在Java中以静态方法访问非静态数据

Question

i am a new java learner and i am making a program where i want to access the value of a not static data member in a static method but rule says that i cant not do this but we can access it after creating a object my question is that if i make a object of that class that old value of that data member is erase why ? how can i use it old value of a not static data member in a static method

import java.util.Scanner;
class emp
{
     String name;
     int rate;
     static String c_name="TCS";


     void setdata(String n,int s)

     {
       name=n;
       rate=s;  

     }
     static void employee_salary_cal(int t)
      {
        int day,rate1,Total;
        day=t;
        emp e2=new emp();
        rate1=e2.rate;
        Total=rate1*day;

        System.out.println("Total salary " +Total);
      }  

     void showData()
     {
       System.out.println("Employee name = " +name);
       System.out.println("Employee pay rate per day = " +rate);
     }


}

class emp_main
{
  public static void main(String args[])
     { 
        int da;
        emp e1=new emp();
        e1.setdata("alex",100);
        System.out.println("Company name = " +emp.c_name);
        e1.showData();
        System.out.println("Enter Work days in months ");
        Scanner sc=new Scanner(System.in);
        da=sc.nextInt();
        emp.employee_salary_cal(da);



     }
}

program output :

Company name = TCS
Employee name = alex
Employee pay rate per day = 100
Enter Work days in months
25
Total salary 0

Answer 1

Just pass your emp object to the static method, instead of creating a new instance of it. The "rule" is that you can't access instance variables and methods, but a static method can receive external objects and fiddle with them.

static void employee_salary_cal(emp e, int t)
{
    System.out.println("Total salary " + e.rate * t);
}

On another note, you are lacking serious programming fundamentals. I recommend you follow some really basic tutorials, again.

Answer 2

why would you use static at all for the function? Use the this context.

 void employee_salary_cal(int day)
  {
    System.out.println("Total salary " + (this.rate * day));
  }  

Then you can just call it as an instance function like so

emp e = new emp();
e.employee_salary_cal(5);

Answer 3

将您要使用的对象作为参数传递给静态函数。

static void employee_salary_cal(emp employee, int t)

Answer 4

It wouldn't make sense to be able to access an instance variable from a static context.

For each class there is exactly 1 set of static fields. However, each class could have any number of copies of each of its instance variables. The JVM would have no idea which copy of the instance variable you were talking about.

Like other have stated, if you want to access an instance from static, you have to tell the static method which instance, by passing the instance into it.

Answer 5

The difference between static and non-static:

class MyClass {
    public int nonStatic=1;
    public static int isStatic = 2;
}

MyClass a=new MyClass();  
a.nonStatic = 3          // nonStatic =3; isStatic=2
MyClass b=new MyClass(); // nonStatic =1; isStatic=2
MyClass.isStatic = 3;    // nonStatic =1; isStatic=3
MyClass c=new MyClass(); // nonStatic =1; isStatic=3

What I want to explain is, If you create N instances of MyClass, you also create N values (or more precise int-pointer in memory) for nonStatic, but only one for isStatic;