在Java中，有逻辑上的原因为什么我不能从命令行说“ java MyClass.class”吗？

Question

Maybe this question may be splitting hairs, but when I compile a file from command line like :

javac MyClass.java

then afterward I cannot run it by saying

java MyClass.class

I have to call:

java MyClass

What is the motivation for this notation?

Answer 1

Because you run a class on the classpath, which may be contained inside a jar for example. You couldn't use your syntax in that case.

Answer 2

Java compiler needs a compilation unit; this is by default (at least) a java source file, with the whole of classes defined in it and its dependencies.

Java interpreter (the jvm) needs a single class with a main method as entry point of the execution - it must start somewhere.

Answer 3

您必须向Sun（现在的Oracle）询问开发历史，但是我确实要指出，对于仅使用Java而不是开发Java的人们来说，“ java DoSomething”比起Java更容易记住和键入。 “ java DoSomething.class”

Answer 4

There is no way to run a Java program that is not a class. For that reason, there is no reason to mandate typing the ".class". You might also invoke a class from within a JAR on your path, or directly, but it's still instantiating a class (possibly a "default" class from the Manifest).

Answer 5

Because the name of the class is MyClass and not MyClass.class . And when running java you specify the CLASS NAME and not the PATH to the actual compiled file. For more in depth knowledge I guess Sun & Oracle will have to answer :)

Answer 6

Imagine that you have a class named package and you have a class named Class , in a package named package ,

--CurrentFolder
    --package
         Class.class
      package.class

so executing java package.class may lead to an undecidability to the compiler!