Java version checker
Minecraft 1.21.1 needs Java 21. Paste what Command Prompt prints and see if you are ready.
Open Command Prompt (press the Windows key, type cmd), run java -version and paste everything it prints.
How it works
The checker reads the version number at the top of the java -version output. Version 21 is what Minecraft 1.21.1 targets. Older versions such as 8, 11 or 17 can stop the game from starting, and a newer version usually works but is not what the game is built for. It also looks for 64-bit, because a 32-bit Java cannot give the game enough memory. If the check says to install Java 21, the Java 21 guide shows how.
Questions
Why does Minecraft need Java 21?
Minecraft 1.21.1 is built for Java 21. Older Java versions cannot run its code.
I installed Java 21 but it still shows an old version.
Another Java earlier in your PATH may be answering. Uninstall the old one, reopen Command Prompt and check again.
Is anything uploaded?
No. The text you paste is read in your browser and never leaves your computer.