How to Efficiently Initialize a HashMap in Java?

How to Initialize a HashMap Directly and Concisely

Many Java developers desire a straightforward way to initialize a HashMap with key-value pairs. Unfortunately, Java's standard library lacks a syntax that directly accommodates this.

Java Versions 9 and Higher

However, for Java 9 and later, there's good news! New factory methods have been introduced to simplify map creation:

• Map.of: Accepts up to 10 key-value pairs in the form of Map.of("key1", "value1", "key2", "value2", ...)
• Map.ofEntries: Handles any number of entries using Map.ofEntries(entry("key1", "value1"), entry("key2", "value2"), ...)

Example:

// Using Map.of for up to 10 elements
Map<String, String> test1 = Map.of("a", "b", "c", "d");

// Using Map.ofEntries for any number of elements
Map<String, String> test2 = Map.ofEntries(entry("a", "b"), entry("c", "d"));

Java Versions 8 and Below

For earlier Java versions, you'll need to manually add each element. Here's a slightly more concise option using an anonymous subclass initializer:

Example:

Map<String, String> myMap = new HashMap<>() {{
    put("a", "b");
    put("c", "d");
}};

Note:

• This approach creates an additional class, potentially affecting memory and performance.
• It may hold a reference to the creating object, preventing garbage collection.

An alternative involving a function:

Example:

Map<String, String> myMap = createMap();

private static Map<String, String> createMap() {
    Map<String,String> myMap = new HashMap<>();
    myMap.put("a", "b");
    myMap.put("c", "d");
    return myMap;
}

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