Java convention on reference to methods and variab

问题:

Section 10.2 of Java conventions recommends using class names instead of objects to use static variables or methods, i.e. MyClass.variable1 or MyClass.methodName1() instead of

MyClass Obj1 = new MyClass();    
Obj1.variable1;
Obj1.methodName1();

There is no explanation of the rationale behind this, although I suspect this has something to do with memory use. It would be great if someone could explain this.

回答1:

I guess you mean "for static methods and variables".

There is no difference regarding memory, except of course if you create the instance just for calling the method. Conventions aren't for memory efficiency but for coder efficiency, which is directly related with the readability of the code.

The rationale is that by reading

MyClass.methodName1()

you know it's a static method and that it can't use or change your Obj1 instance.

And if you write

obj1.variable1; // note the "o" instead of "O", please do follow conventions

then the reader has to read your source code to know if variable1 is static or not.

回答2:

If you use object for static variable access then compiler will replace it with Class Name only.

So

MyClass Obj1 = new MyClass();    
Obj1.variable1;
Obj1.methodName1();

It is same as

MyClass.variable1;
MyClass.methodName1();

Now Why to differentiate? Answer is - It is for better reading If someone see method being called on Class then he immediately come to know that it is static method. Also it prevents generation of one additional object to access the method.

回答3:

This has to do with public static methods and variables. Since these methods/variables are associated with the respective class rather than an instance of the class, it is nice to use refer to these methods or variables as className.methodName() or className.variableName

"Understanding Instance and Class Members" would be a good starting point to learn about the use of the static keyword to create fields and methods that belong to the class, rather than to an instance of the class

回答4:

It is only because, public static method or public static variable is not associated with any object, but the class. Though the language designer has given the flexibility of invoking them on objects, reader of the code would be confused whether those are static variable/methods or instance methods/variables. So readability is the reason behind asking the developers to invoke them on classes.

回答5:

You are allowed to access static members either by using the class name notation or by accessing using an object. It is not recommended to use the object notation since it can be very confusing.

public class TheClass {
    public static final staticValue = 10;
    public static void staticMethod() {
        System.out.println("Hello from static method");
    }

    public static void main(String ... args) {
        TheClass obj = null;

        // This is valid
        System.out.println(obj.staticValue);
        // And this too
        System.out.println(obj.staticMethod());

        // And this is also valid
        System.out.println(((TheClass)null).staticValue);
        // And this too
        System.out.println(((TheClass)null).staticMethod());

    }
}

It is much clearer if the static methods and variables are called with the class name notation.

回答6:

static variable belongs to the class and not to object(instance). A static variable can be accessed directly by the class name and doesn’t need any object. it saves space not having to have variables for the same data for each class.

Syntax : <class-name>.<variable-name>

public class AA{

 static int a =10;


}

You can call

System.out.println(AA.a);
System.out.println(aObject.a);

There is no differen between two calling but maintain coding convention to keep more readbale