为什么静态方法不能直接访问由非静态方法更改的值

Question

public class JustPractice {

    public int points=0;

    public static void main(String args[]) {
        JustPractice ha = new JustPractice();
        ha.end();
        happy();
    }

    public void end() {
        this.points=100;
        System.out.println(points);
    }

    public static void happy() {
        JustPractice object = new JustPractice();
        System.out.println(object.points);
        return;
    }

}

The above is displaying:

100
0

whereas it should be displaying:

100
100

Answer 1

You are looking at two different instances of your class.

Every instance gets their own copy of the instance fields, and they are completely independent from each-other.

JustPractice ha = new JustPractice();
ha.end();    // this one has "100"

JustPractice object = new JustPractice();  // this one has "0"
System.out.println(object.points);   

The static method can only access ha 's instance fields if you provide it with a ha as a parameter.

Answer 2

Make points static . Then you got what you want.

public static int points=0;

make points static will keep only one variable for all instance of your class.

Else each initialization will create separate individual variable and will assign the value to 0

Answer 3

its because of when method is making new object then that time it will having another copy as of the reference object will be having independent copy per object of the class remember the java basic?

and if u will make int as static object then it will give your output what u want and what u asking for simple example from yours is

public static int points = 0;


    public void end() {
        this.points = 100;
        System.out.println(points);
    }

    public static void happy() {
        CheckingClass object = new CheckingClass();
        System.out.println(object.points);
        return;
    }

    public static void main(String args[]) {
        CheckingClass ha = new CheckingClass();
        ha.end();
        happy();
    }

hope it helpful

Answer 4

A static method can access a non-static object, but only if it is passed to the method. Given that the static method in question is the method in which the program begins, you have no ability to pass anything to it.

What it cannot do, within the scope of a non-static class, is access any of the instance members or functions. The problem is not that you're instantiating ha from a static method, it's that you're accessing the member points, which is an instance member that belongs to every instantiation of your class.

In order to fix this, you can either pass points to the instance of JustPractice (ha) that you're creating, and have one of JustPractice's methods return the final value, or you can make points, end() and happy() static as well, in which case all can work happily with one another.

Answer 5

it is displaying as 100 0 instead of 100 100 because you are creating a new instance of class JustPractice in your happy() method, instead of passing reference ie ha of existing instance.

new newly created instance is showing the default value that that u have given in your class, ie 0.

Answer 6

You set value of the points instance variable in end method.and for the object in happy method you don't initialize the points instance variable of object.

So it gets the default value 0.