The reason this causes a problem is that main
is a static method, which means that it has no receiver object. In other words, it doesn't operate relative to some object. Consequently, if you try looking up a non-static field, then Java gets confused about which object that field lives in. Normally, it would assume the field is in the object from which the method is being invoked, but because main
is static this object doesn't exist.
As a general rule, you cannot access regular instance variables from static methods.
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