If you've properly set up Java on your machine you should be able to use the java command to run any code on your machine.

Just write java followed by the path to a file containing Java code.¹

Say we had this code in a file named Main.java

void main() {
    IO.println("Hello");
}

To run this, say java Main.java.

$ java Main.java
Hello

If the file is in a directory that is not your current working directory, you should include the full path.

$ java src/Main.java
Hello