How to initialize a hashmap with values in Java

We will use the Collections.singletonMap() method to initialize it. Since it is an immutable map, we cannot add more than one entry to it. This will result in the UnsupportedOperationException. The HashMaps are key-value-based unordered, unsorted collections. We often use HashMaps in Java and put hard-coded values in it.

This way of initializing the map is not recommended because it creates a new subclass with an anonymous class and an extra class is not a very efficient way to do it. Is in principal valid Java code, but the hash map is not initialized with a reasonable values. Java 9 provides mutiple Map.of() overloaded methods to initialize rutgers new brunswick zip code a Map with upto 10 key-value pairs. You have a defined number of entries or you know what should be the number of values that the map will contain, then it’s recommended to set the ‘initial capacity’ accordingly. The goal of these methods is to ‘reduce the verbosity’ of Java when immutable collections are created.

In the following code an empty map with keys and objects of type Strings is initialized . 2) Obscure, internally uses the instance initializer construct of the anonymous class. We first declare and then initialize a HashMap with the data in the following code.

3) It holds a hidden reference of the enclosing instance, which may cause the memory leak in the Java application. 2) Instantiation and initialization in the same expression. This has the advantage of not creating an Anonymous class.

We use the Stream API to call the of() method that sequentially returns a single element in a Stream. We declare a Map and put the key-value data types as String in the example. //Returns a map which always has the smallest possible initial capacity .

The mapExample is mutable, and we can put another data after the initialization. This type of initialization generates a mutable type of HasMap, and we insert some data in it using the put() method where the first argument is the key. This article describes the need to initialize a HashMap with fitting default values. It is important to know something about the internal implementation of a HashMap to select the best default values for initialization.

The below we can see the HashMap key matching to values like 32 integer value mapping to string object “Deepak”. The approaches discussed above will create a mutable map. But we can also use Java streams to create immutable maps. We can use a two-dimensional object array(with each row containing a key-value pair) to create a stream. Then, we can collect the streamed objects into our map by using the toMap() method.

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