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An object becomes eligible when no strong-reference path leads to it from roots such as live thread stacks, static fields, or JNI references. Eligibility does not guarantee immediate collection or finalization. To diagnose retention, inspect heap dumps and dominator paths; do not rely on System.gc() as a fix.

Enable and inspect unified GC logs for pause duration, cause, heap before/after, promotion, and allocation behavior. Correlate with request latency and container memory. A large live set, allocation bursts, humongous objects, or memory pressure can imply different fixes; tune collector flags only after identifying which resource or workload pattern drives the pauses.

Weak references do not keep an object alive and are useful in structures such as canonicalizing maps when carefully designed. Soft references may be cleared under memory pressure, but their timing is not a predictable cache eviction policy. Phantom references support post-mortem cleanup coordination through a reference queue; they do not let code retrieve the referent.

HashMap uses an array of buckets and computes an index from the hash of the key. Each bucket may hold one or more entries. When collisions happen, Java stores entries in a linked list and later in a tree when the bucket grows large enough. The table resizes when the load factor is exceeded so the map keeps operations efficient. The key idea is balancing fast lookup with controlled memory use.