Can static method access non-static instance variable?

Question

So my understanding was that you can't use static method to access non-static variables, but I came across following code.

class Laptop {
  String memory = "1GB";
}
class Workshop {
  public static void main(String args[]) {
    Laptop life = new Laptop();
    repair(life);
    System.out.println(life.memory);
    }
  public static void repair(Laptop laptop) {
    laptop.memory = "2GB";
  }
}

Which compiles without errors.

So isn't

public static void repair(Laptop laptop) {
laptop.memory = "2GB";
}

accessing String memory defined in class Laptop, which is non-static instance variable?

Since the code compiles without any error, I'm assuming I'm not understanding something here. Can someone please tell me what I'm not understanding?

If you had a non static field in Workshop, you could not access that from the static context. But it's perfectly fine to access the non-static field of the non-static object you created inside the static context. — Fildor
– Fildor, Commented Jul 8, 2016 at 9:48

Thomas · Accepted Answer · 2016-12-22 14:24:49Z

34

A static method can access non-static methods and fields of any instance it knows of. However, it cannot access anything non-static if it doesn't know which instance to operate on.

I think you're mistaking by examples like this that don't work:

class Test {
  int x;

  public static doSthStatically() {
    x = 0; //doesn't work!
  }
}

Here the static method doesn't know which instance of Test it should access. In contrast, if it were a non-static method it would know that x refers to this.x (the this is implicit here) but this doesn't exist in a static context.

If, however, you provide access to an instance even a static method can access x.

Example:

class Test {
  int x;
  static Test globalInstance = new Test();

  public static doSthStatically( Test paramInstance ) {
    paramInstance.x = 0; //a specific instance to Test is passed as a parameter
    globalInstance.x = 0; //globalInstance is a static reference to a specific instance of Test

    Test localInstance = new Test();
    localInstance.x = 0; //a specific local instance is used
  }
}

edited Dec 22, 2016 at 14:24

answered Jul 8, 2016 at 9:49

Thomas

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1 Comment

Tilak M Divakar Over a year ago

One important thing to note is globalInstance is static. doSthStatically method will not be able to access globalInstance.x if it wasn't static.

SCouto · Accepted Answer · 2020-06-28 14:10:08Z

1

Yes, a non-static method can access a static variable or call a static method in Java. There is no problem with that because of static members

i.e. both static variable and static methods belongs to a class and can be called from anywhere, depending upon their access modifier.

For example, if a static variable is private then it can only be accessed from the class itself, but you can access a public static variable from anywhere.

Similarly, a private static method can be called from a non-static method of the same class but a public static method e.g. main() can be called from anywhere

edited Jun 28, 2020 at 14:10

SCouto

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answered Jun 28, 2020 at 13:43

Govinda Hinduja

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Comments

Bhargav Kumar R · Accepted Answer · 2016-07-08 09:48:38Z

0

You can access only with object reference.

Instance variables defined at class level, have to be qualified with object name if you are using in a static context. But it does not not mean that you cannot access at all.

answered Jul 8, 2016 at 9:48

Bhargav Kumar R

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Arun Raaj · Accepted Answer · 2018-04-24 14:25:04Z

0

Static methods cannot modify their value. You can get their current value by accessing them with the reference of current class.

answered Apr 24, 2018 at 14:25

Arun Raaj

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Comments

Zeeshan Adil · Accepted Answer · 2018-09-10 06:09:45Z

-2

try this code

public static void repair() {

    Laptop laptop =new Laptop();

    laptop.memory="2GB";

  }

edited Sep 10, 2018 at 6:09

Zeeshan Adil

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answered Sep 10, 2018 at 5:37

VRadhe

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