Fungible and non-fungible tokens with snapshots in Java

Many blockchain applications exchange tokens, such as bitcoin and ether, or implement them through smart contracts. A trend in blockchain is to apply standards for token interoperability, unchanged, from platform to platform, easing the design challenges with trusted and widely-used specifications. However, the exploitation of the target language semantics can result in technological advantages and more efficient contracts. This paper presents a re-engineering of OpenZeppelin's implementation of the ERC-20 and ERC-721 standards in Takamaka, a Java framework for programming smart contracts. It describes a sound solution to the issue about the types allowed for the token holders and a novel implementation for making snapshots of tokens, based on tree maps, that is possible in Java, but not in Solidity, more efficient than the literal translation in Java from Solidity, within the Java virtual machine. Moreover, it applies to ERC-721 as well, where a snapshot mechanism was previously missing. The same snapshot mechanism can also be applied beyond the smart contracts for tokens.