Since Java 9 the pair Observer-Observable have been declared deprecated.
They don't provide a rich enough event model for applications.
For example, they support only the notion that something has changed,
but they don't convey any information about what has changed.
There are also some thread-safety and sequencing issues that cannot be fixed compatibly.
It's possible to use create an MCP installation for versions of Minecraft where there hasn't been a full MCP release. It takes a little bit of manual setup, but the end result is highly useful.
Download and extract the most recent MCP build from http://www.modcoderpack.com/. (Currently, the latest build is http://www.modcoderpack.com/files/mcp940.zip)
Edit version.cfg in the conf folder, and change ClientVersion and ServerVersion to the version you want (for instance, 1.12.2).
Download the SRG zip for the version you want; these can generally be found at http://mcpbot.bspk.rs/mcp/<version>/mcp-<version>-srg.zip (for example, http://mcpbot.bspk.rs/mcp/1.11.2/mcp-1.11.2-srg.zip) or at http://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp/<version>/mcp-<version>-srg.zip (for example, http://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp/1.12.2/mcp-1.12.2-srg.zip). (For 1.12.1 and 1.12.2, only the minecraftforge link works)
Extract that zip into the MCP conf folder, over