ArrayList vs. HashMap in Java

ArrayList is a part of collection framework and is present in java.util package. It provides us dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed.

HashMap is a part of Java’s collection since Java 1.2. It provides the basic implementation of Map interface of Java. It stores the data in (Key, Value) pairs. To access a value one must know its key. HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to small String that represents same String. A shorter value helps in indexing and faster searches. HashSet also uses HashMap internally. It internally uses a link list to store key-value pairs already explained in HashSet in detail and further articles.

This article discusses the similarities and differences between the ArrayList and HashMap in Java.

Similarity between ArrayList and HashMap in Java

• The ArrayList and HashMap, both are not synchronised. So in-order to use them in the multi-threading environment, it needs to be first synchronised.
• Both ArrayList and HashMap allow null. ArrayList allow null Values and HashMap allows null key and values
• Both ArrayList and HashMap allow duplicates, ArrayList allows duplicate elements and HashMap allow duplicate values
• Both ArrayList and HashMap can be traversed through Iterator in Java.
• Both Somewhere use array, ArrayList is backed by array and HashMap is also internally implemented by Array
• Both are use get() method, ArrayList.get() method works based on index and HashMap.get() method take one parameter key_element of object type and refers to the key whose associated value is supposed to be fetched so both provides constant-time performance.

Difference between ArrayList and HashMap in Java

1. Interface Implemented: ArrayList implement List Interface while HashMap is an implementation of Map interface.

   ArrayList Class Declaration:

   public class ArrayList 
       extends AbstractList 
       implements List, RandomAccess, Cloneable, Serializable
   

   HashMap Class Declaration:

   public class HashMap 
       extends AbstractMap 
       implements Map, Cloneable, Serializable  
   

2. Maintenance of the Insertion Order: ArrayList maintains the insertion order while HashMap does not maintain insertion order. Which means ArrayList returns the list items in the same order while HashMap doesn’t maintain any order so returned key-values pairs any kind of order.

   Below example illustrates this difference:

   Example:

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   import java.util.*;    class GFG {        public static void main(String args[])     {         // Creating ArrayList         ArrayList<String> list             = new ArrayList<String>();            // Adding object in ArrayList         list.add("A");         list.add("B");         list.add("C");         list.add("D");            // Invoking ArrayList object         System.out.println("ArrayList: " + list);            // Creating HashMap         HashMap<Integer, String> hm             = new HashMap<Integer, String>();            // Adding object in HashMap         hm.put(1, "A");         hm.put(2, "B");         hm.put(3, "C");         hm.put(4, "D");            // Invoking HashMap object         // It might or might not display elements         // in the insertion order         System.out.print("HasMap: " + hm);     } }

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   Output:

   ArrayList: [A, B, C, D]
   HasMap: {1=A, 2=B, 3=C, 4=D}
   

3. Memory Consumption: ArrayList stores the elements only as value and maintain internally the indexing for every element. While HashMap stores elements with key and value pair, i.e. two objects. So HashMap takes more memory comparatively.

   ArrayList:

   // String value is stored in ArrayList
   list.add("A");
   

   HashMap:

   // Two String values stored
   // as the key value pair in HashMap
   hm.put(1, "A");
   

4. Duplicates: ArrayList allows duplicate elements while HashMap doesn’t allow duplicate keys but does allow duplicate values.

   Below example illustrates this difference:

   Example:

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   import java.util.*;    class GFG {        public static void main(String args[])     {         // Creating ArrayList         ArrayList<String> list             = new ArrayList<String>();            // Adding object in ArrayList         list.add("A");         list.add("B");            // Add duplicates         list.add("A");         list.add("A");            // Invoking ArrayList object         System.out.println("ArrayList: " + list);            // Creating HashMap         HashMap<Integer, String> hm             = new HashMap<Integer, String>();            // Adding object in HashMap         hm.put(1, "A");         hm.put(2, "B");            // Add duplicates key         // Change value if index exist         hm.put(3, "A");         hm.put(3, "A");            // Add duplicates values         // allow duplicates value         hm.put(4, "A");         hm.put(5, "A");            // Invoking HashMap object         System.out.print("HasMap: " + hm);     } }

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   Output:

   ArrayList: [A, B, A, A]
   HasMap: {1=A, 2=B, 3=A, 4=A, 5=A}
   

5. Easiness while fetching an element: In ArrayList, an element can be fetched easily by specifying the index of it. But in HashMap, the elements is fetched by its corresponding key. It means that the key must be remembered always.

   ArrayList get(index) method always gives O(1) time complexity While HashMap get(key) can be O(1) in the best case and O(n) in the worst case time complexity.

   Below example illustrates this difference:

   Example:

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   import java.util.*;    class GFG {        public static void main(String args[])     {         // Creating ArrayList         ArrayList<String> list             = new ArrayList<String>();            // Adding object in ArrayList         list.add("A");         list.add("B");         list.add("C");         list.add("D");            // Invoking ArrayList object         System.out.println("First Element of ArrayList: "                            + list.get(0));         System.out.println("Third Element of ArrayList: "                            + list.get(2));            // Creating HashMap         HashMap<Integer, String> hm             = new HashMap<Integer, String>();            // Adding object in HashMap         hm.put(1, "A");         hm.put(2, "B");         hm.put(3, "C");         hm.put(4, "D");            // Invoking HashMap object         System.out.println("HashMap value at Key 1: "                            + hm.get(1));         System.out.println("HashMap value at Key 3: "                            + hm.get(3));     } }

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   Output:

   First Element of ArrayList: A
   Third Element of ArrayList: C
   HashMap value at Key 1: A
   HashMap value at Key 3: C
   

6. Storing nulls: In ArrayList, any number of null elements can be stored. While in HashMap, only one null key is allowed, but the values can be of any number.

   Below example illustrates this difference:

   Example:

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   import java.util.*;    class GFG {        public static void main(String args[])     {         // Creating ArrayList         ArrayList<String> list             = new ArrayList<String>();            // Adding object in ArrayList         list.add("A");            // add first null value         list.add(null);         list.add("C");            // add two null value again         list.add(null);         list.add(null);            // Invoking ArrayList object         System.out.println("ArrayList: "                            + list);            // Creating HashMap         HashMap<Integer, String> hm             = new HashMap<Integer, String>();            // Adding object in HashMap         hm.put(1, "A");         hm.put(2, "B");            // add null key         hm.put(null, "C");            // add again null key         // which replace value of first         // insert null key value         hm.put(null, null);            // add second null value         hm.put(3, null);            // Printing the HashMap         System.out.println("HasMap: " + hm);     } }

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   Output:

   ArrayList: [A, null, C, null, null]
   HasMap: {null=null, 1=A, 2=B, 3=null}
   

Difference between ArrayList and HashMap

ArrayList	HashMap
The java ArrayList implements List Interface	The java HashMap is implements Map interface
ArrayList always maintain insertion order of the elements	HashMap does not maintain the insertion order
ArrayList only stores value or element	HashMap stores key and value pairs
ArrayList can contain duplicate elements	HashMap does not contain duplicate keys but contain duplicate values.
We can have any numbers of null elements in ArrayList	We can have only one null key and any number of null values in HashMap
ArrayList get() method always gives an O(1) performance	HashMap get()method can be O(1) in the best case and O(n) in the worst case

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