non-static variable reference from a static context (Java)

Question

I've searched for this problem but still not getting any solution.

I declared this simple program:

public class Test{
  public abstract class Person {
      public void talk(){
        System.out.print("I am a person");
      }
      public abstract void display();
  }

  public class Student extends Person {
    public void talk(){
        System.out.println("I am a student");
    }

    public void display(){
        System.out.println("Nice to meet you");
        super.talk();
    }
  }

  public static void main(String args[])
  {
    Student s = new Student();
    s.display();
  }
}

but it keeps giving me the error :

error: non-static variable this cannot be referenced from a static context

    Student s = new Student();

I've always been declaring objects that way! I don't know what's happening today.

I need to understand what am I doing wrong here?

Other than the `main` method, there seem to be no static method, nor the usage of the `this` keyword. Are you sure you posted the right code? — npinti, Jan 12 '16 at 12:49
it complains about a variable 'this', which you don't have, you also don't use the keyword this, so, make sure your file is saved, and try again — Stultuske, Jan 12 '16 at 12:49
I'll create a new project file and try to copy the code there — Saif Badran, Jan 12 '16 at 12:51
The problems is that Person and Student are declared inside the class Test. Just put them aside and it will work. — Arnaud Denoyelle, Jan 12 '16 at 12:52
@ArnaudDenoyelle now it gave me this message: class Person is public, should be declared in a file named Person.java . should I also separate it in another file? — Saif Badran, Jan 12 '16 at 12:54
@SaifBadran Declare each class in its own .java file : Person.java and Student.java — Arnaud Denoyelle, Jan 12 '16 at 12:55
@ArnaudDenoyelle I declared each class in it's own and it worked properly. But I have a question is there's any other way to make classes in the same file? thanks! — Saif Badran, Jan 12 '16 at 12:59
@SaifBadran: unless they should not be used except for in combination, or are very closely bound, it is not recommended to do so. — Stultuske, Jan 12 '16 at 13:02
@SaifBadran Yes, you can add the keyword `static` : `public static class Student` but I recommend creating a file apart. By declaring them in the same class, you actually made `Student` and `Person` inner-class of class `Test` and it has its consequences that you need to understand (I upvoted Andy Turner 's answer because he explains it well). — Arnaud Denoyelle, Jan 12 '16 at 13:02

Andy Turner · Accepted Answer · 2016-01-12T13:22:09.943

When you declare inner classes:

class Outer {
  class Inner {
  }

  void doSomething() {}
}

there is an implicit reference to an instance of the Outer class held by each instance of Inner. This means that you can write the following in the inner class to refer to the outer instance:

Outer.this.doSomething();

Actually, you can often write simply doSomething() - you only need the Outer.this if the inner class also has a method called doSomething(), and you need to disambiguate.

The long and short of this is that you actually need an instance of Outer to create an instance of Inner:

Outer outer = new Outer();
Inner inner = outer.new Inner();

If you don't actually need to refer to the instance of Outer inside Inner (e.g. you never need to call doSomething()), the simplest solution is just to make the inner class static:

class Outer {
  static class Inner {}

  void doSomething();
}

In fact, I would recommend that you always make you inner classes static, unless you really need them to be non-static.

Inner inner = new Outer().new Inner(); clears everything,,In order to have an instance of inner class you need the instance of outer class — Danyal Sandeelo, Jan 12 '16 at 13:07

Raman Shrivastava · Answer 2 · 2016-01-12T12:58:36.207

1

make your Person and Student Classes static

OR

Create Test Object first to create Student object in main method.

Student s = new Test().new Student();

Reason: As Person/Student classes are non static, they can't exist without Test Object. So either those classes should be static or Test() should be created first to create Student.

edited Jan 12 '16 at 12:58

answered Jan 12 '16 at 12:53

Raman Shrivastava

2,923
15
26

I don't understand what do you mean by Test object? – Saif Badran Jan 12 '16 at 12:56
Yes I got the idea now. but it's just easier to separate the files. Thanks – Saif Badran Jan 12 '16 at 13:01

score 1 · Answer 3 · answered Jan 12 '16 at 12:56

Remove person and student from your Test class:

public abstract class Person { 
    public void talk(){ 
        System.out.print("I am a person"); 
    }
    public abstract void display();
} 

public class Student extends Person    { 
    public void talk(){ 
        System.out.println("I am a student"); 
    }     
    public void display(){ 
        System.out.println("Nice to meet you");
        super.talk(); 
    } 
}

public class Test{ 
    public static void main(String args[]) { 
        Student s = new Student();    
        s.display(); 
    }
 }

score 1 · Answer 4 · answered Jan 12 '16 at 12:56

This works all fine:

     abstract class Person {
        public void talk(){
            System.out.print("I am a person");
        }
        public abstract void display();
    }

     class Student extends Person {
         public void talk() {
             System.out.println("I am a student");
         }

         public void display() {
             System.out.println("Nice to meet you");
             super.talk();
         }
     }

    public class Test {


        public static void main(String args[]) {
            Student s = new Student();
            s.display();
        }
    }

non-static variable reference from a static context (Java)

4 Answers4