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copied!<p>The issues that you need to learn are:</p> <ul> <li>The scope of variables</li> <li>Block declaration and how that creates new scopes</li> <li>Why you should prefer to use <code>{...}</code> block for <code>if</code> statements</li> <li>How to guarantee definite assignment</li> <li>When to use <code>if-else</code> instead of <code>if (something) {...} if (!something) {...}</code></li> </ul> <p>By the way, the idiomatic way to find the difference of two values is:</p> <pre><code>int difference = Math.abs(firstVar - secondVar); </code></pre> <p>Do note that <code>Math.abs</code> has a corner case: when the argument is <code>Integer.MIN_VALUE</code>, the returned value is <code>Integer.MIN_VALUE</code>. That's because <code>-Integer.MIN_VALUE == Integer.MIN_VALUE</code>. The issue here is 32-bit two's complement representation of numbers.</p> <h3>References</h3> <ul> <li><a href="http://java.sun.com/docs/books/jls/third_edition/html/defAssign.html" rel="noreferrer">JLS 16 Definite Assignment</a></li> <li><a href="http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.4.2" rel="noreferrer">JLS 14.4.2 Scope of Local Variable Declarations</a></li> <li><a href="http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.9.2" rel="noreferrer">JLS 14.9.2 The if-then-else Statement</a></li> <li><a href="http://download.oracle.com/javase/6/docs/api/java/lang/Math.html#abs%28int%29" rel="noreferrer"><code>Math.abs(int)</code></a></li> <li><a href="http://en.wikipedia.org/wiki/Two%27s_complement" rel="noreferrer">Wikipedia/Two's complement</a> (read: the most negative number)</li> </ul> <hr> <h3>Attempt #1: Declaring before the <code>if</code></h3> <p>Here's one attempt to fix the snippet, by declaring the variable before the <code>if</code></p> <pre><code>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!) </code></pre> <p>We now have a problem with definite assignment: if <code>first_var == second_var</code>, the variable <code>difference</code> is still not assigned a value.</p> <hr> <h3>Attempt #2: Initializing at declaration</h3> <p>Here's a second attempt:</p> <pre><code>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 </code></pre> <p>Beginners tend to do this, but this precludes the possibility of making <code>difference</code> a <code>final</code> local variable, because it's possibly assigned value twice.</p> <hr> <h3>Attempt #3: <code>if-else</code></h3> <p>Here's a better attempt:</p> <pre><code>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) </code></pre> <p>There are still ways to improve on this.</p> <hr> <h3>Attempt #4: The ternary/conditional operator</h3> <p>Once you're more comfortable with the language and programming, you may use the following idiom:</p> <pre><code>final int difference = (first_var > second_var) ? first_var - second_var : second_var - first_var; </code></pre> <p>This uses the <code>?:</code> 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.</p> <h3>References</h3> <ul> <li><a href="http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.25" rel="noreferrer">JLS 15.25 Conditional Operator <code>?:</code></a></li> </ul>

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