In this post, I’d like to share a tip about creating a Map object in Java with some fixed key-value pairs in just a single line. This tip will save you time in writing Java code, definitely.

You know, normally we create a Mapobject and put some initial pairs of key-value into it, as shown below:

Map<Integer, String> map = new HashMap<>();

map.put(200, "OK");
map.put(201, "Created");
map.put(301, "Moved Permanently");
map.put(403, "Forbidden");
map.put(404, "Not Found");
map.put(500, "Internal Server Error");

This is the trivial way to initialize a Map with some initial key-value pairs, using the Map’s put() method. However, this can be boilerplate and lengthy if there are more number of key-value pairs. There’s should be a better way, as described below.

 

1. Using Map.of() method

Since Java 9, you can use the following static method provided by the Map interface to initialize a Map with some fixed key-value pairs in just a single line:

Map.of(key1, value1, key2, value2, …)

This method creates and returns an immutable object of type Map, which holds the given key-value pairs. You can initialize up to 10 pairs of key-value pairs. So the above code example can be rewritten as follows:

Map<Integer, String> map = Map.of(
		200, "OK",
		201, "Created",
		301, "Moved Permanently",
		403, "Forbidden",
		404, "Not Found",
		500, "Internal Server Error"
	);

You see, this way makes the code more succinct and readable, right? Also it’s easier and more convenient to write.

 

2. Return a mutable Map object

Note that the Map.of() method returns an immutable object of type Map, which means you can’t put new key-value pairs or remove existing ones. Attempt to do so will result in UnsupportedOperationException thrown at runtime.

So if you want to have a modifiable Map object returned, you need to construct a new concrete Map type (e.g. HashMap) that wraps the object returned by the Map.of() method. The follow code example gives you the idea:

Map<Integer, String> map = new HashMap<>(Map.of(
		200, "OK", 
		201, "Created", 
		301, "Moved Permanently",
		403, "Forbidden", 
		404, "Not Found", 
		500, "Internal Server Error"
	));

Then you can put/remove key-value pairs to/from the map later, for example:

map.put(401, "Unauthorized");
map.remove(500);

 

Summary:

You have seen some code examples about how to initialize a Map object with some default key-value pairs in just one line, which is convenient and saves you time. Note that the Map.of() static method takes up to 10 pairs of key-value and it returns an immutable object of type Map. If you want to create a modifiable Map, you need to instantiate a new HashMap object that copies key-value pairs from the object created by Map.of() method.

I recommend you watch the following video to see the coding of Map initialization in action:

 

References:

• Map.of() Javadocs

 

Other Java Collections Tutorials:

• What is Java Collections Framework?
• Java Queue Tutorial
• Java List Tutorial
• Java Set Tutorial
• Java Map Tutorial
• Understand equals and hashCode
• Understand object ordering
• 18 Java Collections and Generics Best Practices