具有main方法的Java继承类

Question

I have 2 objects defined

In Object-1, When I reference the child object it invokes the child method m1() whereas in Object-2 when I reference the child method m1() it references the Parent object..

In both the cases I assume parent object will hold a child type object at runtime...

Object-1

    package pkgB;

    class Parent{
        static int i = 10;
        void m1(){
            System.out.println("In Parent = " + i);
        }

        static void m1(String s){
            System.out.println(s +i );
        }
    }

    public class Child extends Parent{
        static int i = 20;
        void m1(){
            System.out.println("In child" + Child.i);
        }
        public static void main(String[] args){
            Parent p = new Child();
            p.m1();

        }
    }

Object-2

package pkgB;



class Drink{
    public static void m1(){
        System.out.println("Im Drink method");
    }

}

class Coffee extends Drink{
    public static void m1(){
        System.out.println("Im Coffee method");
    }

}

public class test {

    public static void main(String[] args) {
        Drink d = new Coffee();
        d.m1();

    }
}

Output Dobject-1:

In child20

Output Dobject-2:

Im Drink method

Answer 1

static methods are called by their Type while instance methods are called based on the object type. static methods are not overriddedn .

When you say,

  Parent p = new Child();
  p.m1();

You are creating reference type of Parent but object type of Child . m1() is instance method so it is overridden and the Child method is called because object is of type Child

But when you say,

  Drink d = new Coffee();
  d.m1();

here, m1() is static method which are bound to class type. So d.m1() will call the static method as d is of type Drink

Answer 2

In your second example, the m1() methods are static methods. Your compiler should give you a warning when you execute d.m1() , something about referring to a static method in a non-static way.

Anyway, when you call d.m1() in the second example, you are really calling Drink.m1() since d is declared a Drink .

Answer 3

Calling static method using reference variable means you are in state of Sin :)

static methods/variables are solely part of class and can not be overriden.