That would violate the Java specification, i.e. it's not Java. On the other hand, what you could do is only allow allocations to succeed in class initialisation

That would violate the Java specification, i.e. it's not Java. On the other hand, what you could do is only allow allocations to succeed in class initialisation (Java's "static") and throw an error if done outside it. Note, however, that even adding an element to a Map outside of initialisers would fail when using OpenJDK's standard library even if both key and value have been preallocated in initialisers, as it may allocate an internal node and/or a new hash array. So you may want to change some of the standard library to make it more useful with your restrictions. But you may want to ask yourself why you want to do that. OpenJDK's GCs have become really, really good in both throughput and latency, and the main thing you pay in exchange is memory footprint.