Instance variable verification

Question

Reading effective java, it mentions that we need to validate method parameters as a good practice, throw exceptions for public and assert for private methods. But, do we need to take any measure against instance variable.

EG: (dont take example as a use case, main question is Do I need to validate instance var or Not ?. Example was used only for explaining what i mean to ask. )

class Tree {

   private Node root;

   public doSomething() {
      TreeNode node = root;
   }
}

Now lets say root was never initialized and doSomething() was called directly, it would result in NullPtrException.

Do we need to guard against it ? If yes then how ? If no then why not ?

Only if you use node in `doSomething` will throw NullPointerException your example is vague — nachokk, Sep 26 '13 at 17:50
Probably you can check if root is instance of TreeNode using instanceof operator before assigning it... — dganesh2002, Sep 26 '13 at 17:53
@All, please focus on main question rather than the example. — JavaDeveloper, Sep 26 '13 at 17:55
@dganesh2002 From what the OP said, the issue isn't a CastTypeException, it's the fact that root may be null — DeadChex, Sep 26 '13 at 17:55
@DeadChex He in general ask about the validations that we need add for instances so I suggested it. — dganesh2002, Sep 26 '13 at 17:57

score 1 · Answer 1 · answered Sep 26 '13 at 17:51

1

How about:

if (root == null) {
    throw new SomethingException();
}
TreeNode node = root;

Simply put, just perform a null check on the root variable, and if it is null then execute code accordingly to fix that problem or throw a new exception for it.

answered Sep 26 '13 at 17:51

Rogue

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Seems the same issue persists, an Exception is still being thrown, all this seems to do is rename the Exception. Unless there's code further down the path that can also throw a NullPointer and this is to determine what threw it, it seems to be easier to just catch the NullPointer and handle it. – DeadChex Sep 26 '13 at 17:52
I edited in my reasoning, but yes it's more or less a placeholder for "put what you need here". Whether that be a custom exception, a null pointer, or even code that initializes the variable would be up to the design of the OP. – Rogue Sep 26 '13 at 17:53
@Rogue So your answer is that each instance variable needs to be verified ? – JavaDeveloper Sep 26 '13 at 17:57
Personally I would make sure that global variables are set via the class constructor, but if you know that they are not going to be initialized by that point then you should do a check if the global variable is null. – Rogue Sep 26 '13 at 18:00

score 0 · Answer 2 · answered Sep 26 '13 at 17:58

Move any validation code of instance variables to constructors and setters so you check it whenver it will be set or changed and throw any Exceptions if input is invalid BEFORE the change is made.

this keeps the instance variables in a valid state at all times for fully constructed objects so you don't have to sprinkle extra validation everywhere else.

score 0 · Accepted Answer · answered Sep 26 '13 at 19:52

The established practice to check method parameters can be generalized to check correct usage of a class. In the given example it is illegal to call method doSomething() when the instance variable root has not yet been initialized.

If you want a best practice rule, it is not to generally check instance variables, but to check that a certain usage protocol of a class is obeyed. (Unless the class is written in a way that no particular protocol needs to be followed, which is usually the better option.) This check will of course typically involve checking instance variables.

In contrast to throwing an IllegalArgumentException after a negative method parameter check, a protocol violation should lead to an IllegalStateException.

In the given example an IllegalStateException should be thrown, because that instance is not in a state where doSomething() may be called.

Instance variable verification

3 Answers3