to discuss about static factory methods that Java 9 brought to the Collections library. These new methods are added to List, Set and Map to create immutable collection instances. These methods are also referred to as convenience factory methods because it helps to create collection in simpler & compact way.

Check out: Iterate / ofNullable operations in Java 9 Stream

Simpler way of Creating Collections in Java 9...!!! Share on X

In this article, we are going to cover below points:

1. What are Factory methods for Collections?
2. Complete List of Factory methods for List, Set & Map Interface. 
3. Old & New way of creating List, Set & Map with example in Java

Let’s begin:

1. What are Factory methods for Collections?

Factory methods are static methods used to create immutable instances of collections. It is unmodifiable, therefore adding new element will throw java.lang.UnsupportedOperationException

2.  List, Set & Map Interface Factory Methods

LIST Interface Factory Methods

SET Interface Factory Methods

MAP Interface Factory Methods

3. Old Vs New way of Creating List, Set & Map

3.1 Creating LIST

OLD Way (Before Java 9)

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Creating_List_Old_Way_Before_Java9_Example {

    public static void main(String args[]) {

        //Before Java 9 - Creating List
        List <String> list = new ArrayList <> ();

        list.add("Fruit");
        list.add("Car");
        list.add("Animal");

        list = Collections.unmodifiableList(list);

        System.out.println("List values are: " + list);

    }
}

NEW Way (Since Java 9) Immutable List

import java.util.List;

public class Creating_List_New_Way_Java9_Example {

    public static void main(String[] args) {

        //Java 9 - Creating List
        List <String> list = List.of("Fruit", "Car", "Animal");

        //Java 7 - Printing Set
        System.out.println("Printing list using traditional for loop:\n");
        for (String s: list) {
            System.out.println(s);
        }

        //Java 8 - Printing Set
        System.out.println("\nPrinting list using forEach:\n");
        list.stream().forEach(s -> System.out.println(s));

    }
}

3.1 Creating SET

OLD Way (Before Java 9)

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

public class Creating_Set_Old_Way_Before_Java9_Example {

    public static void main(String args[]) {

        //Before Java 9 - Creating Set
        Set <String> set = new HashSet <> ();

        set.add("Fruit");
        set.add("Car");
        set.add("Animal");

        set = Collections.unmodifiableSet(set);

        System.out.println("Set values are: " + set);

    }
}

NEW Way (Since Java 9) Immutable Set

import java.util.Set;

public class Creating_Set_New_Way_Java9_Example {

    public static void main(String[] args) {

        //Java 9 - Creating Set
        Set <String> set = Set.of("Fruit", "Car", "Animal");

        //Java 7 - Printing Set
        System.out.println("Printing set using traditional for loop:\n");
        for (String s: set) {
            System.out.println(s);
        }

        //Java 8 - Printing Set
        System.out.println("\nPrinting set using forEach:\n");
        set.stream().forEach(s -> System.out.println(s));

    }
}

3.1 Creating MAP

OLD Way (Before Java 9)

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class Creating_Map_Old_Way_Before_Java9_Example {

    public static void main(String args[]) {

        //Before Java 9 - Creating Map
        Map <String, String> map = new HashMap <> ();

        map.put("Fruit", "Mango");
        map.put("Car", "Mercedes");
        map.put("Animal", "Dog");

        map = Collections.unmodifiableMap(map);

        System.out.println("Map values are: " + map);

    }
}

First NEW Way (Since Java 9) Immutable Map

import java.util.Map;

public class Creating_Map_New_Way_Java9_Example1 {

    public static void main(String[] args) {

        //Java 9 - Creating Map
        Map <String, String> map = Map.of("Fruit", "Mango", "Car", "Mercedes", "Animal", "Dog");

        //Java 7 - Printing Map
        System.out.println("Print map using traditional for loop:\n");
        for (Map.Entry <String, String> mapObj: map.entrySet()) {
            System.out.println(mapObj.getKey() + " : " + mapObj.getValue());
        }

        //Java 8 - Printing Map
        System.out.println("\nPrint map using forEach:\n");
        map.forEach((k, v) -> System.out.println(k + " : " + v));

    }
}

Second NEW Way (Since Java 9) Immutable Map

import java.util.AbstractMap;
import java.util.Map;

public class Creating_Map_New_Way_Java9_Example2 {

    public static void main(String[] args) {

        //Java 9 - Creating Map
        Map <String, String> map = Map.ofEntries(
            new AbstractMap.SimpleEntry <> ("Fruit", "Mango"),
            new AbstractMap.SimpleEntry <> ("Car", "Mercedes"),
            new AbstractMap.SimpleEntry <> ("Animal", "Dog"));

        //Java 7 - Printing Map
        System.out.println("Printing map using tradition for loop:\n");
        for (Map.Entry <String, String> mapObj: map.entrySet()) {
            System.out.println(mapObj.getKey() + " : " + mapObj.getValue());
        }

        //Java 8 - Printing Map
        System.out.println("\nPrinting map using forEach:\n");
        map.forEach((k, v) -> System.out.println(k + " : " + v));

    }
}

Creating Immutable Collection is literally a very useful addition as part of Java 9. I hope above examples could help you to get better idea on how to implement it.

Simpler way of Creating Collections in Java 9...!!! Share on X

Do you like this Post? – then check my other helpful posts:

• dropWhile / takeWhile operations in Java 9 Streams
• Collection sorting using Lambda in Java 8
• Iterate / ofNullable operations in Java 9 Streams

Other Useful References:

• java.util.stream by Oracle