为什么我们不能在方法之外使用类的对象来调用类的方法？

Question

    public class Basics {  

        Basics b = new Basics(); 



        int instanceVariable = 0; 

        public void behavior() {      
          System.out.println("Print Something");
        }       

        b.behavior();   // Why this line, b.behavior doesn't work?

        public static void main(String[] args){
          Basics b = new Basics();   /* Why are we allowed to create an 
                                          * object of same name again within
                                          * the same class */
          b.behavior(); // This line works
        }


}

In the above class, I am able to create object . But I can't call b.behavior outside any class, but I am able to do that within a method. Why is that so? What is the difference?

public class Basics1 extends Basics{

     Basics b = new Basics();
     Basics1 b1 = new Basics1();

     b = b1.instanceVariable;  // I don't see error in this line, but previous line shows //error.
     b1.instanceVariable // This line doesn't work


}

Why is b1.instanceVariable not working, instanceVariable is the base class instance variable.

Answer 1

A class defines variables and methods. b.behavior(); is a statement that cannot be on its own like that.

Answer 2

You need to understand that a class is a "type definition", not a code block or sequence of statements.

You cannot just write arbitrary statements in a type definition.

Even so, "b1.instanceVariable" is not a statement. "b1.instanceVariable" doesn't mean anything in statement context.

Answer 3

All code needs to be in methods, in field declarations (such as Basics b = new Basics(); in your example) or in "initializer blocks" (which are run as part of constructors or during class initialization).

This is just a syntax rule.

In other languages, you can have this kind of "raw code", to achieve various effects. What do you want to achieve?

• run during compilation (like in Perl): Cannot be done in Java
• run during constructor: use an init block
• run during class loading: use a static init block
• run when the program starts: put it in static void main
• run during method invokation: put it in that method

Answer 4

Write code anywhere and expect it to execute is a form of procedural programming. In this form , you tend to loose context and soon the code becomes a spaghetti code --> methods getting called anywhere , anytime.

With OOP, you are trained to create objects with well defined methods which have a defined context. Just think, when would you like to get the b.behavior(); being called : Before initializing a class, after initializing the class, after execution of main or when the object is destroyed?

Interestingly, Java has defined syntaxes for each of the states.. You can wrap your code in { System.out.println("Hello World "); } and it will execute when the class is instantiate...Also you can use static { System.out.println("Hello World "); } and this will execute when class is loaded. But again, this is part of telling the JVM when to do it - an agreed upon syntax.. But without any marker around your code, when would you actually expect to run?