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HashMap Literal Initialization

Emma Brown
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#java #programming
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Java HashMap init: `new HashMap{...}`? Think you know the answer? Think again. This concise initialization hides a subtle gotcha. Solve the puzzle?

Can a Java HashMap be initialized like `Map test = new HashMap{"test":"test","test":"test"};`? If not, what's the most concise way to initialize it with known, unchanging key-value pairs?

Solution in a Nutshell

Java offers no direct literal HashMap initialization like JSON. However, you can achieve concise initialization using several methods.

Method 1: Using Map.of() (Java 9+)

Best for immutable maps with few entries:

Map<String, Integer> ages = Map.of("Alice", 30, "Bob", 25);

Method 2: Using Map.ofEntries() (Java 9+)

Handles potential collisions better than Map.of():

Map<String, Integer> ages = Map.ofEntries(
    Map.entry("Alice", 30),
    Map.entry("Bob", 25)
);

Method 3: Stream-based initialization

For larger maps or dynamic data:

Map<String, Integer> ages = Stream.of(
    new AbstractMap.SimpleEntry<>("Alice", 30),
    new AbstractMap.SimpleEntry<>("Bob", 25)
).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

Method 4: Traditional Constructor

For maximum flexibility and compatibility with older Java versions:

Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);

Spring Boot Context: These initialization methods work seamlessly within Spring Boot applications. Inject the Map into your services as usual. Choose the method best fitting your map size, immutability needs, and Java version. For larger, mutable maps, the constructor approach offers better performance. For smaller, immutable maps, Map.of() or Map.ofEntries() provide cleaner syntax.