`Long` is a boxed (reference) type, but small numbers are interned so you get the same reference when boxing a primitive value. Larger numbers aren't interned, so you get fresh boxes.
https://stackoverflow.com/questions/1700081/why-is-128-128-f...
Reference comparison it is with a twist: the jvm caches the most common integers. You'll get the same object from -128 to +127 when implicitly assigning value. This is part of the spec, and can be set to a larger pool with -XX:AutoBoxCacheMax (but cannot be turned off).
If you explicitly ask for a new object (Long a = new Long(123l);) you will get a new object and the comparison will fail as expected.
The moral of the story is to always use .equals().
Terms like boxing and reference comparison come to mind but I can't stitch them into a narrative to explain this.