Why is it not syntax error to access member variable in a method not initialized

[Tony Smith]

Today it took me 30 min trying to figure out an issue and it turned out that the instance is not initialized. But the IDE didn't prompt any error. So I had similar code to something below:

public class SomeClass{
    Car myCar;


 public static void main(String[] args) {

  myCar.doSomething();       //IDE will tell me that myCar is not initialized which is good
}



public void someMethod() {

myCar.doSomething();   // However IDE does not prompt error here. Why was it designed this way? 
//What was the reason or principle for not checking that myCar has been initialized or not?

}

}

Thanks!

[Maneesh Godbole]

In future, while posting code, please UseCodeTags. I have added them for you this time. As you can see the code tags, make the code much more easier to read and understand.

In the code you posted, are you sure what you wrote on line 7 is what the IDE is telling you? I would suggest you take a look one more time. Hint: What does static mean?

[Mansukhdeep Thind]

You are breaking a rule laid down by the Java compiler that makes it imperative for you to ensure that static members (methods / variables) should only be accessed in a static context. Read the error message carefully and try and understand the significance of static keyword in Java

[Jeff Verdegan]

Mansukhdeep Thind wrote:You are breaking a rule laid down by the Java compiler that makes it imperative for you to ensure that static members (methods / variables) should only be accessed in a static context.

There is no such rule in the Java language or compiler.

However, it is good practice to minimize access to static members in a non-static context, and to not reference them through a reference variable.

[Jeff Verdegan]

Tony Smith wrote:

public static void main(String[] args) {
  myCar.doSomething();       //IDE will tell me that myCar is not initialized which is good
}

No, it will tell you that you can't access a non-static variable in a static context. It won't say anything about it not being initialized.

public void someMethod() {
myCar.doSomething();   // However IDE does not prompt error here. Why was it designed this way? 
//What was the reason or principle for not checking that myCar has been initialized or not?
}

Member variables get default values at object instantiation, so it's not a compile time error to read those variables before explicitly initializing them, though it will often lead to undesirable results at runtime, such as NullPointerException.

Local variables, on the other hand, do not get any default values, so we have to assign a value to a local before we can read it.

[Campbell Ritchie]

Mansukhdeep Thind wrote: . . . static members (methods / variables) should only be accessed in a static context. . . .

I think you have mistakenly written static when you meant non‑static.

[Mansukhdeep Thind]

Campbell Ritchie wrote:

Mansukhdeep Thind wrote: . . . static members (methods / variables) should only be accessed in a static context. . . .

I think you have mistakenly written static when you meant non‑static.

Yes Campbell, I wanted to say that non static members cannot be accessed from a static context.

[Mansukhdeep Thind]

Jeff Verdegan wrote:

Mansukhdeep Thind wrote:You are breaking a rule laid down by the Java compiler that makes it imperative for you to ensure that static members (methods / variables) should only be accessed in a static context.

There is no such rule in the Java language or compiler.

However, it is good practice to minimize access to static members in a non-static context, and to not reference them through a reference variable.

Sorry Jeff. I meant to say that non static members cannot be accessed from a static context. I mistakenly wrote static where I should have written non static.

[Campbell Ritchie]

Don’t worry. Everybody makes such mistakes every now and again.

[Jeff Verdegan]

Campbell Ritchie wrote:Don’t worry. Everybody makes such mistakes every now and again.

The wonderful thing about this site is that when you do make a mistake, there are a dozen people ready to point it out to you (and to the rest of the world).