Why is there a problem with a non-static variable being read from main?

String name = "Marcus";
static String s_name = "Peter";

public static void main(String[] args) {    
    System.out.println(name);//ERROR
    System.out.println(s_name);//OK
}

ERROR: Cannot ma开发者_开发百科ke a static reference to the non-static field name

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.

To access non-static member variables and functions, you must have a specific object. (e.g. if all that was inside class Bob { ... }, you would need to do something like

Bob bob = new Bob(); 
System.out.println(bob.name);

inside your main.

name is an instance variable in this case, and you are trying to access it without an object created, so technically name variable doesn't exist in memory, but for a static variable(s_name), its a class variable, it comes into existence once the class is created.