Hash-based collections with known capacity should be initialized with the proper related static method.

Why is this an issue?

When creating an instance of HashMap or HashSet, the developer can pick a constructor with known capacity. However, the requested capacity is not fully allocated by default. Indeed, when the collection reaches the load factor of the collection (default: 0.75), the collection is resized on the fly, leading to unexpected performance issues.

How to fix it

As of Java 19, hash-based collections provide a static method that allocates the requested capacity at construction time.

Code examples

Noncompliant code example

private static final int KNOWN_CAPACITY = 1_000_000;

public static Map<String, Integer> buildAMap() {
    return new HashMap<>(KNOWN_CAPACITY); // Noncompliant
}

public static Set<String> buildASet() {
    return new HashSet<>(KNOWN_CAPACITY); // Noncompliant
}

Compliant solution

private static final int KNOWN_CAPACITY = 1_000_000;

public static Map<String, Integer> buildABetterMap() {
    return HashMap.newHashMap(KNOWN_CAPACITY);
}

public static Set<String> buildABetterSet() {
    return HashSet.newHashSet(KNOWN_CAPACITY);
}

public static Set<String> buildABetterSet(float customLoadFactor) {
    return new HashSet<>(KNOWN_CAPACITY, customLoadFactor);
}

Resources

Documentation

• https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/HashMap.html#newHashMap(int)
• https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/HashSet.html#newHashSet(int)
• https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/LinkedHashMap.html#newLinkedHashMap(int)
• https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/LinkedHashSet.html#newLinkedHashSet(int)
• https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/WeakHashMap.html#newWeakHashMap(int)

Message:
Replace this call to the constructor with the better suited static method.

Highlighting:
The infringing constructor call.