Accessing static method through an object instance

问题

I have learnt that if we wish to call static method of another class then you have to write class name while calling static method. In below program, I created object of the Employee class inside Employee_Impl class and using that object, I was still able to access the count method. Why is it allowing me to use count method through an object if static methods are accessed using only class name? Does that mean static methods can be accessed using objects as well as class name, both?

Employee.java

public class Employee{
    static int counter = 0;
    static int count(){
        counter++;
        return counter;
    }
}

Employee_Impl.java

class Employee_Impl
    public static void main(String args[]){
        Employee obj = new Employee();
        System.out.println(obj.count());
        System.out.println(Employee.count());
        System.out.println(obj.count());        
    }
}

output

1
2
3

回答1:

Compiler automatically substitutes this call by call by class name of your variable (not of it's value!). Note, even if your object would be null, it will work, no NullPointerException is thrown.

回答2:

You are allowed to do this because you made an instance of Employee to access the method through.

The point of static methods is to allow access to "utility methods" that can be invoked without the overhead of instantiating a new object. Furthermore, these methods can be shared by any instance of the class Employee and can mutate shared static variables or attributes. For example, pretend the shared prefix of all Employee objects was maintained as a static attribute:

public class Employee {
    private static String EMPLOYEE_IDENTIFIER_PREFIX = "Acme Corporation Employee Number:"

    public static void setEmployeeIdentifierPrefix(String prefix){
         Employee.EMPLOYEE_IDENTIFIER_PREFIX = prefix;
    }
}

If AcmeCorporation was purchased by MultinationalCorporation the prefix can be updated for all Employee objects by using the setEmployeeIdentifierPrefixMethod on the class like so:

Employee.setEmployeeIdentifierPrefix("Multinational Corporation Employee Number:");

回答3:

Static methods are always executes from class...not from objects... here.., even though you are calling count() method on object obj.count() it will executes from Employee class only... first it will identify the class of object then from that object type it will execute the count() method.

来源：https://stackoverflow.com/questions/32039657/accessing-static-method-through-an-object-instance

标签

java

static-methods