How to initialize hashmap java

How to Initialize a HashMap Inline in Java

Examples of creating and adding the key-value pairs to a Java HashMap inline.

Overview

The HashMaps are key-value-based unordered, unsorted collections. We often use HashMaps in Java and put hard-coded values in it. Today, we will cover various ways of creating and initializing Java HashMaps inline. Also, we will learn to create Immutable, Singleton, and Empty maps.

Creating a Simple HashMap

The most basic way of creating a HashMap instance and adding a few elements is to use a constructor to create an empty HashMap and then add elements one by one.
Example of initializing a simple HashMap.

Map map = new HashMap<>(); map.put("color", "black"); map.put("drink", "coffee"); map.put("shape", "slim");Code language: Java (java)

Although this is a two-step operation, the HashMap we created is mutable; thus, we can add, remove, or modify elements.

Creating a HashMap using Anonymous Subclass

We can use an anonymous subclass to instantiate and initialize a Java HashMap in the same line.

Map map = new HashMap<>() < < put("color", "black"); put("drink", "coffee"); put("shape", "slim"); > >;Code language: Java (java)

This way is not recommended for initializing a Java HashMap as it has a lot of overhead and potential memory leak issues.

Creating an Immutable HashMap

The Java Collections class offers the unmodifiableMap() method that creates an immutable HashMap containing the elements from the given Map.
Example of initializing an immutable Java HashMap.

Map map = new HashMap<>(); map.put("color", "black"); map.put("drink", "coffee"); map.put("shape", "slim"); Map immutableMap = Collections.unmodifiableMap(map);Code language: Java (java)

Using the Collections unmodifiableMap() method, we can create an immutable copy of an existing Java HashMap. As the resulting HashMap is immutable, we cannot add, update, or remove elements.

Creating a Singleton HashMap

The singletonMap() method of the Java Collections class creates an immutable HashMap containing the given key-value pair.
Example of creating a singleton HashMap.

Map map = Collections.singletonMap("color", "black");Code language: Java (java)

A singleton HashMap contains one and only one key-value pair. As it is immutable, we cannot modify its contents.

Creating an Empty HashMap

The emptyMap() method of the Java Collections class creates an empty HashMap instance.
Example of creating an empty Java HashMap.

Map map = Collections.emptyMap();Code language: Java (java)

Please note that the resulting HashMap is immutable. Thus, it remains empty forever.

Creating a HashMap using Google Guava

Google’s Guava Library provides several useful abstractions and utility methods to work with the Java Collections framework. Let’s see how the Guava Library helps to initialize mutable and immutable HashMap instances inline.

Immutable HashMap using Guava

Guava’s ImmutableMap class implements Java Map, representing an immutable HashMap. The class provides the of() method, a factory method that creates an immutable Java HashMap containing the given key-value pairs.
Example of initializing an immutable Java HashMap using Google Guava.

Map immutableMap = ImmutableMap .of("color", "pink", "drink", "coffee", "shape", "slim");Code language: Java (java)

Mutable HashMap using Guava

The Maps class of Guava is a static utility class with useful abstractions. The newHashMap() method of the Maps class returns a mutable HashMap instance containing all the elements of the given Map.
Example of initializing a mutable Java HashMap using Google Guava.

Map immutableMap = ImmutableMap .of("color", "pink", "drink", "coffee", "shape", "slim"); Map mutuableMap = Maps.newHashMap(immutableMap);Code language: Java (java)

Although we provide an immutable Map, the newHashMap() method always returns a mutable HashMap.

Creating a HashMap using Stream Collectors

The Java Collectors class provide a collector abstraction that collects the Stream elements and forms a Java HashMap.

Example of initializing a Java HashMap using Stream Collectors.

List colors = List.of("Pink", "Red", "Black"); Map map = colors.stream() .collect(Collectors .toMap(String::toUpperCase, String::toLowerCase)); System.out.println(map); //prints: // Code language: Java (java)

Creating a HashMap using Factory Method

Java 9 introduced factory methods to the Map interface that we can use to do inline initialization of Java HashMap instances.

Using the Factory Method of()

The of() method of the Map interface accepts key-value pairs in the form of varargs and returns a new immutable HashMap containing the elements.
Example of initializing Java HashMap inline using factory methods.

Map immutableMap = Map.of("color", "black", "drink","coffee");Code language: Java (java)

Please note that the Map interface has ten overloaded versions of the of() method.

static Map of (K k1, V v1); static Map of (K k1, V v1, K k2, V v2); static Map of (K k1, V v1, K k2, V v2, K k3, V v3); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9); static Map of (K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10); Code language: Java (java)

This is why we can only use the factory method of() to create an inline HashMap of up to ten key-value pairs.

Using the Factory Method ofEntries()

Alternatively, the ofEntries() method of the Map method accepts n key-value pairs and can initialize a HashMap of infinite elements.

Example of initializing inline Java HashMap using Map ofEntries() method.

Map ofEntries = Map.ofEntries( Map.entry("color", "pink"), Map.entry("drink", "coffee") );Code language: Java (java)

Summary

This article demonstrated different ways of creating mutable and immutable Java HashMap instances and adding key-value pairs in the same line. We used Java Map factory methods, Collections class, and the Guava Library to initialize the Maps inline.

Please refer to our GitHub Repository for the complete source code of the examples.

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Class HashMap

Type Parameters: K — the type of keys maintained by this map V — the type of mapped values All Implemented Interfaces: Serializable , Cloneable , Map Direct Known Subclasses: LinkedHashMap , PrinterStateReasons

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable , except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This implementation provides constant-time performance for the basic operations ( get and put ), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the «capacity» of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put ). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable , this class may use comparison order among keys to help break ties.

Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be «wrapped» using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

Map m = Collections.synchronizedMap(new HashMap(. ));

The iterators returned by all of this class’s «collection view methods» are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

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