1) Can you save java source file by other name than the class name ?

Answer: Yes, if the class in file not pulbic. the figure bellow:

Compiler: javac abc.java

Runtime: java Simple.class

Note: The JRE only has java, not javac but JDK has both.

2) Can you have multi classes in a java source file ?

Answer: yes , the figure bellow

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https://www.javatpoint.com/internal-details-of-hello-java-program