Java is a high-level, well-known and powerful object-oriented language, with a large support library and a comfortable toolbelt. Hence, it has been proposed for writing smart contracts in blockchain. However, its support library is non-deterministic, which is a blocking issue for its application to smart contracts. This paper discusses the kind of (non-)determinism of the methods of the Java library and how a deterministic fragment of that library can be specified. It shows that some relevant parts are deterministic only under specific conditions on run-time values. It concludes with the description of an instrumentation, for the Takamaka blockchain, that enforces such conditions, statically or dynamically, reporting experiments with its implementation.