Question

I need to do something like this,

if (first_var > second_var)
  int difference = first_var - second_var;
if (first_var < second_var)
  int difference = second_var - first_var;

When I try to compile this, an error comes stating the variable ‘difference’ may not have been initialized. Making the variable ‘difference’ global doesn’t help either.

Answer 1

The issues that you need to learn are:

• The scope of variables
• Block declaration and how that creates new scopes
• Why you should prefer to use {...} block for if statements
• How to guarantee definite assignment
• When to use if-else instead of if (something) {...} if (!something) {...}

By the way, the idiomatic way to find the difference of two values is:

int difference = Math.abs(firstVar - secondVar);

Do note that Math.abs has a corner case: when the argument is Integer.MIN_VALUE, the returned value is Integer.MIN_VALUE. That’s because -Integer.MIN_VALUE == Integer.MIN_VALUE. The issue here is 32-bit two’s complement representation of numbers.

References

• JLS 16 Definite Assignment
• JLS 14.4.2 Scope of Local Variable Declarations
• JLS 14.9.2 The if-then-else Statement
• Math.abs(int)
• Wikipedia/Two’s complement (read: the most negative number)

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Attempt #1: Declaring before the if

Here’s one attempt to fix the snippet, by declaring the variable before the if

int difference;
if (first_var > second_var) {
  difference = first_var - second_var;
}
if (first_var < second_var) {
  difference = second_var - first_var;
}

// note: difference is not definitely assigned here!
//           (but at least it's in scope!)

We now have a problem with definite assignment: if first_var == second_var, the variable difference is still not assigned a value.

---

Attempt #2: Initializing at declaration

Here’s a second attempt:

int difference = 0;
if (first_var > second_var) {
  difference = first_var - second_var;
}
if (first_var < second_var) {
  difference = second_var - first_var;
}

// note: difference is in scope here, and definitely assigned

Beginners tend to do this, but this precludes the possibility of making difference a final local variable, because it’s possibly assigned value twice.

---

Attempt #3: if-else

Here’s a better attempt:

final int difference;
if (first_var > second_var) {
  difference = first_var - second_var;
} else {
  difference = second_var - first_var;
}

// note: difference is in scope, and is definitely assigned here,
//       (and declared final)

There are still ways to improve on this.

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Attempt #4: The ternary/conditional operator

Once you’re more comfortable with the language and programming, you may use the following idiom:

final int difference = (first_var > second_var) ? first_var - second_var
                                                : second_var - first_var;

This uses the ?: ternary/conditional operator. Do be careful with this operator; it has some behaviors that may be surprising, and it definitely can be abused. Use carefully, judiciously, idiomatically.

References

• JLS 15.25 Conditional Operator ?: